A great thunderstorm; an extensive flood; a desolating hurricane; a sudden and intense frost; an overwhelming snowstorm; a sultry day, - each of these different scenes exhibits singular beauties even in spite of the damage they cause. Often whilst the heart laments the loss to the citizen, the enlightened mind, seeking for the natural causes, and astonished at the effects, awakes itself to surprise and wonder.
St. John de Crévecoeur

2
•
•
•
•
Weather is the state of the atmosphere at a given time and place.
Extreme weather events threaten lives, disrupt transportation systems, and cause destruction.
The National Weather Service makes over a million daily weather observations that will be used in forecasts by media sources nationwide.
Advanced technology such as weather satellites and
Doppler radar allow the accurate prediction of dangerous weather systems like tornadoes and hurricanes.
No scientific phenomena concern us as much as the daily evolution of weather systems. We live in a culture where weather , the state of the atmosphere at a given time and place, helps us define regional cultural variations. States such as California and Florida are defined in part by their warm, sunny weather. Seattle, Washington, is known for its rain,
North Dakota and Minnesota for cold winter temperatures, and
Oklahoma for tornadoes. Superimposed on the regular rhythms of the atmosphere are more extreme events that threaten lives, disrupt transportation systems, and cause destruction (Figs. 1,
2). Approximately 90% of presidential disaster declarations are weather related. In the dozen years between 1988-1999, there were 38 U.S. weather disasters that generated at least $1 billion in damages. In 1998 alone there were seven billiondollar weather disasters (Fig. 2).
Figure 1.
Extreme weather events in the lower 48 states.
Note hurricanes are found along
Atlantic and Gulf coasts, and tornadoes are most common over the Great
Plains and upper
Midwest.

Figure 2. Top:
Cost of damages associated with weather events,
1998. Total cost was over $16 billion. Bottom:
Proportion of fatalities associated with specific weatherrelated phenomena.
The first half of the chapter is divided into three sections that describe how common weather systems develop across much of the nation. Weather in any region is influenced by the atmospheric changes that occur when masses of air with contrasting properties interact. The characteristics of air masses vary with location ranging from dry and cold to warm and humid. The daily clash of air masses over North America generates our common weather patterns characterized by highand low-pressure systems bounded by warm and cold fronts.
These frontal systems are relatively narrow, curvilinear zones that mark a transition from one air mass to another. Weather experienced over much of the central and eastern U.S. is the result of the west-to-east migration of regional-scale lowpressure systems, termed mid-latitude cyclones, and their associated warm and cold fronts. Mid-latitude cyclones affect much of the continental landmass for up to a week at a time.
Meteorologists attempt to predict the path of these mid-latitude cyclones and their frontal systems by monitoring their associated atmospheric conditions such as moisture, temperature, pressure, and wind direction. Using these characteristics they can predict the potential weather for two to five days in the future. However, these dynamic systems are subject to change, and the short-term, relatively accurate forecast becomes a long-term calculated guess as the forecast extends beyond two or three days.
3

Such is our devotion to understanding how future weather patterns will affect us that millions daily tune in to the cable weather news station, the Weather Channel. Regardless of where we get our information on the weather forecast, almost all of it comes from the same place, the National Weather
Service (NWS). The NWS processes over one million weather observations per day. These basic observations may be reprocessed by commercial weather companies (e.g.,
Accuweather) to generate maps and graphics for public distribution to a variety of media sources.
Figure 3. Early national weather map, created
September 1,
1872, shows a high-pressure system over the
Northeast. Image courtesy of NOAA photolibrary.
The NWS began life on February 9, 1870, as part of the Signal
Service Corps in the Department of War. It initially had the unwieldy title of The Division of Telegrams and Reports for the Benefits of Commerce and was given the charge “to provide for taking meteorological observations at the military stations in the interior of the continent and at other points in the States and Territories . . . and for giving notice . . . of the approach and force of storms.” The fledgling service made its first simultaneous observations at just 24 sites on November 1,
1870. Within two years it was creating national weather maps
(Fig. 3), and by 1878 daily observations were being collected at
284 sites and relayed cross-country by telegraph.
The current NWS mission is to provide “weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national
4

Figure 4.
Geostationary satellites generate thousands of images per day. Image courtesy of NOAA photolibrary.
economy.” Today the NWS uses sophisticated satellite technology to keep tabs on developing weather systems worldwide. The Geostationary Operational Environmental
Satellite (GOES) Program began in 1968 and today has two satellites in synchronous orbit above Earth that provide weather coverage for 60% of the planet's surface (Fig. 4). The NWS has over one hundred Doppler radar sites nationwide that are used to track rapid changes in regional storms. The nationwide expansion of Doppler radar installations resulted in an increase in the warning times given for sudden weather phenomena such as tornadoes and flash floods that claim hundreds of lives annually.
The latter half of the chapter is divided into three sections that review extreme weather events in the U.S., thunderstorms, tornadoes, and hurricanes. Thunderstorms form as warm, humid air is forced aloft, either in advance of cold fronts that are migrating toward the east or as a result of differential warming of air near Earth's surface. The high winds, hail, heavy rains, and lightning associated with these storms claim approximately a hundred lives a year in the U.S. Furthermore, over the much of central and eastern U.S., thunderstorms produce even more violent tornadoes (Fig. 5), the highest velocity winds on Earth. The use of new technology, such as
Doppler radar, has increased the average lead time for tornado warnings in the U.S. from 5 minutes (1986) to 12 minutes in
1998.
Figure 5. A tornado near
Dimmit, west
Texas, 1995.
Image courtesy of NSSL's photo album.
5

Dangerous weather phenomena such as tornadoes and hurricanes cannot be stopped but with detailed observations meteorologists can provide timely warnings to protect people from the onslaught of these hazardous winds. The sheer sizes of hurricanes, hundreds of kilometers across and bigger than most states, mean that they will have significant impact on people and property when they come on shore. The most expensive natural disaster in U.S. history occurred in 1992 when Hurricane Andrew wrecked havoc across southern
Florida, causing $30 billion in damages (Fig. 6). Damages could easily have been doubled if the storm had made landfall in the highly developed areas further north. Continued coastal development makes a future $50 to $100 billion disaster inevitable.
Figure 6.
Property damage in southern
Florida resulting from Hurricane
Andrew, 1992.
Image courtesy of
NOAA.
Think about it . . .
Examine the map at the end of the chapter that illustrates the distribution of extreme weather events for the conterminous U.S. during 2000. What patterns can you identify in the weather characteristics displayed on the map?
6
•
•
•
Air masses are large regions of the lower atmosphere with uniform characteristics that are originally defined by a source area.
Air masses are identified by temperature (polar vs. tropical) and the nature of the source area (continental vs. maritime)
North American weather patterns are dominated by continental polar and maritime tropical air masses.

Figure 7.
Approximate locations of air masses developed over the Northern
Hemisphere (left) and Southern
Hemisphere (right) in July. cA continental
Arctic/Antarctic; cP
- continental polar; cT - continental tropical; mP maritime polar; mT
- maritime tropical. Original globes courtesy of
NOAA's National
Geophysical Data
Center.
• Air masses are modified as they move over areas with different temperatures or topography than the source area.
Air masses represent large regions (1,000s km 2 ) of the lower troposphere with relatively uniform properties (temperature, moisture content). The characteristics of individual air masses are dependent upon the attributes of a source area and the modification of the air mass that occurs as a result of movement from the source region. Weather in any region is influenced by the changes that occur in the air mass over time and the interactions that occur at fronts , the boundaries between contrasting air masses.
An air mass develops when the atmosphere is located above a relatively uniform land or water surface for several days. The lower atmosphere assimilates some of the properties of the underlying surface. Air masses are identified by their temperature (polar/tropical) and the character of the underlying surface (continental/maritime). The latter property is a proxy for moisture content . Air masses that develop above oceans contain much more moisture than those formed over land.
The distribution of air masses is relatively intuitive. Arctic and polar air masses are located at high latitudes (+50 o ) in the
Northern Hemisphere and tropical air masses are located closer to the equator (Fig. 7). Continental air masses are found over land, maritime air masses over ocean. The boundaries between individual air masses vary with seasons. Polar air creeps further south during winter and retreats northward during summer.
7

Heavy rains in the Midwest can result from the interaction between the continental polar air mass and the maritime tropical air mass that pushes northward across much of the eastern U.S. during summer.
Meteorologists use a form of scientific shorthand to label the most common types of air masses (Fig. 7). For example a maritime tropical air mass (warm, moist air formed over the ocean) is identified by mT (m = maritime; T = tropical). The characteristics of five types of air masses are summarized below.
• cA - continental Arctic/Antarctic air forms at high latitudes around the poles above permanently snow-covered ground (or pack ice). These air masses are characterized by extremely cold, dry air that may sweep south across Canada and produce days of bone-chilling cold temperatures over much of the central and eastern U.S. during winter.
• cP - continental polar air forms over the northernmost portions of North America, Europe, and Asia. It shares its basic characteristics (cold, dry) with cA air, without the exceptional cold temperatures. High "lake effect" snowfalls in the upper Midwest are the result of the dry cP air masses picking up moisture as they cross the warmer waters of the
Great Lakes (Fig. 8).
Figure 8. Lake effect snows in
Michigan and northern Illinois and Indiana,
January 1997.
Image courtesy of
NASA GOES.
•
• cT - hot, dry continental tropical air forms over continental interiors such as the dry lands of northern
Mexico and southwestern U.S. (Fig. 9). This air mass disappears from North America in winter but brings scorching summers to the southwest. It will decay as it moves east, absorbing moisture and cooling as it goes.
mP - maritime polar air masses form in the northern
Atlantic and Pacific Oceans and are characterized by cool, moist air that affects states bordering the Atlantic shore of
8

Figure 9. Principal source areas for air masses that influence weather patterns across
North America.
Original globe courtesy of NOAA's
National
Geophysical Data
Center.
• the Northeast and most of the Pacific coastline (Fig. 9).
Temperatures at the ocean surface are less extreme than on land (less cold) so mP air is warmer than cP air.
mT - high temperatures and high humidity distinguish the maritime tropical air masses that move inland from the tropical Pacific, Gulf of Mexico, and tropical Atlantic
Ocean (Fig. 9). The mT air brings hot, humid summers to southeastern states and can form at any time during the year.
The initial characteristics of air masses will inevitably change as the mass of air moves out of its source area and passes over regions with contrasting attributes. The principal factors that will cause modification are the temperature and topography of the underlying surface. Air masses will be heated or cooled from below depending upon the relative temperatures of the original air mass and underlying surface. Heating (cP air moving south) will lead to instability as air near the ground surface rises, mixing the air column. Cooling (mT air moving north) has the opposite effect, because cold air cannot rise but remains in a stable configuration near the land surface.
Orographic lifting forces maritime air upward over mountain ranges in the western U.S., leading to condensation and precipitation that converts the formerly humid air to a much dryer air mass.
9

Think about it . . .
Create a concept map that summarizes the characteristics of the principal air masses and their influence on weather patterns in North America.
•
•
•
•
•
Frontal systems form along the boundaries between colliding air masses of contrasting properties.
Cold fronts and warm fronts are associated with the interaction of continental polar and maritime tropical air masses over the central U.S.
Heavy rainfall, decreasing temperatures, decreasing humidity, and changing wind directions are associated with passage of a cold front.
Light to moderate rain, warmer temperatures, increasing humidity, and changing wind directions follow passage of a warm front.
An occluded front forms when a cold front overtakes a warm front.
Frontal systems represent the meteorological battle that ensues when air masses of contrasting properties clash along their boundaries. As air masses move across Earth's surface they inevitably interact to create relatively narrow, curvilinear
Figure 10. Weather patterns typically encountered with cold and warm fronts associated with a cyclone (low-pressure system) over the central U.S. The occluded front formed where cold and warm fronts coalesced over the northern plains.
Warm maritime tropical air from the Gulf of
Mexico lies between the two fronts. Note that cloud cover occurs in advance of the cold front, adjacent to the warm front, and around the occluded front.
Lines A-B and C-D represent sections through the frontal system (see Figs. 11 and 14).
10

zones that mark a front , a transition from one air mass to another (Fig. 10). Advancing frontal systems bring clouds and precipitation and are accompanied by changes in moisture, temperature, pressure, and wind direction.
The clash between cP and mT air masses over the Great Plains and Midwest is the most common source of frontal systems in the U.S. (For more on the causes of this phenomenon, see the section, Mid-latitude Cyclones.) Weather conditions change in a predictable sequence as warm and cold fronts pass over an area.
Conditions
Pressure
Weather Conditions Associated with a Passing Frontal System
Before Warm Front
Decreasing
Winds South, southeast
Temperature Cool
Clouds Cirrus, cirrostratus, altostratus, nimbostratus
Precipitation Light-moderate, increasing
Between Warm and
Cold Fronts
Small decrease, then small increase
Southwest
Warm
Cumulus, cumulonimbus
None, then heavy rains prior to cold front
After Cold Front
Increasing
West, northwest
Cold
Cumulus, altostratus
Moderate-light, decreasing
Figure 11. Weather conditions associated with cross section A-
B on Figure 10. Warm air (mT) lies between the cold front and warm front. The cold front advances more rapidly than the warm front, forcing warm air to rise, forming thunderclouds and heavy rains. Warm air is forced to rise above the more gently sloping warm front, resulting in the formation of a series of low to high clouds.
Cold, dense continental polar air replaces moist, warm maritime tropical air across the cold front (Fig. 10). People living downwind from the front experience decreasing temperature and humidity and increasing atmospheric pressure with the passage of the cold front. The cold front is pictured as steep in Figure 11 but its actual inclination is ~1 degree toward
11

the warm air (not much, but twice as steep as a warm front).
Warm air will always rise over cooler air so both the cold and warm fronts are inclined toward the warm air mass. Warm air is pushed up and over the advancing cold front, causing relatively rapid cooling and condensation that results in the development of tall cumulonimbus clouds that host heavy but relatively short-lived precipitation (Fig. 11). Rapidly advancing cold fronts may be marked by the growth of a squall line of thunderclouds (Fig. 12).
Changes following the passage of the warm front (Fig. 10) are more benign than the storms that travel with the cold front.
Friction at Earth's surface causes the warm front to slope gently (~½ degree inclination) toward the warm air mass (Fig.
11). Warm, humid air is transported upward over a distance of approximately 1,000 km (625 miles). The first signal of an approaching warm front is the appearance of light, upper-level clouds (cirrus, cirrostratus). Up to 12 hours later, the high clouds will be replaced by lower nimbostratus with associated light to moderate precipitation. Rain associated with a warm front may last longer than precipitation that accompanies a cold front because the warm front typically moves more slowly and extends over a larger area. Temperatures and humidity rise and winds typically shift direction (from south to southwest) with the passage of the warm front.
The cold front moves more rapidly than the warm front (~ 10 km per hour faster) and will eventually close the gap between the fronts, forcing the intervening warm air upward generating additional precipitation (Figs. 13, 14). An occluded front is represented by a combination of warm and cold front symbols
12
Figure 12. A squall line highlighted by intense thunderstorms associated with a rapidly advancing cold front, Gulf of
Mexico. Image courtesy of NASA's
Johnson Space
Center Image
Services.
Figure 13. An occluded front forms when a cold air mass overtakes a warmer air mass.

Figure 14.
Nimbostratus clouds generate precipitation along an occluded front
(see section C-D on Figure 10).
on weather maps (Fig. 10). The occluded front juxtaposes two bodies of cold air; the warmer of the two masses is forced up and over the other. Occluded fronts may be marked by the occurrence of nimbostratus clouds.
Think about it . . .
Examine the map located at the end of the chapter and answer the conceptest questions about frontal systems based on the locations featured on the map. One or two of the questions may require you to read the section that follows on mid-latitude cyclones.
•
•
•
•
Weather in the eastern U.S. is mainly the result of the migration of regional-scale low-pressure systems, termed mid-latitude cyclones.
Mid-latitude cyclones develop where continental polar and maritime tropical air masses collide over the U.S. along the polar front.
Converging surface winds associated with low-pressure systems must be matched with divergent flow in the upper atmosphere.
Cyclones develop from a waveform that originates where irregularities at the surface cause local shearing that distorts the polar front.
Much of the weather experienced over the eastern U.S. is the result of the west-to-east migration of regional-scale lowpressure systems known as mid-latitude cyclones (Fig. 15).
13

These weather patterns are differentiated from tropical cyclones formed over the warm tropical ocean waters that may build into hurricanes. Mid-latitude cyclones (also called wave cyclones) may be 1,000 to 2,000 km (625-1,250 miles) across and can affect much of the continental land mass for periods of three days to as much as a week.
Mid-latitude cyclones develop where continental polar and maritime tropical air masses collide over the U.S. along the polar front . The position of the collision zone migrates south during winter and moves north with summer. The boundary between air masses is initially a stationary front, with airflow in opposite directions on either side. Perturbations in the upper airflow of the jet stream are necessary to promote the growth of a surface low-pressure system. The converging surface winds associated with low-pressure systems must be matched with divergent flow aloft to maintain the cyclone (Fig. 16). (For more on airflow within a cyclone, see Cyclones and
Anticyclones in the chapter, The Atmosphere.)
Cyclones develop from a waveform that originates where irregularities at the surface cause local shearing that distorts the polar front. Features that may induce shearing include
Figure 15. A classic comma-shaped cloud pattern is associated with a mid-latitude cyclone in the central United
States, Christmas
Eve, 1997. A lowpressure center is located over the lower Mississippi
Valley and a warm front spirals over the
Gulf of Mexico.
Image courtesy of
NASA-Goddard Space
Flight Center, NOAA
GOES.
Figure 16. Lowpressure systems
(cyclones) form below regions of divergent flow in the jet stream.
14

Figure 17. Three stages in cyclogenesis, the development of a mid-latitude cyclone over the
U.S. Note west to east track of the cyclone and the merging of warm and cold fronts to form an occluded front in the final image shortly before the cyclone decays.
mountains and contrasting atmospheric properties at land/water boundaries. The waveform becomes exaggerated as warm air pushes northward and cold air moves south, generating counterclockwise rotation typical of cyclones and forming the pairing of warm and cold fronts discussed in the previous section.
Warm air advances along the warm front, at rates of 15 to 20 km per hour (9-13 mph), moving over ground previously covered by colder air. The cold front lies to the west and moves about twice as fast as the warm front (Fig. 17). Warm air is forced aloft as the cold front sweeps across the previously warm sector. The cold front and warm front eventually merge to form an occluded front , merging cold air masses and producing a more stable, stratified atmosphere where cool air lies below warmer air and resulting in the decay of the cyclone.
15

Think about it . . .
Review the Frontal Systems and Mid-latitude Cyclones exercise (see end of chapter) referred to following the previous section. Would you change any of the answers after reading the section above?
•
•
•
•
Thunderstorms form where warm, humid air is forced upward at cold fronts or as a result of differential heating at
Earth's surface.
Latent heat, released during condensation, generates updrafts that maintain upward movement.
Thunderstorms are most frequent over the southeastern
U.S.
The three stages (cumulus, mature, dissipating) in the life cycle of a thunderstorm occur over approximately two hours.
Thunderstorms form where warm, humid air is forced upward to altitudes of up to 15 km. Condensation occurs as the air cools, releasing latent heat and ensuring that the rising air remains unstable (warmer than surrounding air).
Thunderstorms may occur as relatively isolated , short-lived events or as longer-duration severe storms depending upon the conditions that cause the air to rise.
Figure 18. Afternoon thunderstorms form over the Florida peninsula as humid maritime tropical air moves over the warmer landmass.
16

Figure 19. Map of the average number of days per year with thunderstorms featuring damaging winds or wind speeds of more than 50 knots/hour, 1980-
1994.
Isolated afternoon thunderstorms, or cells , are commonplace in warm summer months where moist maritime tropical air masses move over land (Fig. 18). The temperature of the land surface rises to a maximum during the mid-afternoon, warming overlying air parcels and causing them to become unstable enough to rise, generating scattered thunderstorms. (For more on this process, see Clouds and Cloud Formation in the chapter, The Atmosphere).
Severe storms, or supercells , are associated with frontal lifting along the cold front between the continental polar and maritime tropical air masses in mid-latitude cyclones. These storms are most common during spring and early summer, when the contrast in temperatures between air masses is greatest.
Because thunderstorms are associated with mid-latitude cyclones it should come as no surprise that storms are most common over the Great Plains and southeastern U.S. States such as Florida may experience over 100 days a year with thunderstorms whereas such storms are rare (less than 20 per year) in Pacific Coast states (Fig. 19).
17

There are three stages during the life of a typical thunderstorm that rarely lasts for more than two hours:
1.
Cumulus stage - early cloud development when a cumulus cloud expands laterally and vertically as air enters the cloud mass at all levels. Cloud formation is rapid, requiring approximately 15 minutes to grow vertically to heights of
10 km (6 miles; Fig. 20a). Updrafts (~ 4 m/sec near ground surface to ~ 10 m/sec at high levels) within the cloud carry humid air to higher, colder levels where condensation occurs.
Figure 20a.
Cumulus stage in the life cycle of a thunderstorm is characterized by rapid vertical growth of the cloud and condensation.
2.
Mature stage - top of cloud cell may be at altitudes up to
15 km (9 miles; Figs. 20b, 21). Rain and ice formed by condensation become too large to be supported by updrafts and fall to ground. Falling precipitation generates friction within rising air, creating a zone of downdrafts. Descending air warms up, resulting in evaporation that absorbs latent heat and cools the cloud. This stage lasts for 15 to 30 minutes generating rainfall or hail (Fig. 22) at the surface accompanied by gusty winds (downdraft).
Figure 20b.
Downdrafts and precipitation characterize the mature stage in the life cycle of a thunderstorm.
18

Figure 21.
Developing thunderstorm cloud (top) and mature supercell
(below). Images courtesy of NOAA
Photolibrary.
Figure 22. Hail coats the ground after a storm (above right).
Thunderstorms can generate individual hailstones up to 15 cm (6 inches) in diameter (above).
The hail above is ~6 cm across. Images courtesy of NOAA photolibrary.
3.
Dissipating stage - cloud formation processes end as moisture is expended and descending air cools the cloud mass, returning stability to air. The final stage is characterized by diminishing precipitation (light rain) as the cell dissolves (Fig. 20).
•
•
•
•
Tornadoes are narrow, funnel-shaped spirals of wind that rotate at speeds of up to 500 km/hr because of extreme pressure gradients.
Tornadoes are ranked from F
0
(weakest) to F
5
(strongest) using the Fujita Intensity scale.
Most tornadoes move to the east or northeast at an average speed of approximately 50 km/hr.
Tornadoes are associated with thunderstorms and develop in association with mesocyclones within the thunderstorm cell.
19

• The U.S. experiences more tornadoes than any other nation and most occur in tornado alley (Texas, Oklahoma, Kansas,
Nebraska, Iowa, Illinois, and Indiana).
Tornadoes are narrow, funnel-shaped spirals of rapidly rotating air (Fig. 23) that form in association with thunderstorms. Like hurricanes and mid-latitude cyclones, tornadoes are near-circular low-pressure systems . However, the pressure gradient is much more intense for tornadoes.
Pressure differences across mid-latitude cyclones are in the range of 20 to 30 mb (millibars) over hundreds of kilometers
(pressure gradient, 0.02-0.03 mb/km). Hurricanes may experience pressure gradients of more than 100 mb over shorter distances (~0.2-2 mb/km) but large pressure differences in tornadoes occur over distances measured in hundreds of meters. Extreme pressure gradients of up to ~0.1 to 1 mb/m are possible for tornadoes, generating the strongest natural winds on Earth with wind velocities of up to 500 km/hr .
Tornadoes are classified using the Fujita Intensity scale which places tornadoes in one of six categories (F
0
-F
5
) according to level of destruction which is taken as a proxy for wind speed.
The scale divides tornadoes into three subgroups: weak (F
0
,
F
1
); strong (F
2
, F
3
), and violent (F
4
, F
5
). Wind speed cannot be measured directly because the high winds that can blast apart whole buildings (Fig. 24) would make short work of measuring instruments. Scientists use the level of destruction to gauge estimates of wind speed and thus determine the Fujita value for a specific tornado. This makes it difficult to rank winds that touch down in sparsely populated areas.
Figure 23.
Twisting, nearvertical funnelshaped tornado.
Image courtesy of
NSSL's photo album.
Scale
F
0
F
1
F
2
F
3
F
4
F
5
Wind Speed km/hr (miles/hr)
<116 (<72)
116-180 (72-112)
181-253 (113-157)
254-332 (158-206)
333-419 (207-260)
>419 (>260)
Fujita Intensity Scale
Damage
Description
Light
Moderate
Considerable
Severe
Devastating
Incredible
Tornado Class
Weak
Strong
Violent
The funnel of the tornado moves more slowly than the winds that give it shape. Funnels are typically less than 600 m (2,000 feet) wide and average funnel velocities are approximately 50 km/hr, although velocities as high as 200 km/hr (125 mph) have been recorded. Tornado paths follow the direction of movement of their parent thunderstorms that are in turn
% of U.S.
Tornadoes
69%
29%
2%
Time on
Ground
<10 minutes
~20 minutes
> 1 hour
20

Figure 24.
Destruction associated with a violent tornado in a suburb of Oklahoma
City, May 3, 1999.
There were 38 deaths from this single tornado that reached F5 strength along part of its path. Image courtesy of FEMA.
associated with east to northeast-directed mid-latitude cyclones. Scientists have been unable to observe the birth of a tornado because of the difficulty in determining exactly where tornadoes may originate. One hypothesis on tornado formation considers tornado development in three stages (Fig. 25):
•
•
•
Early stage : Friction slows winds at the ground surface, resulting in increasing wind velocity with elevation in the lower troposphere. These contrasting vertical wind speeds generate local winds that rotate about a horizontal axis.
Updraft stage : Updrafts below thunderstorm cell draw the spiraling horizontal winds upward forming a mesocyclone
(a small cyclone) within the larger storm cloud cell. Mesocyclones may be up to 10 km in diameter.
Tornado stage : Rotation within the mesocyclone forms smaller, more intense spiraling winds within a tornado that extend downward from a cloud base toward the ground surface (Fig. 26).
Figure 25.
Stages in the development of a tornado.
Tornado paths are typically 5 to 25 km (3-16 miles) in length but some larger tornadoes may remain on the ground for over
100 km (62 miles). Smaller funnels may skip across the surface like a fickle avenger, destroying one home while leaving neighboring properties undisturbed. Improvements in
21

forecasting methods have reduced the number of fatalities associated with tornadoes (Fig. 27). Approximately two-thirds of U.S. fatalities occur as a result of tornadoes destroying homes; nonpermanent mobile homes are especially susceptible
(Fig. 28).
The U.S. is home to the majority of the world's tornadoes, averaging about 1,000 a year . Tornadoes occur when thunderstorm activity is at an optimum, during the late spring and early summer across much of the nation. The highest frequency of tornadoes per area occurs over the Great Plains states (Texas, Oklahoma, Kansas, Nebraska) and parts of the upper Midwest (Iowa, Indiana, Illinois), a region that has come to be known as tornado alley (Figs. 29, 30).
Figure 26. Left:
Waterspouts are tornado-like phenomena formed over water. Right:
The latter stages in tornado development for an example that touched down in
Enid, Oklahoma.
Images courtesy of
NSSL's photo album.
Figure 27. U.S.
tornado fatalities have declined as forecasting technology improved to provide better tornado warnings.
Figure 28.
Proportion of U.S.
tornado fatalities by location, 1985-1998.
The timing of tornado activity is tied to seasonal movement of the polar front that drives mid-latitude cyclones. Tornadoes move out of the Gulf Coast and southeastern states into the
22

Figure 29.
Average annual distribution of strong and violent tornadoes,
1950-1995.
Figure 30.
Tornado bearing down across open country. Image courtesy of
NSSL's photo album.
Great Plains as the front retreats northward in late spring.
Summer sees tornado activity shifting to the northern Plains states and the upper Midwest.
Think about it . . .
Use the Venn diagram located at the end of the chapter to compare and contrast the characteristics of tornadoes and hurricanes.
•
•
•
Hurricanes are rotating storm systems hundreds of kilometers across that strike the U.S. during summer and early fall.
Hurricanes grow from tropical depressions in regions of convergent winds and warm oceans, and are sustained by divergent airflow in the upper troposphere.
Destruction from high winds, heavy rains, and coastal flooding occurs when hurricanes make landfall.
23

• Hurricanes are divided into five categories by wind speed using the Saffir-Simpson hurricane intensity scale.
Hurricanes are cyclonic storm systems that form over tropical oceans during summer and fall. Also known as typhoons
(Pacific Ocean) and cyclones (Indian Ocean), hurricanes are characterized by high winds (more than 119 km/hr , 74 mph), heavy rainfall (10-25 cm 4-10 inches), and storm surges
(sudden rise in sea level) along coastlines. Hurricanes are smaller (approximately a third to half the size) and less frequent than mid-latitude cyclones that govern most U.S.
weather patterns but they have much more powerful winds.
Unlike mid-latitude cyclones, hurricanes do not originate as a result of perturbations at a boundary between contrasting air masses. Instead they grow from tropical storms generated by disturbances in the belt of equatorial trade winds.
Hurricanes can result in fatalities and substantial property damages as winds and high seas cause structural damage to ocean-going vessels and coastal developments, and heavy rainfall leads to flooding as much as 200 km inland from the coast. An estimated 300,000 people died in Bangladesh (Fig.
31) in 1973 when cyclone pushed onshore from the Bay of
Bengal, generating a 7 meter (22 foot) storm surge and producing widespread flooding of the low-lying coastal plain.
The government of Bangladesh built nearly a thousand concrete shelters in coastal communities with sufficient space to shelter a million residents and improved communications links to try to reduce the danger from future cyclones.
Hurricane Andrew became the costliest natural disaster to affect North America when it decimated much of southern
Florida in 1992, generating up to $30 billion in damages.
Hurricanes develop under a specific suite of conditions including warm surface waters, cyclonic circulation, and divergent flow in the upper troposphere.
• The initial stage in the development of a hurricane is the formation of a tropical depression (low-pressure system) where the trade winds converge near the equator. The location of the convergent winds changes with seasons, lying north of the equator during summer in the Northern
Hemisphere and migrating to southern latitudes during our winter. The rising air cools and condenses to form cumulus
24
Figure 31.
Location of
Bangladesh.

• clouds that will develop into cumulonimbus cells if the rising air is sufficiently warm and humid.
Water temperatures must be at least 27 o C and should extend downward for 50 to 65 meters (165-215 feet) to ensure that colder water won't be drawn to the surface by the developing storm. Warm surface waters typically straddle the equator but are absent south of the equator in the eastern Pacific and Atlantic Oceans due to oceanic circulation patterns (Fig. 32). The North Atlantic hurricane season officially lasts from June 1 to November
30, and most U.S. hurricanes strike in August and
September.
Figure 32.
Hurricanes originate in areas of the world's oceans where water temperatures are greater than
27 o C.
Hurricanes in the Northern
Hemisphere are most common during summer and early fall.
Southern
Hemisphere hurricanes are frequent during our winter (their summer).
•
•
Earth's rotation as reflected in the Coriolis effect imparts a clockwise (Southern Hemisphere) or counterclockwise
(Northern Hemisphere) rotation to the growing storm (Fig.
33). The magnitude of the Coriolis effect increases with increasing latitude and is zero at the equator. Consequently, the necessary rotation is not imparted on storms within 5 degrees either side of the equator.
The inflow of air into the developing low-pressure system must be matched with an outflow of air in the upper troposphere to maintain the pressure gradient in the developing hurricane. If not, the pressure contrast decreases and wind speed declines.
If all of these conditions are met, a tropical depression (wind speed <37 km/hr) forms and has the potential of growing to a tropical storm (wind speed 63-119 km/hr) before developing into a hurricane with wind speeds of at least 119 km/hr. Wind speed increases in inverse proportion to the decrease in pressure in the eye of the hurricane; the lower the pressure the higher the wind velocity. Rising air in the deepening lowpressure system cools and condenses, releasing latent heat and generating a dense spiral of cumulonimbus clouds punctuated
25

by a central eye characterized by clear skies. Bands of clouds spiral outward from the vortex that will continue to grow in size and intensity as long as the underlying water temperature remains above 27 o C. Precipitation is concentrated within a radius of approximately 100 km on either side of the eye, releasing up to 20 billion tons of water per day.
Atlantic hurricanes, driven westward by prevailing winds at rates of 10-25 km/hr (6-16 mph), may turn north parallel to the
U.S. east coast or pass south of Florida to strike along the Gulf
Coast or Caribbean islands (Figs. 34, 35). Florida and Texas experience more hurricane landfalls than any other states (Fig.
35). A hurricane will begin to decay when it passes over land as it experiences greater frictional drag and a dramatic decrease in the water supply that is essential for its maintenance.
Although wind speeds will be reduced to the level of a tropical storm or depression, the storm itself is still capable of dumping large volumes of rain for some distance inland.
26
Figure 33. Four hurricanes in the
North Atlantic
Ocean, 1998.
Image courtesy of
NOAA.

Figure 34.
Selected hurricane tracks for storms that originated in the
Atlantic Ocean.
Figure 35.
Number of hurricane landfalls by state
1900-1996. Blue
- all hurricanes; red - major
(category 3, 4, and 5) storms.
OT (other) includes
Delaware, Maine,
Maryland,
Massachusetts,
New Hampshire, and New Jersey.
Much of the destruction associated with hurricanes isn't caused by high winds or storm surges but is linked to the exceptional precipitation events that can unload 60 cm (24 inches) of rain from a single storm system in just a few days. Heavy rains from Hurricane Mitch (Fig. 36), the most devastating storm to strike North America and Central America in the last two centuries, claimed over 10,000 lives from flooding (both inland and coastal flooding) and landslides. Honduras, one of the hemisphere's poorest nations, took the brunt of the storm. Entire villages were demolished, nearly 20% of the population evacuated their homes, a quarter of the schools were wrecked, water supplies were cut off, and almost all major roads were damaged. The nation's economy was devastated and the loss to sugar, banana, and sugar crops pushed up prices of those commodities worldwide.
27

Figure 36.
Hurricane Mitch approaching the coast of Central
America. Image courtesy of
NOAA.
As hurricanes approach landfall, winds in the northeast quadrant of the storm, north of the eye, are blowing onshore , piling up water in a storm surge (Fig. 37). However, winds in the southwest quadrant of the storm, south of the eye, are blowing offshore , and opposite to the direction of the storm's movement. Offshore winds reduce the local impact of the storm surge. Emergency workers are especially interested in identifying where the hurricane eye comes onshore as it allows them to better gauge the potential for destruction and to distribute their staff accordingly. The National Hurricane
Center's forecasters add an extra 145 km on either side of the projected landfall site issued 24 hours ahead of the storm because of the capricious nature of hurricane motion. Storms may change speeds, remain stationary, or reverse direction, making it difficult to predict exactly where and when to evacuate residents.
Figure 37.
Hurricane Floyd
(1999) threatened much of the southeastern
U.S. before making landfall in North
Carolina. Image courtesy of
NOAA.
28

The largest peacetime evacuation in U.S. history occurred in advance of Hurricane Floyd which endangered Florida and other southeastern states in September 1999 (Fig. 37). Floyd was unusually large and threatened to affect a substantial length of highly developed coastline. Authorities err on the side of caution and evacuate large regions in advance of a hurricane because of the difficulty in accurately predicting the storms future path. Florida residents breathed a sigh of relief when
Floyd turned north, eventually coming onshore in North
Carolina. Thousands of people stranded on crowded, slowmoving highways highlighted the difficulty of evacuating large populations in advance of an impending storm.
Category
1
2
3
4
5
Hurricanes are divided into five categories by wind speed using the Saffir-Simpson hurricane intensity scale. Destruction associated with major (category 3, 4 and 5) hurricanes includes damage to permanent homes, widespread coastal flooding, uprooting of trees, toppling of power lines. Anticipation of such damages prompts evacuation of residents from the area of expected landfall. Hurricane Camille in 1969 was the most recent category 5 hurricane to make landfall in the U.S., coming onshore along the Gulf Coast of Mississippi.
Saffir-Simpson Hurricane Intensity Scale
Wind Speed km/hr (miles/hr)
119-154 (74-95)
155-178 (96-110)
179-210 (111-130)
211-250 (131-155)
>250 (>155)
Pressure
(millibars)
>980
965-979
945-964
920-944
<920
Storm Surge meters (feet)
Damage
Description
1.2-1.5 (4-5) Minimal
1.6-2.4 (6-8) Moderate
2.5-3.6 (9-12) Extensive
3.7-5.4 (13-18) Extreme
> 5.4 (>18) Catastrophic
Camille caused 256 deaths and over a $1 billion in damages
(Fig. 38). The cost of the same storm today would be over $11 billion because of the increased development along the coastline. The most extensive coastal development has taken place in Florida, greatly increasing the potential damages from a major storm. A recent study suggests that an unnamed 1926 category 4 hurricane that smashed into southern Florida before crossing the Gulf of Mexico to Alabama, would generate damages totaling $77 million (twice the cost of Andrew, Fig.
29

39) if it were to occur today. The convention of naming hurricanes didn't begin until the 1950's.
Think about it . . .
1. Use the Venn diagram located at the end of the chapter to compare and contrast the characteristics of tornadoes and hurricanes.
2. You work in a team of disaster specialists for the
Weather Channel. The channel wants to create its own scoring system that better evaluates the potential damage from incoming storms. You and your team are given the assignment to create an evaluation rubric to assess factors that will influence the risk of potential damage from a future hurricane. Go to the end of the chapter to complete the exercise.
30
Figure 38.
Before and after pictures of damage to a hotel resulting from Hurricane
Camille. Thirty people at a
“hurricane party” on the site died.
Images courtesy of NOAA's photolibrary.
Figure 39.
Destruction of homes in a culde-sac in southern Florida, caused by
Hurricane
Andrew, a category 4 storm.
Image courtesy of
NOAA.

1.
Where does the daily weather forecast come from?
Despite what they claim about "exclusive" weather information, the original data for all local weather forecasts comes from observations made by the National Weather
Service. These observations may be used to generate impressive graphics by commercial weather companies but the basic data come from the NWS.
2.
What is an air mass?
An air mass is a large region of the lower troposphere with relatively uniform properties (temperature, moisture content).
An air mass develops when the atmosphere above a land or water surface adopts the characteristics of the underlying surface. Air masses are modified as their properties change when they move over land/water surfaces with contrasting temperatures and/or moisture content.
3.
How are air masses identified?
Air masses are differentiated by their temperatures and moisture content. The most common types are categorized as polar (cold) or tropical (warm), continental (dry) or maritime
(humid). The air masses can be represented by symbols such as cT (continental tropical), cP (continental polar), mT
(maritime tropical), and mP (maritime polar).
4.
Which air masses have the greatest impact on U.S.
weather?
Continental polar and maritime tropical air masses interact over the central and eastern U.S. Continental polar air forms over the northernmost portions of North America. High temperatures and high humidity distinguish the maritime tropical air masses that move inland from the tropical Pacific
Ocean, Gulf of Mexico, or tropical Atlantic Ocean.
5.
What is a frontal system?
Fronts mark the boundaries between air masses of contrasting properties. Fronts come in two basic varieties. Cold fronts form where cold polar air masses move over ground previously occupied by warm air. Warm fronts form where warm air moves over surfaces previously covered by cold air masses.
One consequence of either situation is that warm air rises above the underlying colder air, forming clouds and releasing precipitation.
31

6.
What is the difference between weather at cold and warm fronts?
Heavy rainfall, decreasing temperatures, decreasing humidity and changing wind directions are associated with passage of a cold front. Warm air will always rise above cold air so the front is inclined toward the warmer air mass. Friction at the ground surface causes the front to steepen, forcing warm air aloft more rapidly and leading to the development of tall cumulonimbus clouds. The warm front slopes gently toward the warm air mass and humid air rises gradually over a distance of hundreds of kilometers. Warm fronts are characterized by light to moderate rain, warmer temperatures, increasing humidity, and changing wind directions.
7.
What is an occluded front?
Occluded fronts form when a warm air mass is forced aloft between two bodies of cooler air. The front marks the location on the surface where the colder air masses meet.
8.
What is a mid-latitude cyclone?
Mid-latitude cyclones are low-pressure systems that may be over a thousand kilometers wide and migrate from west to east across North America. The position of the cyclone will change with seasons but typically marks the boundary between cool continental polar and warmer maritime tropical air masses.
9.
How are frontal systems related to mid-latitude cyclones?
Mid-latitude cyclones develop where cold and warm air masses collide along the polar front. The warm air pushes northward and is surrounded by cooler air. The boundaries between air masses represent fronts. The initial boundary is a stationary front that becomes distorted to form a pairing of cold and warm fronts. These fronts travel eastward with the cyclone but the cold front typically moves more rapidly and overtakes the warm front to form an occluded front near the east coast.
10.
Under what conditions do thunderstorms form?
Thunderstorms are characterized by clouds that span the range from low- to high-level clouds and form under unstable atmospheric conditions caused by the rapid rise of warm, humid air. Warm air rises rapidly at cold fronts (associated with mid-latitude cyclones) or where differential heating occurs during hot weather.
11.
What are the stages of thunderstorm development?
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There are three stages in the life of a thunderstorm. A cumulus cloud grows rapidly during the early (cumulus) stage as updrafts carry warm, humid air to high elevations.
Condensation produces rain or hail during the second (mature) stage, and the falling precipitation helps cool the cloud and stop growth. The third (dissipating) stage is marked by the decay of the cloud as the supply of moisture is depleted and the air becomes stable.
12.
Is there any similarity between mid-latitude cyclones, tornadoes, and hurricanes?
All are examples of low-pressure systems where air converges inward from areas of higher pressure. The speed of the winds is greatest for tornadoes where the pressure gradient is greatest
(change in pressure over smallest distance) and is least for midlatitude cyclones that have the smallest pressure gradient.
13.
How do scientists classify tornadoes?
Tornadoes are classified by their wind speed using the Fujita
Intensity scale. The scale divides tornadoes into six categories
(F
0
-F
5
) based upon level of destruction. Destruction is matched to wind speed that is too high to measure with conventional instruments. F
0
and F
1
tornadoes are weak tornadoes with wind speeds up to 180 km/hr. F
2
and F
3
are strong tornadoes with wind speeds from 181 to 332 km/hr. Violent (F
4
, F
5
) tornadoes represent only 2% of all tornadoes and are characterized by estimated wind speeds of more than 332 km/hr.
14.
How do tornadoes form?
Scientists can't get too close to tornadoes because of their intense speeds so direct observations of tornado formation have not been made but there are thought to be three stages to tornado formation. The first stage in tornado formation is the development of horizontally spinning winds just above the ground surface. These horizontal spirals form mesocyclones
(small-scale low-pressure systems) that are pulled into thunderstorm clouds by updrafts of warm air and begin spinning about a near-vertical axis. Rotation within the mesocyclone generates smaller intense twisters that grow downward to the ground surface as narrow funnels.
15.
Why does the U.S. have more tornadoes than any place on
Earth?
The location of tornadoes is directly linked to the passage of the mid-latitude cyclones across the Great Plains and Midwest.
Most tornadoes grow from thunderstorms formed at cold fronts
33

where cold and warm air masses interact. The North American continent at relatively high latitudes and the warm tropical
Atlantic Ocean and Gulf of Mexico provide the ideal breeding grounds for air masses of contrasting properties needed to generate the necessary atmospheric conditions to form tornadoes.
16.
What are the key conditions needed for hurricane formation?
Hurricanes form over warm ocean waters with temperatures of at least 27 o C (80 o F) extending to depths of ~50 meters. In addition, the Coriolis effect must be sufficient to impart rotation on the low-pressure system that will evolve into a hurricane. Combining these two factors requires that storms cannot form at the equator (where the Coriolis effect is zero) and can't form beyond latitudes that are more than 20 degrees north or south of the equator (where waters are too cool).
17.
What hazards are associated with a hurricane?
Hurricanes endanger lives and property because of their high winds, heavy rainfall (and resulting flooding), and storm surges that generate waves of more than 7 meters above normal sea level. The size of a hurricane means that it will affect a large area if its eye comes within a few hundred kilometers of the coastline.
18.
Why do hurricanes affect the east coast and not the west coast?
Hurricanes travel in the direction of the prevailing atmospheric and oceanic circulation systems. Hurricanes move from east to west across the Atlantic and Pacific Oceans following the trade winds. The prevailing wind direction therefore carries Atlantic hurricanes toward a U.S. landfall while transporting Pacific storms away from the West Coast.
34

Examine the maps of extreme weather events for 2000.
1.
What patterns can you identify in the weather characteristics displayed on the maps?
2.
Identify three states that are relatively free of weather hazards.
3.
Identify three states that have the highest risk of weather hazards.
35

Use the map to answer the questions that follow.
1.
The map illustrates the relative positions of a warm front and a cold front associated with a mid-latitude cyclone.
Where is the warm front located?
a) between A and B b) between C and D c) at E
2.
Where is it raining?
a) A and B b) B and C c) C and D d) B and D e) A and C
3.
Which location is in a maritime tropical air mass?
a) A b) G c) E d) H
4.
What direction is the mid-latitude cyclone traveling toward?
a) F b) G c) H
5.
Winds at C would be ________________________.
a) southwesterly b) southeasterly c) northeasterly d) northwesterly
6.
Which location will become warmer in the next 12 hours?
a) A b) B c) C d) D e) E
7.
Which of the images below best represents conditions between A and D on the map?
36

Use the Venn diagram, below, to compare and contrast the similarities and differences between tornadoes and hurricanes.
Print this page and write features unique to either group in the larger areas of the left and right circles; note features that they share in the overlap area in the center of the image.
37

You work in a team of disaster specialists for the Weather
Channel. During discussions about coverage of the upcoming hurricane season your boss states that she doesn't believe that the Saffir-Simpson scale sufficiently reflects the risks associated with hurricanes because it emphasizes one factor
(wind speed). The channel wants to create its own scoring system that better evaluates the potential damage from incoming storms.
You and your team are given the assignment to create a evaluation rubric to assess factors that will influence the risk of potential damage from a future hurricane. One factor is included as an example in the table below, identify five more.
Consider both physical and cultural factors when developing your rubric.
Factors
Wind speed
Low Risk
(1 point)
Low
(category 1, 2)
Moderate Risk
(2 points)
Intermediate
(category 3)
High Risk
(3 points)
High
(category 4, 5)
38

Reviewing your evaluation rubric you realize that some factors are more significant than others. Your team decides to double the score of the most important factor. Which do they choose?
Why?
Read the descriptions of Hurricanes Floyd, Dennis, and Mitch that follow. Would you change any categories in your scoring rubric? Rank these storms using your modified rubric.
39

Floyd brought flooding rains, high winds, and rough seas along a good portion of the Atlantic seaboard from the 14th through the 18th of September. The greatest damages were along the eastern Carolinas northeast into New Jersey, and adjacent areas northeastward along the east coast into Maine. Several states had numerous counties declared disaster areas. Flooding caused major problems across the region, and at least 77 deaths have been reported. Damages are estimated to be $1.6 billion in
Pitt County, North Carolina alone, and total storm damages may surpass the $6 billion caused by Hurricane Fran in 1996.
Although Hurricane Floyd reached category 4 intensity in the
Bahamas, it weakened to category 2 intensity at landfall in
North Carolina. Floyd's large size was a greater problem than its winds, because the heavy rainfall covered a larger area and lasted longer than with a typical category 2 hurricane.
Approximately 2.6 million people evacuated their homes in
Florida, Georgia, and the Carolinas—the largest peacetime evacuation in U.S. history. Ten states were declared major disaster areas as a result of Floyd, including Connecticut,
Delaware, Florida, Maryland, New Jersey, New York, North
Carolina, Pennsylvania, South Carolina and Virginia. There were several reports from the Bahamas area northward of wave heights exceeding 50 feet. The maximum storm surge was estimated to be 10.3 feet on Masonborough Island in New
Hanover County, NC.
The coastal areas of North Carolina had their fourth tropical storm scare in as many years during August 29th and 30th.
Hurricane Dennis developed over the eastern Bahamas on the
26th and drifted northward parallel to the southeast U.S. coast from the 26th through the 30th. Dennis became an immediate threat to southeastern North Carolina on the 29th. The center approached to within 60 miles of the coast early on the 30th as a strong category 2 hurricane with highest sustained winds of
105 miles per hour. Due to the fact that the hurricane never made landfall, damage was only moderate. Rainfall amounts approached 10 inches in coastal southeastern North Carolina and beach erosion was substantial. This area is no stranger to hurricane activity. Category 2 hurricane Bertha and category 3 hurricane Fran hit Brunswick County in 1996 and Hurricane
Bonnie (category 2) followed nearly the same path in 1998.
Prior to 1996, the area had been spared from the direct impact
40

of a hurricane since Charlie (category 1) hit Carteret County in
1986.
Dennis made a return visit in September as a tropical storm, moving west-northwest through eastern and central North
Carolina. The main impact this time was flooding due to heavy rains, with the maximum preliminary report being 13.82 inches in Allisonia, VA. However, due to the storm lingering off the coast for several days, beach erosion and damage to coastal highways was significant. Residents of Hatteras and Ocracoke
Islands were stranded for several days because of severe damage to Highway 12.
Hurricane Mitch will be remembered as the most deadly hurricane to strike the Western Hemisphere in the last two centuries! The death toll currently is reported as 11,000 with thousands of others missing. More than three million people were either homeless or severely affected. In this extremely poor third world region of the globe, estimates of the total damage from the storm are at $5 billion and rising. The
President of Honduras, Carlos Flores Facusse, claimed the storm destroyed 50 years of progress.
Within four days of its birth as a tropical depression on
October 22, Mitch had grown into a category 5 storm on the
Saffir-Simpson hurricane scale. On October 26, the monster storm had deepened to a pressure of 905 millibars with sustained winds of 155 knots (180 mph) and gusts well over
200 mph!
Mitch moved westward and on October 27, the category 5 storm was about 60 miles north of Honduras. Preliminary wave-height estimates north of Honduras during this time at the height of the hurricane are as high as 44 feet, according to one wave model. Although its ferocious winds began to abate slowly, it took Mitch two days to drift southward to make landfall. Coastal regions and the offshore Honduran island of
Guanaja were devastated. Mitch then began a slow westward drift through the mountainous interior of Honduras, finally reaching the border with Guatemala two days later on October
31. Although the ferocity of the winds decreased during the westward drift, the storm produced enormous amounts of precipitation caused in part by the mountains of Central
America. As Mitch's feeder bands swirled into its center from
41

both the Caribbean and the Pacific Ocean to its south, the stage was set for a disaster of epic proportions. Taking into account the orographic effects by the volcanic peaks of Central
America and Mitch's slow movement, rain fell at the rate of a foot or two per day in many of the mountainous regions. Total rainfall has been reported as high as 75 inches for the entire storm. The resulting floods and mud slides virtually destroyed the entire infrastructure of Honduras and devastated parts of
Nicaragua, Guatemala, Belize, and El Salvador. Whole villages and their inhabitants were swept away in the torrents of flood waters and deep mud that came rushing down the mountainsides. Hundreds of thousands of homes were destroyed.
42