Storms
[Type the author name]
Table of Contents
Formation of Precipitation .......................................................................... 3
Homework Assignment #1 ......................................................................... 3
Formation of Clouds ................................................................................... 4
Types of Clouds.......................................................................................... 5
Thunderstorms ............................................................................................ 9
Tornado .................................................................................................... 10
Homework Assignment #2 ....................................................................... 11
Tropical Storms and Hurricanes ............................................................... 12
Winter Storms .......................................................................................... 13
Homework Assignment #3: ...................................................................... 14
Lab: Chasing Hurricane Andrew .............................................................. 15
Intentional Weather Modification............................................................. 20
Formation of Precipitation
Although all clouds contain water, only some produce precipitation.
A raindrop large enough to reach the ground without evaporating contains
roughly a million times the water of a cloud droplet. Therefore, in order for
precipitation to form, millions of cloud droplets must somehow join
together into drops large enough to sustain themselves during their descent.
There are two mechanisms to explain this phenomenon: the Bergeron
process and the collision-coalescence process.
The Bergeron process is named for its discoverer, Tor Bergeron.
Because water suspended in air does not freeze until it reaches nearly 40oC, much colder than it would freeze on land. This water is said to be
supercooled. Airplanes collect ice when they pass through a liquid cloud
made up of supercooled droplets because supercooled water freezes rapidly if agitated. Supercooled droplets
will freeze quickly if they come in contact with freezing nuclei, or particles with a crystal form similar to that
of ice (not unlike condensation nuclei necessary for cloud formation).
Ice crystals are solid, and therefore are held together more tightly than molecules that make up liquid
water. As solid crystals form, they form freezing nuclei for more solid crystals to form. Eventually, the crystals
are large enough to fall. Even a summer rain may have begun as a snowstorm in the clouds overhead.
In clouds below the freezing level, especially in the tropics, precipitation occurs as a result of a different
process called the collision-coalescence process. Droplets of water larger than cloud droplets can form if giant
condensation nuclei (such as sea salt) are present. As large droplets begin to form, they collide with other large
droplets and eventually coalesce into a droplet large enough to fall to the ground without evaporating.
Homework Assignment #1
Fill in the blanks:
A raindrop large enough to reach the ground without evaporating contains roughly ______________ times the
water of a cloud droplet.
The ______________________________________ is the way that rain droplets can form in supercooled
clouds. In this method, supercooled water droplets collect more and more water and get larger.
The _______________________________________ is the way that rain droplets can form below the freezing
point, but giant ________________________________ must be present for water to cling to.
Formation of Clouds
In order for clouds to form water must condense (change from a gas to a liquid) or
sublimate (change from a solid to a gas). Additionally, there must be a condensation nuclei,
which is a solid particle in the atmosphere that provides a solid surface onto which the liquid or
solid water molecules accumulate.
There are four ways that air can be
Change from a
cooled enough to condense or sublimate.
gas to a liquid
1. Adiabatic cooling - temperature
Change from a
cools as the air becomes less
gas to a solid
dense
A solid particle in
the atmosphere to
2. Air cools because it has been
which water
mixed with other air masses
molecules attach
Temperature
3. Air cools because it is lifted by
cooling as a
another air mass into a cooler
result of a
region
decrease in
density
4. Advective cooling Temperature
temperature cools as the air
cooling as a
moves over a cold surface
result of air mass
moving over a
cold surface
Types of Clouds
Clouds are categorized by characteristics such as: Altitude, Appearance, Origin
High Clouds
• Cirrus - high altitude wispy clouds
• Cirrocumulus - distinct patchy and/or wavelike appearance
• Cirrostratus - blanket sky in ill-defined sheets
Middle Clouds
• Altocumulus - distinct cloud elements
• Altostratus – uniform, diffuse coverage
Low Clouds
• Cumulus – puffy, noticeable vertical development.
• Stratocumulus - widely scattered, clustered together, or layered w/ little vertical
development.
• Stratus - lowest of the low clouds, appear as an overcast deck but can be scattered
• Fog - low stratus cloud in contact with the ground
Multi-Layer Clouds
Nimbostratus
• Extends vertically through low and middle levels
• associated with large areas of continuous precipitation
Cumulonimbus
 can produce lightning, thunder, heavy rains, hail, strong winds, and tornadoes
 can span all cloud layers
 anvil-shaped tops form because of stronger winds at higher levels of the atmosphere
RAIN GUAGE – device to measure precipitation
Homework Assignment #5
1. Describe the conditions that are necessary for clouds to form.
2. List the four processes of cooling that can lead to cloud formation. Define each.
3. Compare cirrus, cumulus, and stratus clouds by completing this chart:
Cloud
Altitude
Description of shape Drawing of shape
Cirrus
Cumulus
Stratus
4. What kind of cloud causes thunderstorms?
5. How are clouds and fog different?
6. What instrument measures rainfall amount?
Thunderstorms
Lightning is a huge, hot spark of electricity that
shoots out of a thunderstorm cloud. Thunder is
the sound made when air heated by lightning
explodes.
Look for: Puffy white cumulus clouds that grow
taller and flatten out at the top, forming anvilshaped thunderheads.
Caution: Lightning can be deadly. It is best to go
indoors when lightning is nearby. If caught
outdoors, stay away from high places and do not
go under a tree. Crouch down on two feet, do not
lie flat.
How Far Away?
Light travels 186,000 miles per second, while
sound travels only 1/5 of a mile per second. That
is why you see the lightning before you hear the
thunder. The farther away the lightning occurred, the more time elapses between seeing the lightning and
hearing the thunder. It takes 5 seconds for thunder to travel one mile.
•
•
•
•
•
•
•
•
•
•
Flash Floods/Floods are the number one killer associated with thunderstorms with nearly 140 fatalities a
year.
A thunderstorm is considered severe if it produces hail at least 3/4-inch in diameter, winds of 58 mph or
higher, or tornadoes.
Severe Thunderstorms – long lived thunderstorms; formed in strong vertical wind shears; associated
with gust fronts, high winds, and tornadoes
A severe thunderstorm watch is issued by the National Weather Service when the weather conditions
are such that a severe thunderstorm is likely to develop.
A severe thunderstorm warning is issued when a severe thunderstorm has been sighted or indicated by
weather radar. At this point, the danger is very serious and everyone should go to a safe place, turn on a
battery-operated radio or television, and wait for further information
3 stages of thunderstorm development:
1. Cumulus - Warm humid air rises, cools, and condenses into cumulus cloud(s), Condensing
water releases latent heat making cloud warmer than air around it. Insufficient time for
precipitation to form. Updrafts keep water droplets and ice crystals suspended.
2. Mature - Cell = updraft + downdraft
3. Dissipating - Updrafts weaken and downdrafts dominate. Light precipitation, weaker winds
During a thunderstorm, positive and negative electrical charges build up in the clouds.
The discharge of the electricity between clouds is lightning.
Lightning is as hot as 30,000°C, hotter than the sun.
The heated air expands and explodes producing thunder.
Tornado
Rapidly whirling, funneled-shaped cloud (called a vortex) that reaches down from a storm cloud to touch
Earth’s surface. Since air is invisible, the vortex that we see is water droplets, dust, and debris sucked up from
the ground.
Caution: Take shelter in a basement or in the center of a building immediately when tornado warnings are
issued for your area. Stay away from windows.
• Usually brief, touching the ground approximately 15 minutes or less
• Wind speeds may reach 480 km/hr (230 mph)
• Counterclockwise in both the Northern and Southern Hemispheres
• Stages of Tornado Formation:
• Overshooting top
• Wall cloud descends below base of cloud and slowly rotates
• Funnel cloud descends from wall cloud
• Funnel cloud touches ground
• Tornado Watch – tornadoes are possibly in your area
• Tornado Warning – tornadoes have been seen in the sky or on weather radar
• Occur most often in the US
• Approx. 800 tornadoes a year
• Tornado Alley - Warm humid air mass moves north from Gulf of Mexico and meets a cold dry mass that
is moving south from Canada; SD, Iowa, Nebraska, Kansas, Missouri, Oklahoma, TX, NM, and
Arkansas
• Enhanced Fujita Scale- Measuring intensities of tornadoes based on a scale developed by Dr. Theodore
Fujita at the University of Chicago in the 1960s
•
• EF0 Gale
65-85mph
• EF1 Weak
86-110 mph
• EF2 Strong
111-135 mph
• EF3 Severe
136-185mph
• EF4 Devastating 186-200 mph
• EF5 Incredible
over 200 mph
The deadliest tornado on record in the United States was the tri-state tornado outbreak of March 18,
1925. Several tornadoes demolished portions of Missouri, Illinois, and Indiana, killing 747 people and
injuring over 2000 more.
Homework Assignment #2
1. What should you do if you’re caught outside during a lightning storm?
2. Why do you see lightning before you hear thunder?
3. What is the difference between a severe thunderstorm watch and a severe thunderstorm warning?
4. In what direction do tornadoes spin in NJ?
5. What is the difference between a tornado watch and a tornado warning?
6. Where is tornado valley?
Tropical Storms and Hurricanes
•
•
•
•
•
•
•
•
•
•
Tropical storms form over warm ocean waters. They begin as a tropical disturbance which is nothing
more than a cluster of big thunderstorms. When this growing storm begins to rotate, it becomes a
tropical depression. When wind speeds reach 40 miles per hour it is called a tropical storm.
Hurricanes are tropical storms that have winds of 119 km/hr
Usually occur between June and November in the eastern US
Hurricanes that occur in western US are called typhoons.
Size can be 220-700 km in diameter
Eye – center of a hurricane
Storm Surge - Hurricane pulls up ocean water an average of 1 cm per 1 mb of air pressure drop; Regions
along shores and coasts are inundated with water being pushed ahead of the hurricane
Stages of Hurricane Development:
1. Tropical Disturbance
Mass of thunderstorms begin to
organize
Light wind circulation
2. Tropical Depression
Wind speed 20-34 knots
Central low pressure developing
with rotation of thunderstorms
3. Tropical Storm
35-64 knots
Strong central low pressure
Increasing wind speeds
Forward movement across
oceans
4. Hurricane
64 knots or more
Well developed central low
pressure; eye may be visible
Moving to west along with global winds (NE or SE Trades)
Can move up to 50 knots over open ocean
Highest wind speed on the forward traveling side
Saffir Simpson Scale – measures hurricane intensity by comparing wind speed and air pressure
• Tropical Storm – 39-73 mph winds
• Category 1 – 74-95 mph
• Category 2 – 96-110 mph
• Category 3 – 111-130 mph
• Category 4 – 131-155 mph
• Category 5 – 156 mph and up
Naming Hurricanes:
•
Initial tracking of hurricane was by latitude and longitude
•
•
•
WWII – use of military terms to identify individual hurricanes (Alpha, Bravo, Tango…)
Early 1950s – use of female names
Late 1970s – Male and female names are used; alternate male/female name for Pacific or
Atlantic hurricanes
Winter Storms
•
•
•
•
•
•
•
Ice storms – when temperatures below a raining cloud are very cold, the raindrops become supercooled
which means they cool to below 32 degrees F and freeze when they hit the ground and other objects.
Called freezing rain, precipitation from these ice storms covers streets, houses, and trees with layers of
heavy ice.
Sleet forms when falling snow melts and then refreezes before it hits the ground. It consists of small
pellets of ice that bounce and make tapping sounds when they hit the ground.
Hail is a mixture of liquid and frozen precipitation. Formed inside cumulonimbus clouds, hailstones are
composed of layers of ice and can be quite large when strong gusts of upward moving air keep them
inside the cloud. As they blow around in the cloud, they collide with raindrops, adding layers and
growing before they fall to the earth.
Frost – ice crystals that form on a surface such as the ground or the leaves of plants are called frost.
Frost forms when the air temperature drops below freezing and the water vapor in the air freezes into ice
crystals. Frost is devastating to crops. When temperatures drop below freezing, the water inside leaves
and stems freezes. Some farmers burn large drums of oil in their orchards on cold nights to ward off
frost.
Snow forms when cloud temperatures are below freezing. Depending on temperature nd humidity,
snowflakes can have lots of shapes. Snow that starts and stops, sometimes many times over, is called a
flurry or snow shower. More intense brief snowfall, sometimes accumulating to several inches or more,
is called a snow squall.
Blizzards are caused by the combination of heavy snowfall, cold temperatures, and strong winds.
Nor-Easters – most of the snow in New England and the mid-Atlantic states comes from northeasters
which are storms with strong northeast winds that form over the Atlantic Ocean. These storms often
dump large amounts of snow.
Homework Assignment #3:
1. Where do tropical storms form?
2. List the stages of hurricane development:
3. What is a storm surge?
4. What is the difference between a tropical depression and a tropical storm?
5. What is the difference between a tropical storm and a hurricane?
6. How does freezing rain form?
7. How does sleet form?
8. How does hail form?
9. How does frost form?
10. How does snow form?
Lab: Chasing Hurricane Andrew
Background
Hurricanes are the most destructive storms on earth. They develop from tropical storms (cyclones) and are
Classified as hurricanes when their winds reach 64 knots ( ~ 71 mph or 119 kph). Hurricanes include
a small central region known as the eye, where the winds are light and there are few clouds. Moving out
from the eye, a narrow band of intense thunderstorms, heavy rains, and strong winds is encountered. This band
is called the eye wall. Beyond the eye wall are strong but diminishing spirals of the same weather. Hurricanes
are huge storms. Typically they are about 500 km in diameter, and they usually last for a week or more.
Hurricanes contain tremendous amount of energy. They gather this energy from warm ocean waters in
the tropics. As the warm, humid air rises, it cools and condenses, releasing heat (called latent heat). This heat
warms the surrounding air, making it lighter and causing it to rise farther. As the warm air rises, cooler air
flows in to replace it, causing wind. This cooler air is warmed by the ocean, and the cycle continues. This heat
from warm ocean water is the fuel that hurricanes run on. For this reason, hurricanes diminish and die when
they move inland or move into colder waters.
In addition to the high winds – gusts up to 172 knots (about 192 mph or 320 kph) – and the torrential
rains, hurricanes produce what is known as a storm surge. The circular winds, together with the low-pressure
eye and high-pressure outer regions of a hurricane, create a mound of water in the center of a hurricane. The
storm surge causes considerable flooding and is responsible for most hurricane damage and deaths.
Weather satellites in orbit above the Earth can easily detect hurricanes. Satellite data, along with data
from radar and aircraft, is used to follow developing hurricanes. Through tracking, we can tell where a
hurricane has been. We also can estimate where it will go in the near future. When it appears that a hurricane
is moving toward land, the National Weather Service (NSW) issues watches and warnings. A hurricane watch
means that hurricane conditions are likely in the watch area within 36 hours. A hurricane warning means that
these conditions are likely within 24 hours. People living in low coastal areas that could be affected by a storm
surge need to evacuate as soon as watches and warnings are issued.
In August 1992, Hurricane Andrew caused a tremendous amount of human suffering and billions of
dollars of damage in the Bahamas, the Southern tip of Florida, and parts of Louisiana. This hurricane was
unusual because it stuck the United States twice. After coming ashore in Florida, it passed over the Gulf of
Mexico – regaining strength in the warm Gulf waters – then hit the coast of Louisiana. This activity contains
the actual tracking data collected on Hurricane Andrew.
Procedure
1. Look at the data in the different parts of the table marked “Track of Hurricane Andrew.” It contains three types of
information:
a. Date/Time: Data was collected on Andrew every six hours beginning August 16 through August 28. Only a
portion of the data is presented here. Time is given in the military convention; for example, 1200 is 12:00 noon,
and 1800 is 6:00 pm.
b. Position: This is the position of the eye of the hurricane by latitude and longitude. It is important to remember
that the storm is much bigger than the eye. The winds extend out beyond the eye about 100 km in all directions
(about ½ the area of one 5o longitude-latitude square on the map).
c. Wind Speed: This is the maximum speed of the winds in the hurricane, not the speed with which the hurricane
is actually moving. Wind speed is given in knots (kt). 1 kt = 1.15 mph = 1.85 kph.
2. Plot the data given in the tracking table on the map your teacher has supplied. Make a dot for each position of
Andrew, and then connect the dots. For each position at the beginning of a day (time = 0000), draw a small star or
asterisk over the dot. You will be asked to stop and plotting data periodically and issue hurricane warnings and watches
based on the path of the hurricane you have plotted. REMEMBER: A hurricane warning means hurricane conditions are
likely for a location within 24 hours. A hurricane watch means hurricane conditions are likely for a location within 36
hours.
Questions/Conclusions
1. Where did Andrew do the most damage before striking Florida?
2. Describe the motion of the storm displayed on your tracking map from the first point you plotted to the last.
3. What happens to the direction of Andrew after it struck Louisiana?
4. What happened to the wind speed in Andrew after it came aground in Louisiana? Why did this happen?
5. Judging from the wind speed, when did Andrew become a hurricane and when should it have been
downgraded to a tropical storm?
6. In terms of damage done, why was it so devastating for Andrew to hit the southern part of Florida? Why
might it have been less destructive it if had hit Farther north on the coast of the United States; for instance,
Georgia or South Carolina?
Position
Wind speed
Date/Time
Lat. (oN) Lon. (oW)
(knots)
________________________________________________________
Aug 21/0000
23.2
62.4
45
0600
23.9
63.4
45
1200
24.4
64.2
50
1800
24.8
64.9
50
Aug 22/0000
25.3
65.9
55
0600
25.6
67.0
60
1200
25.8
68.3
70
1800
25.7
69.7
80
Aug 23/0000
25.6
71.1
90
Stop! Question 1: Based on how far the storm has traveled over the last 24 hours and its
direction so far, for which locations would you issue hurricane warnings and watches? You can tell how far the
hurricane has traveled in the last 24 hours by looking at the distance between the last two stars or asterisks you
have drawn on the map. Don’t forget that the size of the hurricane is much larger than the dots you have drawn.
Position
Wind speed
Date/Time
Lat. (oN) Lon. (oW)
(knots)
________________________________________________________
Aug 23/0600
25.5
72.5
105
1200
25.4
74.2
120
1800
25.4
75.8
135
Aug 24/0000
25.4
77.5
125
Stop! Question #2. Based how far the storm has traveled over the last 24 hours and its direction
so far, which locations would you issue hurricane warnings and watches?
Position
Wind speed
Date/Time
Lat. (oN) Lon. (oW)
(knots)
________________________________________________________
Aug 24/0600
25.4
79.3
120
1200
25.6
81.2
110
1800
25.8
83.1
115
Aug 25/0000
26.2
85.0
115
Stop! Question #3. Based how far the storm has traveled over the
last 24 hours and its direction so far, which locations would you
issue hurricane warnings and watches?
Position
Wind speed
Date/Time
Lat. (oN) Lon. (oW)
(knots)
________________________________________________________
Aug 25/0600
26.6
86.7
115
1200
27.2
88.2
115
1800
27.8
89.6
120
Aug 26/0000
28.5
90.5
120
Stop! Question #4. Based how far the storm has traveled over the
last 24 hours and its direction so far, which locations would you
issue hurricane warnings and watches?
Position
Wind speed
Date/Time
Lat. (oN) Lon. (oW)
(knots)
________________________________________________________
Aug 26/0600
29.2
91.3
115
1200
30.1
91.7
80
1800
30.9
91.6
50
Aug 27/0000
31.5
91.1
35
Stop! Question #5. Based how far the storm has traveled over the
last 24 hours and its direction so far, which locations would you
issue hurricane warnings and watches?
Intentional Weather Modification
Weather modification is deliberate human intervention to influence and improve the atmospheric
processes and events that constitute the weather – that is, to aim the weather at human purposes. From earliest
recorded times, people have used prayer, wizardry, dancing, and even black magic in attempts to alter the
weather. Until modern times, however, most attempts at weather modification remained largely in the realm of
the mystic. By the nineteenth century such devices as smudge pots, sprinklers, and wind machines to fight frost
were in use. During the American Civil War, observations that rainfall apparently increased following some
battles led to experiments in which cannons were fired into clouds to bring more rain. Unfortunately, these
experiments, as well as many others, proved unsuccessful.
Weather modification strategies fall into three broad subdivisions. The first relies on the injection of
energy by force. The use of powerful heat sources or the intense mechanical mixing of the air (such as by
helicopters) are both examples of techniques of fog dispersal attempted at some airports that fall into this
category. The second subdivision involves the alteration of land and water surfaces in order to change their
natural interactions with the lower atmosphere. One commonly discussed but still theoretical example of this
technique is the blanketing of a land area with a dark substance. If it were done, the amount of heat absorbed by
the surface would increase and lead to stronger upward air currents that, in turn, might aid cloud formation.
Finally, the third subdivision involves triggering, intensifying, or redirecting the atmosphere’s natural energies.
The seeding of clouds with such agents as dry ice and silver iodide for many purposes, including precipitation
enhancement, represents the primary example in this category. Because it offers a relatively inexpensive and
easily used technique, cloud seeding has been the main focus of modern weather modification technology.
Modern weather modification is used for various purposes. First, it can be used to disperse fog and
clouds to improve visibility. The US Air Force has practiced weather modification for many years at various
airbases, and commercial airlines have modified weather at selected airports in the northwest US. Additionally,
weather modification can be used to suppress hail and reduce the damage caused by it. Finally, weather
medication can be used to prevent frost from killing crops.
The first scientific breakthrough in intentional weather modification came in 1946 when Vincent J.
Schaefer discovered that dry ice dropped into a supercooled cloud spurred the growth of ice crystals. Shortly
after Schaefer’s discovery, it was learned that silver iodide could also be used for cloud seeding. The similarity
in the crystalline structures of silver iodide and ice accounts for silver iodide’s ability to initiate the growth of
ice crystals. Thus, unlike dry ice, silver iodide crystals act as freezing nuclei rather than as a cooling agent. This
substance has an advantage over dry ice in that it can be supplied to clouds from burners on the ground as well
as from aircraft. If either substance is to be successful, certain atmospheric conditions must exist. Clouds must
be present, for seeding cannot generate them. In addition, at least the top portion of the cloud must be
supercooled to below 0oC.