Unit 13: WEATHER SYSTEMS
The image above shows a mid-latitude cyclone that was called the “1993 Superstorm” and “The
Storm of the Century”. This mid-March cyclone brought blizzard conditions to the E. coast. Midlatitude cyclones are easy to identify in satellite images by their “comma” appearance.
Source: http://www.nc-climate.ncsu.edu/edu/k12/.mlc
OBJECTIVES
• Demonstrate the importance of migrating weather
systems in the global weather picture
• Discuss the significant weather systems of lower
latitudes, including tropical storms and hurricanes.
• Explain weather systems of mid- and high-latitudes,
including the formation of cyclones and the
weather patterns associated with them.
• Examine the transformation of energy and moisture
within weather systems.
Weather Systems: Tropical Systems
Full-disk image from GOES-14 weather satellite at 1:31 pm EST on 17 August 2009. This thermal infrared
image measures radiation emission from Earth at a wavelength of 10.7 micrometers; coldest
temperatures are bright white, and hottest temperatures are black. The Inter-Tropical Convergence Zone
appears as a discontinuous band of thunderstorms. Tropical Storm Claudette is rapidly losing energy
along the eastern Gulf Coast, Tropical Depression Ana is approaching Puerto Rico, and Hurricane Bill is
building in the central North Atlantic Ocean.
Easterly Waves
Easterly waves are disturbances within the trades that can produce heavy
rainfall and potentially can form into tropical storms and hurricanes.
Source:
http://www.geography.hun
ter.cuny.edu/~tbw/wc.note
s/11.hurricanes/easterly_w
ave_atl.htm
The trough axis marks where the trades are converging with rising motion behind the westward
moving wave, and where divergence and sinking motion occur ahead of the wave axis. Heaviest
precipitation occur in the convergence zone, while mostly sunny conditions precede the axis.
Tropical Storms and Hurricanes
Tropical cyclones can form over land or water, however hurricanes always
develop over warm oceans. As cyclones increase their energy, their winds
increase. Storms are defined by wind speed:
Tropical Depression-winds below 40 mph
Tropical Storm-winds 40 to 74 mph
Hurricane-winds exceed 74 mph
Hurricanes may form if
1. They occur over warm water-80F or 28C
2. There is an initial disturbance (ITCZ, convergence, mid-latitude cyclone
3. There is sufficient Coriolis force. Most hurricanes form in 8-15 degrees
latitude
4. There are weak upper air winds. Strong flow destroys formation.
Hurricanes can form over all tropical oceans where these conditions
are met.
Fig 14.2
The eye of Hurricane Mitch is still over water, but this 1998 storm is poised to
strike Central America and will become the costliest natural disaster in the
modern history of the western hemisphere. Honduras will be hit hardest, with
nearly 10,000 deaths When it is over, nearly one-quarter of the country’s 6.4
million people will be homeless, and most of the agricultural economy will be
ruined.
Fig 14.3
Cross-sectional view of a hurricane showing its mechanics and component parts.
Saffir–Simpson Hurricane Wind Scale
Category
Sustained Wind Speed
Description of Effects
1
119–153 km/h
Damage primarily to unanchored mobile homes, shrubbery, tree branches;
(74–95 mi/h)
power lines blown down
154–177 km/h
Damage to roofing material, doors and windows; small trees blown down;
(96–110 mi/h)
unprotected marine craft break moorings
178–209 km/h
Damage to small buildings; large trees blown down; mobile homes destroyed;
(111–130 mi/h)
flooding near coast destroys many structures
2
3
Source: Information from NOAA,
National Hurricane Center.
4
210–249 km/h
Extensive roof failures on houses and smaller commercial buildings; major
(131–155 mi/h)
damage to doors and windows, and lower floors of near-shore structures;
flooding up to 9 km (6 mi) inland from coast
5
>249 km/h
Widespread roof failures and destruction of many smaller-sized buildings; major
(155 mi/h)
damage to all structures less than 15 ft (4.5 m) above sea level; flooding up to 16
km (10 mi) inland from coast
Storm Surge
Click on Image to Play Video
Surge animation with shallow continental shelf (Click on Image to Play
Video) Source: http://www.nhc.noaa.gov/surge/
Historic Tropical Storm and Hurricane Tracks
Storm tracks of weak and severe tropical cyclones from 150 years through 2006. Weaker
systems, shown in blue and yellow, occur near the Equator, during their early stages of
development, and over land and in the middle latitudes as they lose energy and weaken. The
steering of these systems by the easterly trade winds within the tropics, and the westerlies as
they move into the middle latitudes is apparent. Category 4 and 5 hurricanes are most common
in the western North Pacific Ocean.
North Atlantic Hurricane Season
Source: http://www.nhc.noaa.gov/climo/images/peakofseason.gif
Fig 14.5
Some of the worst devastation caused by Hurricane Andrew in the suburbs south of Miami on
August 24, 1992. This is part of Cutler Ridge, located just inland from where the eye wall passed
over the Florida coast.
Fig 14.6
We need to verify we have permission to use
this image.
Mitch’s 4500-km (2800-mi) track, from its birth in the western Caribbean off Columbia to its
demise, 15 days later, in the Atlantic east of Florida. Honduras took the worst beating as the
storm made landfall there as a Category 5 hurricane, and then took more than 48 hours to
traverse the full length of the country.
Rossby Waves in the Upper Atmosphere Westerlies
500-mb map for North America in November. The lines show the height of the 500-mb
level in meters above the surface (ex. 92 equals 5920 m). There is a ridge along the W. coast
with troughs over central and E. coast US. Dashed lines are temperatures in Celsius.
Polar Jet Stream and Surface Weather
Relationship between the Polar jet
stream and surface pressure patterns.
(A) Position of the jet stream 9000 m
(30,000 ft) above North America. (B)
Associated air movement around points
X and Y in vertical cross section. (C)
Resulting surface pressure conditions.
Life Cycle of a Mid-latitude Cyclone
Source:
http://www.geography.hunt
er.cuny.edu/~tbw/wc.notes
/9.weather.patterns/mid_cy
clone_stages.htm
Mature Stage of a Midlatitude Cyclone
Mature stage of a midlatitude cyclone over the southeastern United States. (A) Pressure fields, windflows, and fronts. (B)
Cross-sectional view along the dashed line mapped in (A). The vertical scale is greatly exaggerated. Cold fronts typically
rise only 1 m for every 70 m horizontal extent; for warm fronts the ratio is about 1:200. (C) Summary of surface weather
conditions along the cross-sectional transect.
Mid-latitude Cyclone and Vertical Circulation
(A) February 2, 2011 GOES-13
satellite image of a massive
winter midlatitude cyclone
moving across the eastern US.
The cyclone is about 2000 km
(1240 mi) in diameter; a large
comma cloud extends from the
Midwest to New England. A dry
slot formed by the dry conveyor
belt is faintly evident to the west
of the cold front.
(B) An oblique view of a midlatitude
cyclone showing the relative
positions of the warm conveyor
belt, the cold conveyor belt, and
the dry conveyor belt.
(B)