Unit 13

The image above shows a midlatitude cyclone that was called the “1993 Superstorm” and
“The Storm of the Century.” This mid-March cyclone brought blizzard conditions to the E.
coast. Mid-latitude cyclones are easy to identify in satellite images by their “comma”
Source: http://www.nc-climate.ncsu.edu/edu/k12/.mlc
• 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 middle 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 InterTropical 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.
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 occurs 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
they occur over warm water--80F or 28C.
there is an initial disturbance (ITCZ, convergence, midlatitude cyclone.
there is sufficient Coriolis force. Most hurricanes form in 8-15 degrees latitude.
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
Sustained Wind Speed
Description of Effects
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
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
>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
Source: Information from NOAA, National Hurricane Center.
Storm Surge
Click on Image to Play Video
Surge animation with shallow continental shelf (Click on Image to Play
Source: http://www.nhc.noaa.gov/surge/
Historic Tropical Storm and Hurricane
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
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.
Rossby Waves in the Upper Atmosphere
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 9,000 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 Midlatitude Cyclone
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.
Midlatitude Cyclone and Vertical
(A) February 2, 2011, GOES-13 satellite
image of a massive winter
midlatitude cyclone moving across
the eastern US. The cyclone is
about 2,000 km (1,240 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.

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