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Chapter 11 Notes
Hurricanes
Tropical Storms Boris and Christiana Together-2008
Profile of a Hurrican
• Most hurricanes form between the latitudes of 5O and 20O over all the
tropical oceans except the South Atlantic and eastern South Pacific
• These tropical storms are known in various parts of the world by different
names. In the western Pacific they are called typhoons; in the Indian
Ocean they are called cyclones; the tropical storms in the Atlantic are
called hurricanes.
• By international agreement a hurricane has sustained wind speeds of at
least 119 kilometers per hour (74 mph) and a rotary circulation. They can
range in diameter from 100-km up to about 1500-km. From the outer edge
of the hurricane to the center, the barometric pressure can drop 60
millibars.
Rare Hurricane in South Atlantic-2004
A hurricane is a heat engine that is fueled by the latent heat liberated
when huge quantities of water vapor condense. The release of latent
heat warms the air and provides buoyancy for its upward flight. The
upward movement of air reduces the pressure near the surface, which
results in a more rapid inflow of air
Hurricanes develop most often in the late summer when ocean waters
have reached temperatures of 27OC (80OF) or higher and are able to
provide the necessary heat and moisture to the air
Hurricanes do not form within 5 degrees of the equator because the
Coriolis force is too weak to initiate the necessary rotary motion
Easterly Wave in Subtropical Atlantic
Hurricane Formation
In the Atlantic, most hurricanes occur in the west along the coast of North
America and in the Caribbean Sea. The origin of these hurricanes is far
to the east.
In the eastern Atlantic, disorganized arrays of clouds and thunderstorms,
called tropical disturbances, sometime develop and exhibit weak
pressure gradients and little or no rotation. Occasionally, this
disturbances grow larger and develop a strong cyclonic rotation.
Tropical disturbances that produce many of the strongest hurricanes
often begin as large undulations or ripples in the trade winds known as
easterly waves.
To the east of the wave the air converges, rises and clouds form. Tropical
disturbances are located on the east side of the wave.
Hurricane Katrina, August 28
Not all tropical storms strengthen to hurricane status. If a temperature inversion
forms surface air will not be able to rise, and thunderstorms will not develop.
Another factor that can inhibit hurricane formation is the presence of a strong
wind aloft. This wind will disperse the latent heat created when clouds form
which limits cloud growth.
In order for a tropical storm to achieve hurricane status, the combination heat
released by the thunderstorms and the decrease in surface pressure cause
surface winds to increase bringing in more moisture which nurtures the storm
growth. A cycle of water vapor condensation and the adiabatic cooling of rising
air fuels storm growth.
By international agreement, le4sser tropical cyclones are given different manes
based on the strength of their winds:
Tropical depression – strongest winds do net exceed ;61 kilometers per hour
Tropical storm – sustained winds between 61 and 119 kilometers per hour.
Hurricane Katrina, August 29
Hurricane Decay
Hurricane diminish in intensity whenever they:
1. Move ocean waters that cannot supply warm, moist tropical air
2. Move onto land
3. Reach a location where the large-scale flow aloft is unfavorable
Whenever a hurricane moves unto land, it loses energy quickly. The
primary reason for this is the fact that the storm’s source of warm,
moist air is cut off.
Storm Surge Damage from Hurricane Katrina
Hurricane Destruction
Based of the study of past storms, the Saffir-Simpson scale was established to rank the
relative intensities of hurricanes. When a tropical storm becomes a hurricane, the
National Weather Service assigns its scale (category) number. Category assignments
are based on observed conditions at a particular stage in the life of a hurricane. As
conditions change, the category of the a storm is reevaluated.. A rating of 5 on the
scale represents the worst storm possible, and a 1 is the least severe.
Damage caused by hurricanes can be divided into three classes: (1) storm surge, (2)
wind damage, and (3) inland freshwater flooding.