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Hurricanes & Thunderstorms
What are hurricanes?
Where do they form?
How do thunderstorms occur?
What are their stages?
Hurricanes
• They are TROPICAL low pressures
– They form in the tropics
– They form over warm ocean waters (<27oC)
– Usually form between 5oN(or S) and 20oN (or S)
• Why don’t they normally form further North or South? (Think
about Coriolis Force and Sea Surface Temps)
– They form in low vertical wind shear
• Upper winds are weak with respect to surface winds
– There is usually a pre-existing tropical disturbance or
low pressure at the surface
Hurricanes
The Eye
• Located at the hurricane's center and can measure 20 to
30 miles wide (10 to 65 km).
• An eye will usually develop when the maximum
sustained wind speeds go above 78 mph (130 kph).
• A cloud free area of sinking air and light winds that
usually doesn't exceed 15 mph (24 kph).
• The calmest part of the storm.
• Generally shrinks in size when the storm strengthens.
Hurricanes
Eye Wall
• Completely or partially (at least 50%) surrounds the eye
of a mature hurricane and is an area where winds may
gust to more than 200 mph (320 kph).
• Consists of a ring of tall thunderstorms that produce
heavy rains and very strong winds.
• Has the most destructive section of the storm on the side
where the wind blows in the same direction as the
storm's forward motion.
Hurricanes
RAIN BANDS
• Curved bands of clouds that trail away from the eye wall
in a spiral fashion.
• Capable of producing heavy bursts of rain and wind,
perhaps one-half or two-thirds the strength of those
associated with the eye wall.
• May cause a hurricane's diameter to extend outward up
to 340 miles (548 km).
Hurricanes
Formation
• The hurricane season in the Atlantic, Caribbean, Gulf of Mexico, and
Central Pacific is between June 1 and November 30. August and
September are typically the peak months of the season.
• The hurricane season in the Eastern Pacific is between May 15 and
November 30.
• Hurricane development often begins over tropical areas of the ocean
near the equator where the water is at least 80º F (27º C). As the
heat and moisture rise due to convection, cluster thunderstorms are
formed, creating a tropical disturbance.
Hurricanes
Formation (cont)
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These cluster thunderstorms may then begin to rotate due to the Coriolis
force, resulting in an organized mass of thunderstorms that move in a
particular circular direction (counterclockwise in the northern hemisphere
and clockwise in the southern hemisphere).
As the swirling clouds and rain become more organized and begin to
circulate around a center with sustained wind speeds of less than 38 mph
(61 kph/33 kt), the storm is classified as a tropical depression. Once the
winds gain speed and become constant at 39 mph, the tropical storm is
given a name to identify and track it.
The tropical storm may then gain energy as it is fueled by water vapor that
is pulled up from the warm ocean surface by upper level winds. With an
increase in energy, the storm winds may reach a constant speed of 74 mph
(119 kph/64 kt) and develop into a hurricane.
Hurricanes can last for more than two weeks over open waters and can run
a path across the entire length of the eastern coast. When a hurricane
crosses land or cooler waters, it loses its source of power, and its winds
gradually slow until they are no longer of hurricane force.
Hurricanes
Movement
•
Prevailing Wind Currents - The prevailing wind currents that surround a storm
determine the movement of a hurricane. The storm is guided in the direction of the
wind currents, which also determine the forward traveling speed of the storm. When
the steering winds are strong, it is easier to predict where a hurricane will go. When
the steering winds are weak, a storm often follows an erratic path that makes
forecasting very difficult.
•
Circulating Winds - The movement of a hurricane affects the speed of the winds that
circulate around the eye of the storm. On one side of the storm, where the circulating
winds and the entire storm are moving in the same direction, the forward movement
of the storm increases the wind speed. While on the opposite side of the storm, the
forward motion decreases the circulating wind speed. For example, in the Northern
Hemisphere a hurricane's strongest winds are usually found in its right-front side.
Hurricanes
Movement (cont)
• Tropical Trade Winds - The average hurricane moves from east to
west due to the tropical trade winds that blow near the equator.
• High-Pressure Zones - The clockwise rotation of air associated with
high-pressure zones is the driving force that causes many
hurricanes to stray from their east-to-west movement and start
northward.
• Coriolis Force - The Coriolis force can have an effect on the
movement of some storms. For example, in the northern
hemisphere, the Coriolis force may cause a tropical system to curve
to the north and then move eastwards (turns to the right of motion).
• Wind Shear - Tropical systems weaken when their sources of heat
and moisture are cut off or when they encounter strong wind shear.
Hurricanes
Hurricane Decay
• No warm ocean waters for evaporation and latent heat to power the
storm.
– For instance over land or over colder waters
• Potential friction over land slowing down the winds
• Vertical wind shear (this is a change in wind direction or speed with
height in the atmosphere) It can shear a hurricane apart.
Hurricanes
• Where don’t they occur?
– South Atlantic ….why?
• Ocean water temperatures too low
• Upper winds create wind shear that is unfavorable for the
vertical structure of the storm.
• Exception: Hurricane “Catarina” in 2004
– First hurricane in recorded history in South Atlantic
– Brazilian government refused to even call it a hurricane for a
long time.
– No official name for it because hurricanes just don’t occur
there!
– Southeast Pacific ….why?
• Ocean water temperatures too low
Thunderstorms
• Stages
• Stage 1: Cumulus Stage (formative)
– Air gets forced up by any of the four lifting mechanisms or a
combination of them!
– As the air is forced up, the water vapor eventually cools and condenses
into liquid water droplets which is a warming process (remember?)
– This warming of the surrounding air causes additional updrafts or
upwards movement of the air which starts a convection cell (like Hadley
cells or mantle convection cells). This in turn causes lower pressure at
the surface which results in more upward movement of air (this is like a
miniature version of the mid-latitude cyclone).
– This process continues until you reach the mature stage
Thunderstorms
Stages
•
Stage 2: Mature
– Once the storm reaches its mature stage, it eventually reaches a “cap”
where the clouds can no longer grow vertically
• With strong to severe storms this can be the Tropopause (the top of the
Troposphere)
– The water droplets that have formed in the clouds begin to combine with
each other, the clouds become heavier and the water begins to freeze
into particles of ice. These particles of ice become raindrops when they
melt as they fall from the clouds.
Thunderstorms
Stages
•
Stage 2: Mature (cont)
– Thunderstorms produce thunder and lightning in their mature phase
because of the mixture of updrafts and downdrafts within their clouds.
• The heat and the flow of air around the thunderstorm produces a
continuous updraft while the falling rain produces a downdraft.
• With this mixture, an internal turbulence within the thunderstorm is
born, giving rise to strong winds, positive and negative charges
which create lightning and the possibility of tornadoes.
– Hail may also occur during this stage when the ice particles are continuously
picked up by updrafts and sent flying up into the storm to accumulate more
layers of water on them which then freezes creating more layers of ice.
Thunderstorms
• Stages
•
Stage 3: Dissipation
– This stage typically occurs when the downdrafts from precipitation and the
cooling effect from the precipitation overcome the updrafts.
– Convection cells within large thunderstorms typically last from 30mins to 2
hours but as old cells die out, new ones are born.
– Notice the weak updrafts (compared to downdrafts) and the decay of the
cumulonimbus cloud in the diagram below.