1. Hurricanes & Thunderstorms
What are hurricanes?
Where do they form?
How do thunderstorms occur?
What are their stages?

2. 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

3. 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.

4. 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.

5. 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).

6. 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.

7. Hurricanes Formation (cont)
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.

8. 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.

9. 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.

11. 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.

12. 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?

13. 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

14. 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.

15. 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.

16. 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.