Earth Science: Unit 1 (mini-unit)

Hurricanes

and Global Winds

Hurricanes

Whirling tropical cyclones –intense low pressure storms that produce winds of at least 74 miles per hour are known in the United States as hurricanes.

Hurricanes form ONLY ON WARM WATER!• Water must be >80 F. Because this is where

they get their energy from: between about 5 and 20 degrees north and south latitude.

(The North Pacific has the greatest number of storms, averaging 20 per year, called typhoons)

2Air Mass needed for fueling the energy of a hurricane.

Maritime Tropical Air

• Maritime tropical air masses are warm, loaded with moisture, and usually unstable*

• Warm air rises, cools and condenses, releasing heat (energy) in the form of latent heat of condensation.

• . *Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

Note the location of mT air masses

The Role of Airflow Aloft

Upper Air Winds

More often than not, air high up in the atmosphere fuels a middle-latitude cyclone

---(These are called “low pressure storms”) In hurricanes, fast upper-level winds sheer

off the tops of thunderstorms, preventing the growth of tropical waves, depressions or storms.

Development of Thunderstorms and Waves

Severe Storm Development

Occurrence of Thunderstorms-

• At any given time, there are an estimated 2000 thunderstorms in progress on Earth. The greatest number occur in the tropics where Air is warm, moist, and unstable.

• Thunderstorms form when warm, humid air rises. • Moving off Africa, clusters of storms are called

tropical “WAVES” –these often stay together and become a tropical depression- especially in late summer.

A wave exits Africa and begins to circulate in the Eastern, Tropical Atlantic.

• A wave starts to get together:

Review of the Conditions needed for Hurricanes to develop:

• Warm, humid air (over the water)• Warm water (>80 degrees F) ..greater than!

• Thunderstorms to coordinate/circulate

(“Waves” coming off of Africa)

• Little to no wind shear (in upper atm.)

EYEEYE WALLFEEDER BANDS CENTRAL DENSE OVERCAST -OR “CDO”

Parts of a hurricane:

Identify the following in the picture of Hurricane Floyd.

Satellite View of Hurricane Floyd

Cross Section of a Hurricane

Parts of a storm:

•Rain FEEDER bands ”Spiral arms”) are curving lines of thunderstorms attached to the center.

• The eye wall is a doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane. Outside the eye wall is the CENTRAL DENSE OVERCAST

• The eye is a zone of scattered clouds and calm averaging about 20 kilometers in diameter at the center of a hurricane.

Evolution and life cycle of a Hurricane• Hurricanes develop most often in the late

summer when water temperatures are warm enough to provide the necessary heat and moisture to the air.

• Tropical Wave…Tropical Depression…• Tropical Storm (39 mph)• Hurricane (74 mph) is when an eye can form.• Eventually, the storm will die (Land friction

and/or loss of energy)

Hurricane Intensity

• The Saffir-Simpson scale describes the intensity of the hurricane. It is based on sustained wind speed.

• A storm surge is the abnormal rise of the

oceans surface. (Think of it as a higher high tide like a spring tide)

- This is due to the lower air pressure pushing down on the water and also as a result of strong winds.

Classifying Hurricanes

Saffir-Simpson Hurricane Scale

Eye of hurricane (Not always seen)

Hurr. Bill- Sat. Aug 22, ’09Why will Ct./New England be spared from a direct hit?

Sat

Hurricane Bill –Aug 23rd races past Nova Scotia and Newfoundland.

Aug 25

Aug 26 (Wed)

Wed 26th