Part 6 Thunderstorms Tornadoes and Hurricanes

Severe Storms : 

Severe Storms Thunderstorms Tornadoes Hurricanes

Severe Storms : 

Severe Storms Thunderstorms Thermal instability (unstable conditions) lead to the development of thunderstorms Thunderstorms are storms that generate lightening and thunder The frequently produce gusty winds, heavy rain, and hail

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Severe Storms Thunderstorms Can be produced by a single cumulonimbus cloud for a small area or clusters of cumulonimbus clouds that stretch for miles along a cold front

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Severe Storms Occurrence of thunderstorms At any given time there are an estimated 2,000 thunderstorms in progress on Earth Greatest number occur in the tropics About 45,000 occur each day More than 16,000,000 occur each year The USA experiences about 100,000 each year Most are in Florida and the eastern gulf region Most areas of the country have 30 – 100 / year The western margin of the USA has the least storms

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Severe Storms Development of a thunderstorm Form when warm, humid air rises in an unstable environment There are three stages Cumulus stage Mature stage Dissipating stage

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Severe Storms Stage 1: Cumulus Stage Strong updrafts supply moist air that causes the cloud to grow vertically Each new surge of warm air rises higher than the last and causes the cloud to grow vertically

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Severe Storms Stage 2: Mature Stage Occurs usually within an hour of the initial updraft The amount and size of precipitation is too great for the updrafts to support so heavy precipitation is released This is the most active stage Gusty winds, lightening, heavy precipitation, and sometimes hail are produced

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Severe Storms Stage 3: Dissipating Stage Eventually downdrafts dominate throughout the cloud The cooling effect of falling precipitation and the flowing of cooler air from high above cause the storm to die down

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Severe Storms Thunderstorms The life span of a single cumulonimbus cell within a thunderstorm is only about an hour or two As the storm moves, though, f4rresh supplies of warm, humid air generate new cells to replace those that are scattering

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Severe Storms Tornadoes A tornado is a violent windstorm that takes the form of a rotating column of air called a vortex The vortex extends downward from a cumulonimbus cloud Sometimes, especially in very strong storms, a tornado can have more than one vortex within the main funnel They are only about 10 m (31 ft) in diameter This is why sometimes one building can be demolished and another building only a few feet away suffers little or no damage

Severe Storms : 

Severe Storms Occurrence of tornadoes Approximately 770 tornadoes are reported each year in the USA Most occur from April through June The least frequent occurrences are in December and January

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Severe Storms Development of tornadoes Most form in association with severe thunderstorms An important process in the formation of many tornadoes is the development of a mesocyclone A mesocyclone is a vertical cylinder of rotating air that develops in the updraft of a thunderstorm

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Severe Storms Development of a Mesocyclone Three steps First: Stronger winds aloft cause lower winds to roll along the ground Second: Updrafts tilt the rolling air so that it becomes nearly vertical Third: When the rotating air is completely vertical, the mesocyclone is established

Severe Storms : 

Severe Storms Tornado Intensity Pressure inside of a tornado is very low Some tornadoes have pressures as much as 10% lower than the atmosphere immediately outside the tornado The low pressure causes air near the ground to rush into the tornado from all directions Because of the tremendous amount of pressure change in strong tornadoes, winds can sometimes be nearly 500 mph

Severe Storms : 

Severe Storms Measuring tornado Intensity One common scale to measure intensity is the Fujita Intensity Scale The Fujita scale measures intensity from F0 to F5 with F5 being the worst Because tornado winds cannot be measured directly, intensity is determined by assessing the worst damage caused by the storm

Severe Storms : 

Severe Storms

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Severe Storms Hurricanes Whirling tropical cyclones that produce winds of at least 119 km/h (74 mph) In the USA they are called hurricanes. Other parts of the world refer to them as typhoons, cyclones, or tropical cyclones Hurricanes are the most powerful storms on Earth

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Severe Storms The power of a hurricane At sea they can generate 15 m (49 ft) waves Depending on where they hit land, they can cause destruction 100s of miles away Most damage is caused by flooding, storm surges, and strong winds A storm surge is a dome of water about 40 – 50 miles wide that sweeps across the coast where a hurricane’s eye moves onto land

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Severe Storms Occurrence of hurricanes Most form between 5 and 20 degrees north and south latitude The North Pacific has the greatest number of storms (about 20/yr) Coastal regions of the southern and eastern USA experience less than 5/yr on average

Severe Storms : 

Severe Storms Development of a hurricane Develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air Start off as tropical disturbances consisting of disorganized clouds and thunderstorms If an inward rush of warm, moist surface air moves toward the core of these storms, a hurricane can begin to develop

Severe Storms : 

Severe Storms Development of a hurricane The air then turns upward and rises in a ring of cumulonimbus clouds This donut shaped wall that surrounds the center of the storm is called the “eye wall” The greatest winds and rainfall occur here Surrounding the eye wall are curved bands of clouds that trail away from the center

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Severe Storms Development of a hurricane At the very center of the storm is the eye Precipitation ceases and winds subside in this area The air within the eye gradually descends and heats by compression making the eye the warmest part of the storm

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Severe Storms Development of a hurricane A hurricane weakens when it moves over cool ocean waters that cannot supply adequate heat and moisture or when it moves over land because there is not sufficient moisture, and friction with land surfaces cause winds to subside When airflow is unfavorable, the hurricane will die out

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Severe Storms Measuring hurricane intensity The Saffir-Simpson Hurricane Scale is used Hurricanes are classified by categories ranging from Category 1 to Category 5 with 5 being the most dangerous

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Severe Storms