CHAPTER 20.3Severe Storms

THUNDERSTORMS

A thunderstorm is a storm that makes lightning and thunder. They also produce strong winds, heavy rain, and hail.

A thunderstorm may be produced by a single cumulonimbus cloud and only affect a small area. Or it may have clusters of cumulonimbus clouds that stretch long distances along a cold front.

OCCURRENCE OF THUNDERSTORMS

At any given time, there are about 2000 thunderstorms going on on Earth. Most occur in the tropics.

Most thunderstorms in the US are in Florida and the Gulf Coast region.

DEVELOPMENT OF THUNDERSTORMS

Thunderstorms will form when warm, humid air rises in an unstable environment.

There are 3 stages: the cumulus stage – strong updrafts supply moist air. This causes clouds to form and grow vertically.

The mature stage is when the amount and size of the precipitations is too large for the updraft to support. This is the most active stage with gusty winds, lightning, heavy precipitation, and sometimes hail produced.

The final stage is the dissipating stage which is when the storm dies down.

TORNADOES

Tornadoes are violent windstorms that take the form of a rotating column of air called a vortex. The vortex moves downward from a cumulonimbus cloud.

OCCURRENCE AND DEVELOPMENT OF TORNADOES

About 770 tornadoes occur in the US each year. Most tornadoes occur from April to June.

Most tornadoes will form with a severe thunderstorm.

Part of the development is the development of a mesocyclone. This is a vertical cylinder of rotating air that will occur in the updraft of a thunderstorm.

TORNADO INTENSITY

Tornadoes have lower pressure than the pressure outside the storm. This causes air to rush into a tornado from all directions. Because of this the winds can sometimes reach 480 km/hr.

We use the Fujita tornado intensity scale to help determine the tornado intensity.

TORNADO SAFETY

The Storm Prediction Center located in Norman, Oklahoma monitors different kinds of severe weather. They want to provide timely and accurate forecasts and watches for severe thunderstorms and tornadoes.

Tornado watches alert people to the possibility of a tornado in a specific area for a certain time period.

HURRICANES

Hurricanes are whirling tropical cyclones that produce winds of at least 119 km/hr in the United States. In other parts of the world these storms are called typhoons, cyclones, and tropical cyclones.

These storms can produce 15 meter waves. On land they can cause strong winds and extensive flooding.

OCCURRENCE OF HURRICANES

Most hurricanes form between 5 and 20 degrees north and south latitude.

DEVELOPMENT OF HURRICANES

Most hurricanes will develop in late summer when water temperatures are warm enough to provide heat and moisture to the air.

Hurricanes start as disorganized clouds and thunderstorms where there is little or no rotation to these storms.

When a hurricane develops an inward rush or warm, moist surface air. This creates a doughnut-shaped wall that surrounds the center of the storm. This wall is called an eye wall and it has the greatest wind speeds and heaviest rainfall.

At the very center of the storm is the eye. This is where precipitation stops and winds die down. This is the warmest part of the storm.

HURRICANE INTENSITY

The Saffir-Simpson scale is used to show hurricane intensity.

The most devastating damage from a hurricane is caused by storm surges. This is when a dome of water sweeps across the coast where a hurricane’s eye move onto land.

A hurricane will die down if it hit cool ocean waters or when it hits land.