TornadoesTornadoes

Spinning LowsSpinning Lows

Low Pressure SystemLow Pressure System

• Warm air has low density and it rises (forced upwards by surrounding denser air)

• As the air rises it cools• As the air cools the relative humidity

increases – eventually reaching the dew point– At the dew point condensation takes place and

clouds form

• Low pressure systems usually form along fronts where warm air and cold air meet

• Low pressure systems cause most of the weather in the US.

• A low pressure system leads to precipitation in the form of rain, snow, hail, sleet.

Moving FrontsMoving Fronts

• When we witness a change in the weather from day to day it is due to the movement of air masses. It is the movement and collision of air masses that causes the weather to change.

What is Severe Weather?What is Severe Weather?

• Severe weather are events that fall outside of the ordinary weather patterns.

• Severe weather can include the following:– Thunderstorms– Lightning– Tornadoes– Hurricanes– Floods– Droughts– Blizzards– Fog– Ice Storms

TornadoesTornadoes• Tornadoes are violent funnel-shaped storms with

whirling winds that move in narrow paths over land.• Tornadoes form from severe thunderstorms• Tornado producing thunderstorms involve the rapid

upward movement of warm moist air• The upward moving air begins to rotate (why the

rotation starts is still not understood)• As the speed of the rotation increases more warm

moist air is drawn into the low pressure at the center• A funnel shaped cloud extends from the bottom of

the cloud sometimes touching ground– As it touches ground it picks up dirt and debris

that give it the characteristic dark gray, black color.

• Tornadoes are one of the most destructive types of storms.

Strength of Tornadoes:Fujita ScaleStrength of Tornadoes:Fujita Scale

Scale Wind Speed (mph)

Expected Damage

F0 <72 Light Damage: broken branches, billboards.

F1 72 – 112 Moderate Damage: mobile homes overturned, roofs.

F2 113 – 157 Considerable Damage: roofs off, trees snapped, uprooted.

F3 158 – 206 Severe Damage: trains overturned, cars thrown.

F4 207 – 260 Devastating Damage: houses leveled, large missles.

F5 >260 Incredible Damage: houses lifted, trees debarked, auto missles.

Tornado AlleyTornado Alley