1. 20.3 Thunderstorms and tornadoes
Objectives
Describe the conditions necessary for the formation of thunderstorms and tornadoes.
Describe the hazards of thunderstorms and tornadoes, and discuss related safety measures.

2. Introduction
Thunderstorms: storms with lightning, thunder, rain, and sometimes hail only a few kilometers across

3. Thunderstorms Cumulonimbus clouds Thunderstorms
Form in warm, moist, unstable air
May attain heights of up to 20 km
Thunderstorms
Often happen in the afternoon b/c surface warming throughout the day causes the air to become unstable

4. Thunderstorms (cont’d)
Something forces air up
Reaches condensation level
Heat released
Makes air warmer and less dense
Cumulonimbus cloud forms and thunderstorms begins

5. Thunderstorms (cont’d)
Made of 1 or more convection cells
Often form along fronts b/c the frontal boundary forces air to rise
Associated with large-scale pressure systems
May start and stop for days

6. Thunderstorms (cont’d)
Frontal thunderstorms often occur in lines hundreds of km long along the frontal surface
Squall lines: happen in lines ahead of the front
Strong winds often come before
Falling rain cools air

7. Thunderstorms (cont’d)
Downdraft: sinking air within a thunderstorm
Supercells: very large single-cell thunderstorms with particular strong updrafts
Can cause a tornado to develop