Chapter 13

The Nature of Storms

Thunderstorms• Three conditions required:

– Source of moisture– Rising air mass– Unstable atmosphere

• Air mass t-storms– Orographic (mountain)– Sea breeze– Convection

• Frontal t-storms– Advancing cold fronts (squall

line)

Sea breeze

T-storm days per year

Three Stages of Thunderstorm Development

Lightning and Thunder

• Transfer of electricity (similar to static electric shock)

• 5 times hotter than the sun’s surface• Thunder is the sonic boom made by

expanding air• Forest fires, injuries, and deaths• Lightning causes more annual deaths (on

average) than tornadoes, hurricanes or floods

Lightning Strikes

Supercells

• Severe t-storm• Fastest winds

on Earth• Can last for

several hours• Strong winds• Hail• Downbursts• Tornadoes

There are about 100,000 thunderstorms in the United States each year. About 10% of those are severe; fewer still are supercells.

Tornado Development

• Horizontal rotating air is lifted vertically by t-storm updraft

• 80% of all tornadoes are weak (F0 or F1)

Tornado Classification

Tornado Frequency Map

• “Tornado alley”—Texas, Oklahoma, Kansas, Missouri

• Most common in spring during late afternoon and evening

• Only about 1% of all tornadoes are violent (F4 or F5)

• Safety: Shelter in a basement or interior room; NOT in a car or mobile home

Hurricanes/Tropical Storms

• Begin to form over warm tropical oceans

• Air rises and pulls in more warm humid air

• Coriolis effect causes cyclonic rotation

• Wind speeds are used to classify tropical storms/hurricanes

Floods and Droughts

• Floods and flash floods can occur after storms or a hurricane moves through an area

• Coastlines, floodplains, and narrow river valleys are most susceptible

• Drought—extremely dry weather—can also cause damage

• Droughts are usually caused by sustained high pressure systems

Heat Waves/Cold Waves

• Heat waves are often produced by high pressure systems that cause droughts

• High humidity adds to the discomfort of high temperatures (see next slide)

• Cold waves are also produced by large high pressure systems

• Windchill magnifies the effect of cold weather for people

Heat Index