Chapter 13 Notes
Section 1- Thunderstorms
3 conditions must be present for a thunderstorm to form
1. An abundant source of moisture in the lower levels of the
atmosphere
2. Some mechanism must lift the air in order for it to condense
3. Atmosphere around the rising air mass must be unstable
Limitations to storm growth
1. Rising air mass reaches a stable layer of air thus hindering uplift
2. Rising air mass encounters air that is warmer than it is therefore
hindering condensation
Types of Thunderstorms
1. Air Mass Thunderstorm- caused by unequal heating of the earth’s
surface
a. Mountain Thunderstorm
b. Sea Breeze Thunderstorm
2. Frontal Thunderstorm- caused by collision of advancing cold and
warm fronts
Life Cycle of a Thunderstorm- 3 phases
1. Cumulus
a. Warm air begins to rise
b. Updrafts are created by rising air
c. Rising air condenses when it is cooled in upper levels of the
atmosphere
d. Latent heat is released when warm air condenses
e. Water droplets coalesce to form precipitation
2. Mature
a. Precipitation begins to fall
b. Falling precipitation cools the air around it
c. Cooled air sinks creating downdrafts
d. Equal amounts of updraft and downdraft in this stage
3. Dissipation
a. Updrafts slow as the lower atmosphere cools
b. Absence of warm air leads to fewer updrafts
c. Remaining precipitation falls from system
Section 2- Severe Weather
Tornado Formation
1. Wind speed and direction suddenly change with height
2. This creates a funnel cloud that spins horizontally.
3. Strong updrafts tilt the funnel to a vertical position
4. The pressure gradient between the inside (low pressure) and
outside (high pressure) creates strong winds
Fujita Scale- used to classify tornadoes
1. F0-F1
a. 80% of all tornadoes
b. path of up to 3 miles
c. wind speeds of 60-115 mph
d. lasts 1-10 minutes
2. F2-F3
a. 19% of all tornadoes
b. path up to 15 miles
c. wind speeds of 110-205 mph
d. lasts 20+ minutes
3. F4-F5
a. 1% of all tornadoes
b. path of 50+ miles
c. wind speeds of 200+ mph
Section 3- Tropical Storms
Hurricanes
Certain factors must be present for hurricanes to form
1. An abundant source of warm moist air
2. This warm air must be forced up
3. As air rises (evaporates) latent heat is stored
4. When this air condenses, latent heat is released, giving the
storm energy
5. A low-pressure system is created at the surface of the ocean
6. More warm air moves toward the low-pressure center of
the storm to replace the rising air
7. The Coriolis effect causes the moving air to start rotating
8. As air approaches the low-pressure center it continues to
rise into the rotating wind above.
9. When it rises and condenses, more energy is released into
the storm
Classification of Hurricanes
Saffir-Simpson hurricane scale
5 Categories
1. Sustained winds of 74-95 mph
2. Sustained winds of 96-110 mph
3. Sustained winds of 111-130 mph
4. Sustained winds of 131-155 mph
5. Sustained winds of >155 mph
Section 4- Recurring Weather
Floods- occur when storms move slowly and drop large amounts of rain in
one area
Droughts
1. Extended periods with well-below-normal rainfall
2. Usually the result of shifts in global wind patterns allowing large
high pressure systems to persist over long periods of time
3. There is little rising air thus no condensation occurs
4. Will persist until global wind patterns shift enough to move the
high pressure system
5. Heat waves- often associated with droughts
Cold waves- extended period of below-normal temps