Severe Weather
Flash Floods
•Flash floods occur when rainfall or snowmelt exceeds the lands
infiltration rate and/or the carrying capacity of a river.
•Flash floods can occur in desert landscapes, valleys, coastal areas,
and flood plains along rivers.
•Floods kill more people than any other disaster.
Thunderstorms
• Characterized by: Dark cumulonimbus clouds that produce
heavy rain, sounds of thunder, flashes of lightning, strong
winds, and sometimes hail.
Thunderstorms often
occur in the summer
when the air is warm,
moist, and unstable.
Where do thunderstorms form?
• Thunderstorms form along fronts because air is
forced to rise and ahead of fronts called squall
lines. Supercells are large single-cell
thunderstorms with extremely strong updrafts,
the updrafts rotate to form tornadoes.
How does hail form?
• A
tornado is a violently rotating column of air that forms
from a mesocyclone, when it touches the ground it becomes a
tornado. Air pressure on the center of a tornado is very low and
air is sucked into the funnel cloud.
Where is “tornado alley”?
• A tornado can form in any state during any time of
year, but they are most common in “tornado alley”
during the spring and summer months.
A tornado is measured using the Fujita
based on wind speed.
scale, it is
Hurricanes
• A hurricane starts out
as a tropical storm. It
begins to gain
strength from warm
ocean water, which
evaporates and then
condenses to make
storm clouds. Winds
help push warm, moist
air upwards and help
the hurricane get
bigger as it rotates and
moves.
Hurricane diagram
• The coriolis effect
causes a
hurricane to
rotate counterclockwise.
A hurricane in the South Pacific is
called a typhoon.
• A hurricanes strength is measured using the
Saffir-Simpson Scale, it is based in wind
speeds, category 1-5.
***Notice how
air pressure
decreases when
the wind
speeds and
storm surge
increases?
Blizzards
• The winds must exceed 35 mph or 56 kmph.
• The temperature must be below 19°F or -7°C.
• The blowing snow must reduce visibility by less than ¼
of a mile.
• Conditions must persist for more than 3 hours.