Ch.13 – The Nature of
Storms
Thunderstorms
• Thunderstorms
• Violent disturbances in the atmosphere, which involve sudden changes in air
pressure.
• Heavy rain storms with thunder and lightning.
• Form in cumulonimbus clouds, or thunderheads.
• Form when warm air is forced upward by a cold front, or during the afternoon when
the air is hot and humid.
• How thunderstorms form… 3 ways.
• Moisture
• Lifting
• Stability
• Limits… are the same.
Thunderstorms
• Types of Thunderstorms
• Air-Mass
• Frontal
• Thunderstorm development… 3 Stages
• Cumulus stage
• Mature Stage
• Dissipation Stage
Thunderstorms
• What causes lightning?
• Electrical discharge between clouds or the ground.
• What causes thunder?
• Super heating of the air around lightning causes rapid expansion and an
explosion.
• Safety
• Get inside!
• Stay away from metal, tall structures, wide open spaces and get off of bodies
of water.
• When inside, avoid using phones, electrical appliances, and plumbing fixtures.
• Cars are safe, but stay inside and don’t touch metal.
Severe Weather
• Main characteristics of thunderstorms.
• Wind
• Rain
• Lightning
• Weather Cells
• Supercells
• Rotating updrafts.
• 150 mph winds.
• Produce Tornadoes.
• Downbursts
• Straight downdrafts.
• 150 mph winds.
• Produce Micro & Macrobursts.
Severe Weather
• How does hail form?
• Water droplets freeze inside a thunderhead.
• Updrafts and down drafts exist side by side.
• How does a tornado form?
• A change in wind direction and speed creates horizontal rotation(like a rolling
pin).
• Updrafts tilt the rotating air into a vertical position.
• Updrafts stretch the tornado and it spins faster.
• Air is removed from the center, which drops air pressure inside, causing winds
to increase.
Severe Weather
Severe Weather
• Tornado Distribution in the U.S.
• Most common in the Central U.S., due to large temperature contrasts.
• Tornado Safety
• Watches & Warnings
• Watch means conditions are right, but no tornado…yet.
• Warning means a tornado has been spotted.
• Indoors- Get into the basement if possible, if no basement is available, find an area away
from windows and doors.
• Outdoors- Get inside! Or lie in a ditch, or low lying area.
Tropical Storms
• Tropical Cyclones
• Hurricanes, Typhoons, & Cyclones…
• Need two ingredients to form…
• Lots of warm ocean water and lift.
• Movement is toward the west, unpredictable, and slows as it approaches
land.
• Three stages of cyclone formation.
• Formative
• Mature
• Dissipation
Tropical Storms
• Three main dangers of a Hurricane.
• Damage
• Winds
• Storm Surge
• Eye of the Hurricane
• It is the center of a hurricane, an area of calm in the storm, but it is
surrounded by the storms most powerful winds.
• Safety
• Evacuate! If you do stay, treat it like tornado safety.
Tropical Storms
Tropical Storms
Recurrent Weather
• Floods
• Unexpected floods which occur when so much water pours into a stream that it
overflows its banks and covers the land on either side of the channel.
• Most are due to large amounts of rainfall in a short period of time.
• Flood Safety
• Predicting a flood…
• Weather forecasting
• Flood watches
• In an emergency…
• Get to higher ground
• Evacuate
• Hazards
• Polluted water, mudslides, structural damage.
Recurrent Weather
• Droughts
• Extended periods of below average rainfall.
• Dust Bowl = the most famous U.S.
• The result of persistent high pressure.
• Heat & Cold Waves
• Read pgs.362-365.