Starter 1.In which atmospheric layer of Earth do thunderstorms occur? a.Thermosphere b.Mesosphere c.Stratosphere d.Troposphere 2.Which gas makes up the largest component of Earth’s atmosphere? a.Argon b.Oxygen c.Nitrogen d.Carbon Dioxide 3.Which is a major source of air pollution created by humans? a.Hydroelectric plants b.Industrial factories c.Reforestation d.Nuclear power 4.Which statement describes the general movement of air masses? a.They move from areas of high pressure to areas of low pressure. b.They move from areas of high altitude to areas of low altitude. c.They move from areas of low humidity to areas of high humidity. d.They move from areas of low temperature to areas of high temperature.

Objectives  Explain the formation of a thunderstorm  Describe the conditions needed for a tornado to form  Identify the conditions that must exist for a hurricane to form

20.3 Severe Storms  A thunderstorm is a storm that generates lightning and thunder. Thunderstorms frequently produce gusty winds, heavy rain, and hail.

20.3 Severe Storms  Occurrence of Thunderstorms At any given time, there are an estimated 2000 thunderstorms in progress on Earth. The greatest number occur in the tropics where warmth, plentiful moisture, and instability are common atmospheric conditions.  Development of Thunderstorms Thunderstorms form when warm, humid air rises in an unstable environment.

Stages in the Development of a Thunderstorm During the cumulus stage, warm moist air is supplied to the cloud. Heavy precipitation falls during the mature stage. The cloud begins to evaporate during the dissipating stage.

20.3 Severe Storms  Tornadoes are violent windstorms that take the form of a rotating column of air called a vortex. The vortex extends downward from a cumulonimbus cloud.  Occurrence and Development of Tornadoes Most tornadoes form in association with severe thunderstorms. A mesocyclone is a vertical cylinder of rotating air that develops in the updraft of a thunderstorm.

Formation of a Mesocyclone A mesocyclone can occur before the formation of a tornado. First, stronger winds aloft cause lower winds to roll. Updrafts tilt the rolling air so that it becomes nearly vertical. When the rotating air is completely vertical, the mesocyclone is made.

20.3 Severe Storms  Tornado Intensity  Tornado Safety Tornado watches alert people to the possibility of tornadoes in a specified area for a particular time. Because tornado winds cannot be measured directly, a rating on the Fujita scale is determined by assessing the worst damage produced by the storm. A tornado warning is issued when a tornado has actually been sighted in an area or is indicated by weather radar.

Fujita Tornado Intensity Scale < >260

SCALEWIND SPEEDPOSSIBLE DAMAGE Enhanced, Operational Fujita Scale F mph Light damage: Branches broken off trees; minor roof damage EFO mph F mph Moderate damage: Trees snapped; mobile home pushed off foundations; roofs damaged EF mph F mph Considerable damage: Mobile homes demolished; trees uprooted; strong built homes unroofed EF mph F mph Severe damage: Trains overturned; cars lifted off the ground; strong built homes have outside walls blown away EF mph F mph Devastating damage: Houses leveled leaving piles of debris; cars thrown 300 yards or more in the air EF mph F mph Incredible damage: Strongly built homes completely blown away; automobile-sized missiles generated EF5 over 200 mph Fujita Scale of Tornado Intensity

20.3 Severe Storms  Whirling tropical cyclones that produce winds of at least 119 kilometers per hour (74 mph) are known in the United States as hurricanes.  Occurrence of Hurricanes Most hurricanes form between about 5 and 20 degrees north and south latitude. The North Pacific has the greatest number of storms, averaging 20 per year.

Satellite View of Hurricane Floyd This satellite image of Hurricane Floyd shows its position off the coast of Florida a few days before the hurricane moved onto land. Floyd eventually made landfall near the Cape Fear River mouth in North Carolina (Southport, NC).

20.3 Severe Storms  Development of Hurricanes Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air. The eye wall is a doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane. The eye is a zone of scattered clouds and calm averaging about 20 kilometers(13 mi.) in diameter at the center of a hurricane.

20.3 Severe Storms  Hurricane Intensity The intensity of a hurricane is described using the Saffir-Simpson scale. A storm surge is the abnormal rise of the sea along a shore as a result of strong winds.

Saffir-Simpson Hurricane Scale >155

Concept Map (page 57) Two air masses meet to form can be front warm cold stationary occluded