GEO 125

Physical Geography

Thunderstorms

Generic Atmospheric Conditions Associated with the Occurrence of Thunderstorms

• Vertical air movement
• Unstable atmospheric conditions
• High temperature
• High relative humidity
• Cumulonimbus clouds

Causes of Thunderstorms

Thermal convection

• Major cause of rain in the tropical regions of the world
• Major cause of rain in the summer months in many parts of the midlatitudes

Passage of cold fronts in the midlatitudes at any time of the year

Occurrence of Thunderstorms in the Midlatitudes

Temporal characteristics

• Mainly occur during the summer months
• Normally in the afternoon

Spatial variation in the occurrence of thunderstorms

• Over the land rather than water
• Over flat land rather than in mountainous areas
• Over agricultural land rather than forested land

Geographic Distribution of Thunderstorms

United States

• Florida has the most thunderstorm days per year (about 90)
• Southeastern United Sates have between 50 to 70 thunderstorm days per year
• Most locations on the west coast have less than 5 thunderstorm days per year

Worldwide

• The rain of tropical regions is associated with thunderstorms caused by convectional lifting
• Indonesia is the country in the world with the greatest number of thunderstorm days

Stages in Thunderstorm Development

• Cumulus stage dominated by updrafts
• Cumulonimbus stage having updrafts, downdrafts, and violent weather
• Dissipating stage dominated by downdrafts

Hazards Associated with Thunderstorms

• Lightning (more people are killed in an average year by lightning in the United States than by any other meteorological hazard)
• Flash floodsHail
• High winds
• Macrobursts
• Tornadoes

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