Thunderstorms develop in clouds and need three conditions: moisture, usually from humid air near the Earth's surface; instability, caused by mixing warm and cold air; and lift, usually from a cold or warm front moving in, a mountain, or the collision of surface winds moving the air upwards.

A cold front is the leading edge boundary of a cold air mass where cold air replaces warm air. A warm front is the leading edge of an air mass where warm air rises over the denser cold air. Each front is reflected by a symbol, a half moon for a warm front and a triangle for a cold front.

FORWARD

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Curator: Tom.Benson@grc.nasa.gov
Responsible Official: Kathy.Zona@grc.nasa.gov