The Recipe for Thunderstorm
All thunderstorms require three main ingredients:
1. Moisture
Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans, as well as the Gulf of Mexico. Water temperature also plays a large role in how much moisture is in the atmosphere. Evaporation is higher in warm ocean currents and therefore they put more moisture into the atmosphere than with cold ocean currents at the same latitude.
2. Instability
Air is considered unstable if it continues to rise when given a nudge upward (or continues to sink if given a nudge downward). An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft.
In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As this parcel rises it cools and some of the water vapor will condense forming the familiar tall cumulonimbus cloud that is the thunderstorm.
3. A Lifting Mechanism
Typically, for a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. This upward nudge is a direct result of air density.
Some of the sun's heating of the earth's surface is transferred to the air which, in turn, creates different air densities. The propensity for air to rise increases with decreasing density. This is difference in air density is the main source for lift and is accomplished by several methods (differential heating, terrain, fronts, dry lines and outflow boundaries)
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