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There are two atmospheric conditions that are resposible for the development of severe thunderstorms. They are the following:
- Vertical wind shear, which can be one of two things: 1. Directional shear, wind direction changing with height, or 2. Speed shear, wind speed changing with height. Vertical wind shear has a strong influence on what form the thunderstorm might take. As wind shear becomes stronger, the strength and the duration of thunderstorm development increases. Wind shear plays an extremely important role in the generation of thunderstorms. In addition, speed shear helps in the generation of strong gusty winds. Often strong winds in thunderstorms are the result of fast moving upper level winds being mixed down to the surface.
- An unstable thermodynamic structure of the atmosphere. In other words, an atmosphere that will support rising air and force the development of convection. The stronger the buoyancy of the air, the greater the convection. Thermodynamic structure strongly influences the vertical accelerations within a storm.
It is important to know that thunderstorms require strong columns of rising air in order to form. When the air begins to rise, it expands and cools adiabatically due to the decreasing pressure. Eventually it will become saturated, condense and form clouds. The stronger the vertical velocity, the more concentrated and higher the clouds are. Eventually precipitation results. There are several situations that the air can be made to rise. They include:
1. Warm air at the surface or cold air aloft. Cold air is heavier than warm air. Therefore if a bubble of warm air is surrounded by cold air, that warm air bubble will rise. The greater the temperature difference, the faster the air parcel will rise. This is why thunderstorms are more common on hot, summer days.*
2. Moist air at the surface or dry air aloft. Dry air is heavier than moist air. The mechanisms to create rising motion are very similiar to that of cold and warm air. This is why thunderstorms are so common on humid, summer days.*
3. A trigger mechanism, that forces the air to rise even in a stable atmosphere. Examples include cold fronts, dry lines, outflow boundries, and orographic lifting.
4. Positive Vorticity Advection at 500mb, which in turn forces air to rise at the ground.
*Denotes a thermodynamically unstable atmosphere
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