A thunderstorm is caused by a low pressure system, usually extratropical in nature. An extratropical low is one that is not tropical. A tropical low is usually a tropical storm or hurricane. Furthermore, extratropical lows are usually asymmetrical, and most rainfall with them comes with frontal boundaries. There are two main types of frontal boundaries, cold and warm. For convenience, a brief description is described below.
A cold front is an area of cold air advancing against an area of warm air. The warm air, being lighter, is pushed up. At this boundary, moisture forms, causing clouds. In a cold front, the clouds start low, and billow upward, as the cold air pushes higher up into the sky. The clouds often reach five miles in height and are then called cumulonimbus clouds. Heavy rain or even hail fall form these clouds. Then, as the boundary moves on, the clouds expend all their moisture into rain and dissipate.
A warm front is an area of warm air advancing against an area of cold air. Unlike a cold front, the effects of a warm front start at high altitudes as the warm air pushes the cold from above. The first clouds are usually high-level, such as cirrus or cirrostratus. Then, as the warm front continues to advance, the clouds come at lower altitudes, typically ending in the low altitude nimbostratus cloud. Lighter rain, or, if in winter, snow, can fall from these clouds. Then, as the clouds empty out their moisture, the clouds thin, sometimes ending in a white stratus cloud.
There are other types of fronts as well, that are not as well known. One is the stationary front, which is as boundary of cold and warm air in which neither is advancing. There is usually little storm activity associated with these fronts. Another rarer type of front is the occluded front, where a cold front catches up and combines with a warm front, ending in a boundary of cold and cool air. Since the difference in air temperature is less prominent in these systems, the fronts usually subsequently dissipate.
An area of thunderstorms form in a line, usually along a cold frontal boundary. Along this front, a squall line forms. A squall line is a thin area of heavy rain and wind that trails from a low pressure system, often hundreds of miles away.
A low pressure over northwestern Pennsylvania and an accompanying squall line to the south.
The internal structure of the thunderstorm is one of rising air. Usually warm air on the surface rises, cools, forms a few cumulus clouds, and then sinks when it cools to past the temperature of the surrounding air. However, in a cold frontal boundary, abnormally warm air is rising into cold air, and it therefore does not stop. The clouds continue upwards until they reach cumulonimbus stage, where the air finally cools and sinks. A healthy thunderstorm is a series of updrafts and downdrafts balancing each other out. The downdrafts help water particles to condense and become rain. However, although contrary to what would be thought at first glance, hail forms during the warmer seasons, when there are thunderstorms. Hail forms when water droplets form in the part of the cloud where the temperatures are below freezing. The updrafts send the ice higher into the cloud, until the plummet, attach to other droplets in the lower atmosphere, and then are swept up again. This continues until the ice droplets get heavy enough to fall. In strong updraft conditions, hail can reach over half a foot in diameter and weigh over a pound.
Another uncommon occurrence with thunderstorms is thundersnow, which is a thunderstorm at below freezing temperatures. These are rare, because thunderstorms are prevalent in the warm seasons. This is because there is a greater difference between ground temperatures and atmospheric temperatures and air can more easily stay warmer than surrounding air. As a result, cold fronts are generally strong in summer and weak in winter, whereas warm fronts are generally strong in winter and weak in summer.
Most of the time, thunderstorms are relatively simple systems. However, when a tornado forms, the situation is much more complex. Tornadoes are discussed in the next part of this post, see here. For more information on particularly intense thunderstorms called derechos, click here.
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