This week we learn about how thunderstorms form, how hot and cold air move within storm clouds, and how this weather phenomenon can break those dreaded summer heat waves.
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In order to understand most weather phenomena, it is important to understand how the temperature of air affects its density and humidity. Warm air is less dense than cool air, so a pocket of warm air will rise when surrounded by cool air. Warm air also holds more water than cool air, so it can reach a higher maximum humidity before the water begins to condense and fall out of the air. Remember these facts for later, they are integral to understanding the mechanics of a thunderstorm.
During a heat wave, a layer of warm air covers a region, and there is little air movement because the air is all the same temperature. In order for a thunderstorm to occur, a mass of warm, moist air must somehow be forced upward. The sun may heat a bubble of air to a higher temperature than the surrounding air, for example the air over a hot parking lot will be warmer than the air over a grassy field. The mass of warm air may be pushed up and over a mountain by incoming wind, which is why many mountain ranges have a wet side and a dry side. And finally the warm air might be displaced by an incoming cold front. In this case the temperature on the ground will cool due to the colder incoming air, and it is the breaking heat wave that causes the thunderstorm instead of the other way around.
Even if the thunderstorm is not caused by an incoming cold front, the air near the ground will often cool following the storm. To understand this phenomenon we need to track what’s happening to the bubble of warm air during the thunderstorm. As the mass of warm air rises and moves further from the warm earth, it cools. The cooler air now contains more water than it can hold, so some of the water condenses onto particles of dust in the air, forming a cloud. When water vapor condenses, it releases heat energy, just as liquid water must absorb heat energy to evaporate. This release of energy warms the mass of air again, which moves higher yet into the atmosphere, where it cools further. This cycle of cooling, condensation, warming, and rising continues, forming a tall cumulonimbus cloud, until the mass of air hits the tropopause. The air temperature above the tropopause increases with altitude, so the mass of warm air will no longer rise.
Things are chaotic within the cumulonimbus cloud. Pockets of warm air cause updrafts after depositing their water into the cloud. The deposited water droplets and ice particles begin bumping into each other, coalescing, and falling due to gravity, carrying some air with them in a downdraft. These updrafts and downdrafts, along with any horizontal wind present, carry ice particles with them, and friction between these particles causes the buildup of static electricity that is released as lightning. Eventually, as the warm air deposits more and more water, and cools further and further, the updrafts weaken, and the rain or hail falls to the ground, bringing the cool air to the ground with it, which is exactly where cool air wants to be.