A thunderstorm is a storm that is accompanied with lightning and thunder, along with heavy rainfall or hail. Typically, these storms are relatively brief, resolving themselves within a few hours, but they can do a great deal of damage, especially in the case of supercells, massive thunderstorms that can pack a serious punch. Because of the potential danger, people are usually advised to stay indoors during such storms, and severe weather warnings may be issued to put emergency services on alert if a thunderstorm looks extremely dangerous.
In order for a thunderstorm to form, warm moist air must be present near the surface of the Earth. Because warm air is lighter than cool air, it rises up into the atmosphere. If the higher air is cooler, the moist air starts to condense, forming clouds. When conditions in the atmosphere are unstable, those clouds can turn into towering cumulonimbus, with a classic anvil shape. Eventually, the clouds release their moisture, generating a torrent of rain or hail in very cold conditions.
The unstable conditions also generate electrical charges that become lightning. As the lightning strikes, it generates thunder, which is caused by the superheated air generated by lightning as it streaks through the atmosphere. As this air expands, it forms a shock wave, which registers as thunder in human ears.
In a single cell thunderstorm, there is only one major source of warm, moist air, and the storm typically ends quickly because it essentially runs out of ammunition. Multi-cell storms have multiple sources, and many form up into a cell cluster or squall line, a series of storms that hammer an area repeatedly until all of the storms pass over. In the case of a supercell, the cloud actually breaks up through the troposphere, and the resulting storm has tremendous energy, which can in turn create hurricanes and tornadoes.
Thunderstorms are most common in the tropics, where warm, moist air abounds, and in some regions, small ones occur almost every day. These storms can potentially occur anywhere, however, even in the frigid arctic, and it is not always possible to predict them. Often, weather conditions suggest that a thunderstorm may be likely, and meteorologists can alert people to the danger, but in other cases, one seems to come out of nowhere, and to dissipate just as unexpectedly. As a general rule, anvil-shaped clouds, a heavy sky, and a feeling of tension are all good reasons to go indoors for a bit in case a serious storm develops.