Thunderstorm

Thunderstorms develop in the same process that forms the puffy clouds of summer skies, cumulus clouds. These clouds form when humid air (that is, air with an abundance of water vapor) near the surface is pushed up by being forced over a mountain range, a front, strong solar heating of the surface, or some other means. As the air rises through the atmosphere, it expands and cools. Eventually the rising air cools to the point where its water vapor condenses to form droplets of liquid water. A huge collection of these tiny suspended droplets forms a cloud. At this stage the rising air is visible as a cumulus cloud, called a convective cloud since it forms by convection (vertical air movement). During fair weather the convective clouds stop their vertical growth at this point and do not bring rain.

To form a thunderstorm from a convective cloud several conditions are necessary. Most importantly the atmosphere must be unstable. In an unstable atmosphere the air temperature drops rapidly with height, meaning any bubble of air that begins rising and cooling will remain warmer than its surroundings. At every point in its ascent the rising air acts like a hot air balloon: since it is warmer and less dense than the surrounding air it continues to rise.

A second requirement for a strong thunderstorm is plenty of humid air. This condition supports the growth of cloud droplets and actually fuels the rising air through latent heat. The water vapor in the air comes from the evaporation of liquid water somewhere—most likely the oceans. To evaporate the water into vapor, energy is required just as heat must be added to a kettle to make its water boil. This energy carried with the water vapor wherever it goes is latent or hidden heat. If and when the vapor condenses to form liquid water, the latent heat will be released back into the environment. Thus when the water vapor in rising air condenses to form water droplets a significant amount of heat is released to the surrounding air. Heating the air makes it less dense and increases the tendency of the air bubble, now a cloud, to rise.

As the air continues to rise and cool, droplets within the cloud begin to grow by coalescence (sticking together). In the clouds of colder climates droplets may freeze to form ice crystals, which grow as more and more water vapor condenses on them. The droplets or ice crystals, known as precipitation particles, only grow as long as they can be supported by the updrafts. When they grow too large, they begin to fall out of the cloud as drizzle or raindrops. If the updrafts in the cloud are vigorous enough, much larger precipitation will be formed. In a thunderstorm the uplift process is so strong that the cloud grows to the height of the entire lower atmosphere (about 40,000 ft [12 km] above the surface) allowing large raindrops and hailstones to form.

At least two distinct types of thunderstorms can be observed. Over warm humid areas such as the Gulf of Mexico the air-mass thunderstorm is the most common. These thunderstorms grow from converging cumulus clouds that rise and cool as described above. As the storm matures, rain begins to fall from the upper part of the cloud. The falling precipitation causes downdrafts. This downward moving air eventually overwhelms the rising air. The downdrafts effectively shut off the uplift necessary for the storm to grow, so the storm dissipates as the air sinks and no more rain is formed. These types of thunderstorms are common over the Florida peninsula bringing showers and lightning strikes but rarely any hail or damaging winds unless frontal action is nearby.

Potentially more severe thunderstorms form in temperate regions such as the central and eastern United States. Called frontal thunderstorms these storms often form ahead of the advancing edge of a cold air mass (a cold front). In the summer months the air ahead of the cold front is usually warm humid air that is highly unstable. The denser cold air forces the warmer lighter air ahead of it to rise forming convective clouds which eventually rain. As in an air mass thunderstorm, the falling rain causes downdrafts in the cloud. Unlike the air mass A nighttime thunderstorm. © Keith Kent/Photo Researchers. Reproduced by permission. storm, a frontal thunderstorm is arranged so that it is intensified by the downdrafts. The downdrafts become strong gusts of down-flowing air. When they reach the ground the downdrafts spread out and force more warm humid air to begin rising into the thunderstorm. This provides the storm with more latent heat, strengthening the cloud's updrafts, increasing its wind speeds, and improving the chances of heavy rain and hail. The storm advances into the warm air, vacuuming up humid air, and transforming it into a very organized system of powerful updrafts and downdrafts. After the storm and the front passes, the affected area is often affected by the cold air behind the front where temperatures and humidities are usually much lower.