Thunderstorms often form during warm and sunny days; most commonly during the months of spring, summer and fall. Thunderstorms usually go through three distinct stages during their development: a formation stage, a mature stage and a dissipation stage. For a thunderstorm to form, it needs moisture, unstable air and some means by which these two weather elements can rise into the atmosphere. An unstable atmosphere is characterized by a drop in air temperature as it rises up in the atmosphere. Many thunderstorms form along zones of wind convergence, where contrasting air temperature, humidity and density are found.
Earth´s surface heating
On warm sunny days, heat is absorbed by the Earth´s surface. The air at surface level is heated by the radiation received from the Sun. Heated air becomes lighter than the surrounding air and rises higher into the atmosphere. Heated air gradually mixes with cooler drier air by convection. When the heated air reaches saturation levels at higher altitudes in the atmosphere, it condenses into clouds, releasing latent heat into the surrounding air. In this way, convection and the release of latent heat combine together to form clouds which might be able to reach 20 km (12 miles) in altitude.
Convection
Some regions in the Earth´s surface absorb the sun´s energy more efficiently than others. The air above these regions becomes warmer and less dense than its surroundings. This causes a parcel of air to rise, and as it rises, it expands and cools. At higher latitudes in the atmosphere, the parcel of air mixes with the cooler air by the process of convection. During water vapor condensation, the latent heat released warms the surrounding air, causing it to rise higher and allowing warmer, humid air from below add itself to the cloud, forming a vertical cumulus cloud.
Growth stage
In this stage, as a parcel of warm and moist air rises, it cools by convection when coming in contact with drier, colder air above. As the cloud begins to form by condensation, latent heat is released when water vapor turns into a liquid or solid. This maintains the rising air in the cloud warmer than the air surrounding it. The cloud continues to grow being pumped by the rising air underneath. In a few minutes, a huge vertical cloud may develop. During the growth stage, the updrafts maintain the ice crystals and water droplets suspended inside the cloud. Moreover, during this stage there is usually no rain or lightning.
Mature stage
When a cloud reaches maturity, its top usually expands, attaining an anvil shape, which usually occurs when a cloud reaches the stratosphere. At higher latitudes, freezing temperatures cause the formation of hail and sleet, which usually fall down and touch the ground before melting into liquid. In the mature stage, water droplets have become sufficiently big to overcome the warm rising air and they begin to fall as precipitation, forming a cold downdraft. When the cold downdraft touches the Earth´s surface, the air spreads horizontally in every direction, forming a gust front. Updrafts and downdrafts form in the center of the cloud, creating severe turbulence. Different electric charges on both extremes of the cloud produce lightning and thunder.
Dissipation stage
From 15-20 minutes after a thunderstorm has entered its mature stage, it begins to dissipate. This stage occurs when the gust front subsides, preventing updrafts from entering the cloud. During the dissipation stage, downdrafts prevail, preventing the formation of humid updrafts, which is what ultimately drives a storm. After warm, humid air is no longer able to penetrate the storm, a storm is gradually brought to a halt. As the storm gradually dies, water droplets no longer form and the only droplets present evaporate rapidly, leaving only the high altitude cirrus, anvil shaped, cloud.
A thunderstorm usually goes through its three stages in about an hour or less. Typically, rapid rising parcels of air are needed to produce a thunderstorm. Rapid rising parcels of air are formed in an unstable atmosphere. Generally, the warmer the air near the surface of the Earth, the colder the air is at higher altitudes. In addition to temperature, moisture on the surface of the ground may add to the atmosphere´s instability. According to the National Weather Service, during the summer, thunderstorms develop in the afternoons when the Sun heats the surface air. In an unstable atmosphere, parcels of air will rise and produce clouds, precipitation and lightning.
