Basics of Thunderstorms

Thunderstorms, also referred to as electrical storms are famous by their presence of lightning and their meeter thunder produced in a cumulonimbus cloud. Thunderstorms generally have an abundance of moisture and therefore generally produce heavy rainfall, along with strong winds, hail, and occasional tornadoes. During the winter season thunder can sporadically be herd within a heavy snowfall event — generally within snowsqualls off the Great Lakes. This is known as thundersnow.

Thunderstorms form when moist, unstable air is lifted vertically into the atmosphere. Lifting of this air produces condensation and the release of latent heat (the energy required to change a substance to a higher say of matter (solid to liquid to gas). These processes that initiate vertical lifting can be caused by:

(i) Unequal warming at the earth’s surface: Warmer air at the surface and cooler air aloft (upper atmosphere) causes a parcel of air to rise. Depending on the lapse rate, (rate of the parcel of air rising) cumulonimbus clouds will form developing into thunderstorms.

However, whether the air aloft is warmer or the same temperature as the surface a capping inversion develops. This means that a parcel of air will not rise, disallowing a thunderstorm development. As the cap weakens, (which can be due to moistening of the atmosphere, cooling the upper atmosphere, or warming of the surface) thunderstorms might develop. Thunderstorms are generally strongest when maximum heating has occurred and capping weakens in the mid to late afternoon hours.

(ii) Uplift of an air masses because of a topographic impediment (mountains): The uplift also causes the cooling of the air mass and therefore whether enough cooling occurs condensation can occur and form into orographic precipitation such as thunderstorms.

(iii) Dynamic lifting: Occurs with the presence of a frontal zone, such as a warm front (separates warmer more humid air from drier cooler air) and a cold front (a separation of warmer more humid air from drier cooler air).

After lifting has begun, the rising parcel of air will start to cool because of adiabatic expansion (a reduction in air pressure). At a certain level, (depending on the atmospheric set up) the parcel of air will reach its dewpoint and condensation will start to form. This yields clouds, and whether the uplift continues due to unstable conditions, these clouds form into cumulonimbus clouds and consequently thunderstorms.

Thunderstorms once developed have stages they go through as well. These stages can be described as the following:

(i) Cumulus Stage: This is the initial stage of thunderstorm development, which occurs when the updraft reaches the condensation point in the region (the formation of water droplets) and a cumulus cloud begins to form. During this stage the cumulus cloud will expand both vertically (higher up into the atmosphere) and laterally (covering more area on the ground by growing outwards in all directions). Cumulus clouds might also merge together forming a thunderstorm cell many miles wide, with cloud tops reaching about 30,000 feet.

(ii) Mature Stage: The updraft has begun to penetrate higher levels of the atmosphere, and the pressure of an abundance of cover crystals and water droplets leads to the formation of precipitation. The appearance of precipitation marks the beginning of this stage. The precipitation falling towards the surface is one contributor to the development of the downdraft in the storm. At the beginning, the downdraft might only be present in the middle and lower levels of the storm, which gradually increases in a lateral and vertical extent but it never extends to the top of the cloud. Strong winds at these altitudes cause the tops of the clouds to level off, and take an anvil shape. The resulting cloud is called cumulonimbus incus. The “anvil” is so high and temperatures are so low that the top of the cloud is composed entirely of cover crystals. Cloud tops during this stage might reach upwards of 60,000 feet or higher.

(iii) Dissipating Stage: A gradual termination of rainfall occurs because the updraft has been cut off (no more air is being fed into the thunderstorm cell). This is when precipitation falls through the cloud, breaking it up. During the dissipating stage, the humidity in the air drops and the precipitation ends.

To recap thunderstorms need certain ingredients to from and once they have formed they can become very violent very swiftly producing strong winds, hail, flooding rains and tornadoes. The presence of thunder and rain marks the mature stage of a thunderstorm — the most hazardous part of the storm which occurs at the end of the mature stage as the storm collapses.