A thunderstorm forms when moist, unstable air is lifted vertically into the atmosphere. Lifting of this air results in condensation and the release of latent heat. The process to initiate vertical lifting can be caused by:
- Unequal warming of the surface of the Earth.
- Orographic lifting due to topographic obstruction of air flow.
- Dynamic lifting because of the presence of a frontal zone.
Immediately after lifting begins, the rising parcel of warm moist air begins to cool because of adiabatic expansion. At a certain elevation the dew point is reached resulting in condensation and the formation of a cumulus cloud. For the cumulus cloud to form into a thunderstorm, continued uplift must occur in an unstable atmosphere. With the vertical extension of the air parcel, the cumulus cloud grows into a cumulonimbus cloud. Cumulonimbus clouds can reach heights of 20 kilometers above the Earth's surface. Severe weather associated with some these clouds includes hail, strong winds, thunder, lightning, intense rain, and tornadoes.
Generally, two types of thunderstorms are common:
- Air mass thunderstorms which occur in the mid-latitudes in summer and at the equator all year long.
- Thunderstorms associated with mid-latitude cyclone cold fronts or dry lines. This type of thunderstorm often has severe weather associated with it.
The most common type of thunderstorm is the air mass storm. Air mass thunderstorms normally develop in late afternoon hours when surface heating produces the maximum number of convection currents in the atmosphere. The life cycle of these weather events has three distinct stages. The first stage of air mass thunderstorm development is called the cumulus stage (Figure 3). In this stage, parcels of warm humid air rise and cool to form clusters of puffy white cumulus clouds. The clouds are the result of condensation and deposition which releases large quantities of latent heat. The added heat energy keeps the air inside the cloud warmer than the air around it. The cloud continues to develop as long as more humid air is added to it from below. Updrafts dominate the circulation patterns within the cloud.
The mature thunderstorm begins to decrease in intensity and enters the dissipating stage after about half an hour. Air currents within the convective storm are now mainly downdrafts as the supply of warm moist air from the lower atmosphere is depleted. Within about 1 hour, the storm is finished and precipitation has stopped.
Thunderstorms form from the equator to as far north as Alaska. They occur most commonly in the tropics were convectional heating of moist surface air occurs year round. Many tropical land-based locations experience over 100 thunderstorm days per year. Thunderstorm formation over tropical oceans is less frequent because these surfaces do not warm rapidly. Outside the tropics, thunderstorm formation is more seasonal occurring in those months where heating is most intense.
Figure 6 describes the annual average number of thunderstorm days across the United States. According to this map, the greatest incidence of thunderstorms occurs in the southeast and in parts of Colorado, Arizona, and New Mexico. This particular spatial distribution suggests that extreme solar heating is not the only requirement for thunderstorm formation. Another important prerequisite is the availability of warm moist air. In the United States, the Gulf of Mexico supplies adjacent continental areas with moist maritime tropical air masses. These air masses are relatively unstable, quickly forming cumulonimbus clouds when surface heating is intense. The secondary maximum found in Colorado and New Mexico is due to another climatic factor. All of these areas are mountainous. Mountain slopes in these areas that face the sun absorb more direct solar radiation and become relatively warmer creating strong updrafts that form into cumulus clouds. If the differential heating is also supplemented by winds from the east, the cumulus clouds are further enhanced by orographic lifting and become thunderstorms. Notice that only a few thunderstorms occur along the west coast of the United States and Canada. This region is frequently under the influence of cooler maritime air masses that suppress convectional uplift over land.
The figure above shows the average number of thunderstorm days each year throughout the U.S. The most frequent occurrence is in the southeastern states, with Florida having the highest number 'thunder' days (80 to 100+ days per year).
It is in this part of the country that warm, moist air from the Gulf of Mexico and Atlantic Ocean (which we will see later are necessary ingredients for thunderstorm development) is most readily available to fuel thunderstorm development. (via noaa.gov)
Most thunderstorms are of the variety described above. However, some can form into more severe storms if the conditions exist to enhance and prolong the mature stage of development. Severe thunderstorms are defined as convective storms with frequent lighting, accompanied by local wind gusts of 97 kilometers per hour, or hail that is 2 centimeters in diameter or larger. Severe thunderstorms can also have tornadoes.
In most severe thunderstorms, the movement of the storm, in roughly an easterly direction, can refresh the storm's supply of warm humid air. With a continual supply of latent heat energy, the updrafts and downdrafts within the storm become balanced and the storm maintains itself indefinitely. Movement of the severe storm is usually caused by the presence of a mid-latitude cyclone cold front or a dry line some 100 to 300 kilometers ahead of a cold front. In the spring and early summer, frontal cyclones are common weather events that move from west to east in the mid-latitudes. At the same time, the ground surface in the mid-latitudes is receiving elevated levels of insolation which creates ideal conditions for air mass thunderstorm formation. When the cold front or dry line of a frontal cyclone comes in contact with this warm air, it pushes it like a bulldozer both horizontally and vertically. If this air has a high humidity and extends some distance to the east, the movement of the mid-latitude cyclone enhances vertical uplift in the storm and keeps the thunderstorms supplied with moisture and energy. Thus, the mid-latitude cyclone converts air mass thunderstorms into severe thunderstorms that last for many hours. Severe thunderstorms dissipate only when no more warm moist air is encountered. This condition occurs several hours after nightfall when the atmosphere begins to cool off.
Figure 7 illustrates the features associated with a severe thunderstorm. This storm would be moving from left to right because of the motion associated with a mid-latitude cyclone. The upper-level dry air wind is generated from the mid-latitude cyclone. It causes the tilting of vertical air currents within the storm so that the updrafts move up and over the downdrafts. The green arrows represent the updrafts which are created as warm moist air is forced into the front of the storm. At the back end of the cloud, the updrafts swing around and become downdrafts (blue arrows). The leading edge of the downdrafts produces a gust front near the surface. As the gust front passes, the wind on the surface shifts and becomes strong with gusts exceeding 100 kilometers per hour, temperatures become cold, and the surface pressure rises. Warm moist air that rises over the gust front may form a roll cloud. These clouds are especially prevalent when an inversion exists near the base of the thunderstorm.
Some severe thunderstorms develop a strong vertical updraft, commonly known as a mesocyclone. Mesocyclones measure about 3 to 10 kilometers across and extend from the storm's base to its top. They are also found in the southwest quadrant of the storm. In some cases, mesocyclones can overshoot the top of the storm and form a cloud dome (Figure 7). About half of all mesocyclones spawn tornadoes. When a tornado occurs, the mesocyclone lengthens vertically, constricts, and spirals down to the ground surface. Scientists speculate that mesocyclones form when strong horizontal upper air winds interact with normally occurring updrafts. The shearing effect of this interaction forces the horizontal wind to flow upward intensifying the updraft.