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A thunderstorm is a local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder. Thunderstorms and cumulonimbus clouds contain many of the most severe atmospheric hazards for the balloon pilot. They are almost always accompanied by strong gusty winds, severe turbulence, heavy rain showers, and lightning. These hazards may extend well away from the central core of the thunderstorm mass, sometimes as much as 30 miles or more.
Ceilings and visibility in the precipitation areas under the thunderstorms are normally poor. Because of the heavy precipitation, the ceiling reported is at best an estimate of where the pilot may break out into visual contact with the surface. The weather observer determines the vertical visibility into the precipitation, which may be significantly different from the slant-range of the pilot.
Potentially hazardous turbulence, as well as many other hazards associated with thunderstorms, make a thunderstorm a dangerous weather formation. The safe balloon pilot avoids any conditions that may expose him or her to potential thunderstorm activity. He or she exercises discretion and good judgment when potential thunderstorm conditions exist.
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Figure 4-24. Congestive cumulus cloud.
Figure 4-25. Mature thunderstorm.
Structure of ThunderstormsConvective Cells
The fundamental structural element of the thunderstorm is the unit of convective circulation known as a convective cell. A mature thunderstorm contains one or more of these cells in different stages of development, each varying in diameter from one to five miles. By radar analysis and measurement of drafts, it has been determined that each cell is generally independent of surrounding cells in the same storm. Each thunderstorm progresses through a life cycle from 1 to 3 hours, depending upon the number of cells contained and their stage of development. In the initial stage (cumulus), the cloud consists of a single cell. As the development progresses, however, new cells may form as older cells dissipate.
Stages in Thunderstorm Development
The life cycle of each thunderstorm cell consists of three distinct stages:
• Cumulus,
• Mature, and
• Dissipating.Cumulus Stage
Although most cumulus clouds do not become thunderstorms, the initial stage of a thunderstorm is always a cumulus cloud. [Figure 4-24] The chief distinguishing feature of the cumulus or building stage is an updraft that prevails throughout the entire cell. This updraft may vary from a few feet per second to as much as 6,000 fpm (65 knots) in mature cells. As an updraft continues through the vertical extent of the cell, water droplets grow in size and raindrops are formed.
Mature Stage
The beginning of surface rain and adjacent updrafts and downdrafts initiates the mature stage. [Figure 4-25] By this time, the average cell has attained a height of 25,000 feet. As the drops begin to fall, the surrounding air begins a downward motion because of frictional drag. This descending air will be colder that its surroundings and its rate of downward motion is accelerated, forming the downdraft. The downdraft reaches maximum speed a short time after rain begins to fall in the cloud. Downdrafts occur at all levels within the storm, and their speed ranges from a few feet per minute to about 2,500 fpm (25 knots). Significant downdrafts never extend from the top of the cell because moisture is not sufficient in the upper levels for raindrops to form. At these high levels, only ice crystals, snowflakes, and supercooled water are present. Therefore, their rate of fall is insufficient to cause appreciable downdrafts. The mature cell generally extends far above 25,000 feet—in some cases up to 70,000 feet. In the middle levels, around 14,000 feet, strong updrafts and downdrafts are adjacent to each other. A shear action exists between these drafts and produces strong and frequent gusts.
Dissipating or Anvil Stage
Throughout the life span of the mature cell, more and more air aloft is entrained by the falling raindrops. Consequently, the downdraft spreads out to take the place of the weakening updrafts. As this process progresses, the entire lower portion of the cell becomes an area of downdraft. Since updrafts are necessary to produce condensation and release latent heat energy, the entire structure begins to dissipate.
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Figure 4-26. Dissipating thunderstorm.
[Figure 4-26] The strong winds aloft carry the upper section of the cloud into the familiar anvil form (cumulonimbus cloud). However, the appearance of the anvil does not always indicate that the thunderstorm is dissipating.
A significant thunderstorm hazard is the rapid change in wind direction and wind speed immediately prior to storm passage at the surface. These strong winds are the result of the horizontal spreading of the storm’s downdraft current as they approach the surface of the earth. This initial wind surge, as observed at the surface, is known as a first gust. The speed of this first gust may exceed 75 knots and vary 180° in direction from the previously prevailing surface winds. First-gust speeds average about 15 knots over prevailing velocities, and average an approximately 40° change in the direction of the wind. First gusts usually precede the heavy precipitation, and strong gusts may continue for approximately 5 to 10 minutes with each thunderstorm cell. First gusts are not limited to the area ahead of the storm’s movement. They may be found in all sectors, including the area back of the storm’s movement.
　
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