The paramount responsibilities of the forecaster include providing forecasts of severe weather conditions and timely warnings to aircraft and ships to ensure the safety of their operations, as well as the safety of their personnel.

This chapter discusses some of these phenomena and methods that may be used to forecast severe weather conditions.

LEARNING OBJECTIVES: Recognize phenomena associated with thunderstorm activity. Forecast the movement and intensity of thunderstorms.

The thunderstorm represents one of the most formidable weather hazards in temperate and tropical zones. The turbulence, high winds, heavy rain, and occasionally hail that accompany thunderstorms are a definite threat to the safety of flight and to the security of naval installations. It is important that the forecaster be acquainted with the structure of thunderstorms and their associated weather, as well as the knowledge to accurately predict their formation and movement.

The AG2 TRAMAN, volume 1, covered thunderstorm formation and movement. Therefore, in this chapter, we will discuss, in more detail, the weather phenomena associated with thunderstorms and various methods of forecasting their intensity and movement.

THUNDERSTORM TURBULENCE AND WEATHER

Tlmnderstorms are characterized by turbulence, moderate to extreme updrafts and downdrafts, hail, icing, lightning, precipitation, and, under most severe conditions (in certain areas), tornados.

Downdrafts and updrafts are currents of air that may be continuous over many thousands of feet in the vertical, and horizontally as large as the extent of the thunderstorm. The velocity of the downdrafts and updrafts is relatively constant as contrasted to gusts. Gusts are primarily responsible for the bumpiness (turbulence) normally encountered in cumuliform clouds. A downdraft or updraft maybe compared to a river flowing at a fairly constant rate, whereas a gust is comparable to an eddy or other type of random motion of water in a river. 

Studies of the structure of the thunderstorm cell indicate that during the cumulus stage of development, the updrafts may cover a horizontal area as large as 6 miles. In the cumulus stage, the updraft may extend from below the cloud base to the cloud top, a height greater than 25,000 feet. During the mature stage, the updrafts cease in the lower levels of the cloud, although they continue in the upper levels where cloud tops may exceed 60,000 feet. These drafts are of considerable importance in aviation because of the change in altitude that may occur when an aircraft flies through them. In general, the maximum number of high velocity gusts are found at altitudes of 5,000 to 10,000 feet below the top of the thunderstorm cloud, while the least severe turbulence is encountered near the base of the thunderstorm. The characteristic response of an aircraft intercepting a series of gusts is a number of sharp accelerations or "bumps" without an accompanying change in altitude. The degree of bumpiness or turbulence experienced in flight is related to both the number of such abrupt changes encountered in a given distance and the strength of the individual changes.

Hail is regarded as one of the worst hazards of flying in thunderstorms. It usually occurs during the mature stage of cells that have updrafts of more than average intensity, and is found with the greatest frequency between 10,000 and 15,000 feet. As a general rule, the greater the vertical extent of the thunderstorm, the more likely hail will occur.

Although encounters by aircraft with large hail are not too common, hail can severely damage an aircraft in a very few seconds. The general conclusion regarding hail is that most midlatitude storms contain hail sometime during their cycle. Most hail will occur during the mature stage. In subtropical and tropical thunderstorms, hail seldom reaches the ground. It is generally believed that these thunderstorms contain less hail aloft than do midlatitude storms.

Thunderstorms contain considerable quantities of moisture that may or may not be falling to the ground as rain. These water droplets may be suspended in, or moving with, the updrafts. Rain is encountered below the freezing level in almost all penetrations of fully developed thunderstorms. Above the freezing level, however, there is a sharp decline in the frequency of rain. There seems to be a definite correlation between turbulence and precipitation. The intensity of turbulence, in most cases, varies directly with the intensity of precipitation. This relationship indicates that most rain or snow in thunderstorms is held aloft by updrafts.

Where the air temperatures are at or below freezing, icing should be expected in flights through thunderstorms. In general, icing is associated with temperatures from 0 to 20C. Most severe icing occurs from 0C to 10C. The heaviest icing conditions usually occur in that region above the freezing level where the cloud droplets have not yet turned to ice crystals. When the thunderstorm is in the cumulus stage, severe icing may occur at any point above the freezing level. However, because of the formation of ice crystals at high levels and the removal of liquid water by precipitation, icing conditions are usually somewhat less in the mature and dissipating stage.

THUNDERSTORM ELECTRICITY AND LIGHTNING

The thunderstorm changes the normal electrical field, in which the earth is negative with respect to the air above it, by making the upper portion of the thunderstorm cloud positive and the lower portion negative. This negative charge then induces a positive charge on the ground. The distribution of the electric charges in a typical thunderstorm is shown in figure 5-1. The lightning first occurs between the upper positive charge area and the negative charge area immediately below it. Lightning discharges are considered to occur most frequently in the area roughly bounded by the 0C and the -9C temperature levels. However, this does not mean that all discharges are confined to this region, for as the thunderstorm develops, lightning discharges may occur in other areas, and from cloud to cloud, as well as cloud to ground. Lightning can do considerable damage to aircraft, especially to radio equipment.

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