THUNDERSTORM ELECTRICITY AND LIGHTNING

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. Rain 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. Icing 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 –20°C. Most severe icing occurs from 0°C to –10°C. 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 0°C and the -9°C 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. THUNDERSTORMS IN RELATION TO THE WIND FIELD During all stages of a cell, air is being brought into the cloud through the sides of the cloud. This process is known as entrainment. A cell entrains air at a rate of 100 percent per 500 hPa; that is, it doubles its mass in an ascent of 500 hPa. The factor of entrainment is important in establishing a lapse rate within the cloud that is greater than the moist adiabatic rate, and in maintaining the downdraft. When there is a marked increase with height in the horizontal wind speed, the mature stage of the cell may be prolonged. In addition, the increasing speed of the wind with height produces considerable tilt to the updraft of the cell, and in fact, to the visible cloud itself. Thus, the falling precipitation passes through only a small section of the rising air; it falls thereafter through the relatively still air next to the updraft, perhaps even outside the cell boundary. Therefore, since the drag of the falling water is not imposed on the rising air currents within the thunderstorm cell, the updraft can continue until its source of energy is exhausted. Tilting of the thunderstorm explains why hail is sometimes encountered in a cloudless area just ahead of the storm. RADAR DETECTION Radar, either surface or airborne, is the best aid in detecting thunderstorms and charting their movements. A thunderstorm’s size, direction of movement, shape and height, as well as other significant features, can be determined from a radar presentation. Radar interpretation is mentioned in chapter 12 of this manual, and for a more detailed discussion, refer to the Federal Meteorological Handbook No. 7, Part B, and the Federal Meteorological Handbook No. 11, Part B. THUNDERSTORM FLIGHT HAZARDS Thunderstorms are often accompanied by extreme fluctuations in ceiling and visibility. Every thunderstorm has turbulence, sustained updrafts and downdrafts, precipitation, and lightning. Icing conditions, though quite localized, are quite common in thunderstorms, and many contain hail. The flight 5-2