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, either surface or airborne, is the best aid in detecting thunderstorms and charting their movements. A thunderstorms 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.

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

Figure 5-1.-Location of electric charges inside a typical thunderstorm cell.

conditions listed below are generally representative of many (but not necessarily all) thunderstorms.

. The chance of severe or extreme turbulence within thunderstorms is greatest at higher altitudes, with most cases of severe and extreme turbulence about 8,000 to 15,000 feet above ground level (AGL). The least turbulence may be expected when flying at or just below the base of the main thunderstorm cloud. (The latter rule would not be true over rough terrain or in mountainous areas where strong eddy currents produced by strong surface winds would extend the turbulence up to a higher level.)

. The heaviest turbulence is closely associated with the areas of heaviest rain.

. The strongest updrafts are found at heights of about 10,000 feet AGL or more; in extreme cases, updrafts in excess of 65 feet per second occur.

. Downdrafts are less severe, but downdrafts on the order of 20 feet per second are quite common.

. The probability of lightning strikes occurring is greatest near or slightly above the freezing level. Because of the potential hazards of flying in a thunderstorm, it is obviously nothing short of folly for pilots to attempt to fly in thunderstorms, unless operationally necessary.

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