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[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. 

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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. 

During the passage of a thunderstorm, rapid and marked surface variations generally occur. These variations usually occur in a particular sequence characterized by: 

• An abrupt fall in pressure as the storm approaches. 

• An abrupt rise in pressure associated with rain showers as the storm moves on and the rain ceases. 

All thunderstorms are similar in physical makeup, but for purposes of identification they are divided into two general groups: frontal and air mass. This division gives the balloon 

pilot an indication of the method by which the storms are formed and the distribution of the clouds over the area. The specific nomenclature of these thunderstorms depends upon the manner in which the lifting action occurs. 

## Frontal Thunderstorms 

Thunderstorms may occur within the cloud system of any front: warm, cold, stationary, or occluded. Frontal thunderstorms are caused by the lifting of warm, moist, conditionally unstable air over a frontal surface. Thunderstorms may also occur many miles ahead of a rapidly moving cold front, and are called prefrontal or squall line thunderstorms. 

Warm front thunderstorms are caused when warm, moist, conditionally unstable air is forced aloft over a colder, denser shelf of retreating air. Because the frontal surface is shallow, the air is lifted gradually. The lifting condensation level is normally reached long before the level of free convection, thus producing stratiform clouds. The level of free convection is normally reached in isolated areas along the frontal surface. This is the area where the greatest amount of water vapor is present in the warm air being lifted. Therefore, warm front thunderstorms are generally scattered. When the level of free convection is reached, warm front thunderstorms may form. These thunderstorms may prove particularly hazardous, as they are frequently obscured by the surrounding stratiform clouds. 

Cold front thunderstorms are caused by the forward motion of a wedge of cold air under a mass of warm, moist, conditionally unstable air (cold front), increasing the possibility for thunderstorms to develop. The slope of a typical cold frontal surface is relatively steep, so the lifting condensation level and the level of free convection are usually near the same altitude. Cold front thunderstorms are typically positioned along the frontal surface in what appears to be a continuous line. These storms are easily recognized from the air, because they are partly visible from the front and rear of the storm line. However, if the slope of the frontal surface is shallow, the lifting action is not sufficient to produce thunderstorms in lines (line squalls). With a shallow front, the thunderstorms form behind the surface front and are widely scattered. Such storms may be concealed by the surrounding cloud layers. 

Lines of thunderstorms frequently develop ahead of rapidly moving cold fronts. These are known as prefrontal squall lines, and frequently form parallel to the cold front. Prefrontal squall line thunderstorms are usually more violent than cold front thunderstorms. They are most active between noon and midnight. The cold front cloud system usually weakens during the period of the greatest prefrontal squall line activity because the warm air displaced by the frontal surface has 

