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. [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. 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.  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. 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  During the passage of a thunderstorm, rapid and marked  surface variations generally occur. These variations usually  occur in a particular sequence characterized by:  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  4-26 •  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. Figure 4-26.  Dissipating thunderstorm. Frontal Thunderstorms