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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
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lost its moisture and energy in the prefrontal thunderstorms. In the United States, tornadoes are frequently associated with strong prefrontal squall lines. Pre-frontal squall line thunderstorms are indicated on a surface weather map by an alternate dash-dot-dash line (display).
The distribution of stationary front thunderstorms is controlled by the slope of the frontal surface. Steeply sloped stationary fronts tend to have lines of storms, whereas shallow stationary fronts tend to have widely scattered storms.
Occluded front thunderstorms are associated with the two types of occluded fronts (warm front and cold front occlusions), and are usually cold front thunderstorms that have been moved into the area of warm frontal weather by the occlusion process. They are found along the upper front, and are normally strongest for a distance of 50 to 100 miles north of the peak of the warm sector. Air Mass Thunderstorms
The two types of air mass thunderstorms are locally convective and orographic. Both types form within air masses, and are randomly distributed throughout the air mass.
Convective thunderstorms are often caused by solar heating of the land, which provides heat to the air, thereby resulting in thermal convection. Relatively cool air flowing over a warmer water surface may also produce sufficient convection to cause thunderstorms. The land-type convective thunderstorms normally form during the afternoon hours, after the Earth has gained maximum heating from the sun. If cool, moist, conditionally unstable air is passing over this land area, heating from below will cause convective currents, thereby resulting in towering cumulus or thunderstorm activity. Dissipation usually occurs during the early evening hours, as the land begins to lose its heat to the atmosphere. Although convective thunderstorms form as individual cells, they may become so numerous over a particular geographical area that continued flight cannot be maintained.
　
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