Some of the weather conditions most adverse to fire control, such as strong, gusty winds, turbulence, and lightning storms, occur in frontal zones. Sometimes there is insufficient moisture in the warm air mass, or inadequate lifting of this mass, so that no precipitation occurs with the front. Strong, gusty, and shifting winds are typical of a dry frontal zone, adding greatly to the difficulty of fire control.

Types of fronts are distinguished by the way they move relative to the air masses involved. If a front is moving so that cold air is replacing warm air, it is a cold front. If the warm air is advancing and replacing cold air ahead, the front is a warm front. If a front is not moving, it is a stationary front. Cold fronts are indicated on weather maps by pointed cusps, and warm fronts by semicircles, on the side toward which they are moving. A stationary front is indicated by a combination of both.

In a frontal zone, the warmer air mass, being lighter, will be forced over the colder air mass. The rotation of the earth deflects the movement of both the cold and the warm air masses as one tries to overrun or underride the other, and prevents the formation of a horizontal discontinuity surface. Instead, the frontal surface slopes up over the colder air. The slope varies from about 1/50 to 1/300. A 1/50 slope means that for every 50 miles horizontally, the front is 1 mile higher in the vertical. The amount of slope is dependent upon the temperature contrast between the two air masses, the difference in wind speed across the front, and the relative movements of the air masses involved; that is, whether cold air is replacing warm air at the surface or warm air is replacing cold air. On a surface weather map, only the intersection of the frontal surface with the earth is indicated. The contrast between the air masses is strongest near the earths surface, and decreases upward in the atmosphere.

The central portions of air masses are usually associated with areas of high pressure, but fronts are formed in troughs of low pressure. From a position on a front, we find that the pressure rises both toward the warmer air and toward the colder air. Because the gradient wind in the Northern Hemisphere always blows with high pressure on the right, as one faces downstream, this means that the wind blows in one direction in the cold air and a different direction in the warm air. At a given location the wind shifts in a clockwise direction as a front passes--for example, from southeast to southwest or from southwest to northwest.

The wind-shift line and pressure trough line provide good clues to the weatherman for the location of fronts, but there are other indications to consider. A temperature discontinuity exists across a front. As a rule, the greater and more abrupt the temperature contrast, the more intense the front. Weak fronts are characterized by gradual and minor changes in temperature. The moisture contrast between air masses on different sides of a front may be indicated by the dew-point temperatures. Usually the cold air mass will be drier than the warm air mass. Other indications of front location are cloud types, pressure changes, and visibility changes.

The leading edge of an advancing cold air mass is a cold front. It forms a wedge, which pushes under a warm air mass forcing the warm air to rise. Because of surface friction, the lowest layers of the cold air are slowed down. This increases the steepness of the frontal surface and causes a cold front to have a blunted appearance when viewed in cross-section. The slopes of cold fronts usually vary from 1/50 to 1/150.

There are wide variations in the orientation and speed of cold fronts. Usually, they are oriented in a northeast-southwest direction, and they move to the east and southeast, at speeds varying from about 10 to 40 m.p.h. and faster in the winter.

As a cold front approaches, the southerly winds increase in the warm air ahead of the front. Clouds appear in the direction from which the front is approaching. The barometric pressure usually falls, reaches its lowest point as the front passes, then rises sharply. Winds become strong and gusty and shift sharply to westerly or northwesterly as the cold front passes. Temperature and dew point are lower after the cold front passes. In frontal zones with precipitation, the heaviest precipitation usually occurs with the passage of the front. Then it may end quickly and be followed by clearing weather.

There are many exceptions to the foregoing general pattern of cold-front passages. The severity of the weather associated with cold fronts depends upon the moisture and stability of the warm air, the steepness of the front, and the speed of the front. Since cold fronts are usually steeper and move faster than warm fronts, the accompanying band of weather is narrower, more severe, and usually of shorter duration than with warm fronts.

With slow-moving cold fronts and stable warm air, rain clouds of the stratus type form in a wide band over the frontal surface and extend for some distance behind the front. If the warm air is moist and conditionally unstable, thunderstorms may form, with the heaviest rainfall near the frontal zone and immediately following. If the warm air is fairly dry and the temperature contrast across the front is small, there may be little or no precipitation and few or no clouds.

With rapidly moving cold fronts, the weather is more severe and occupies a narrower band. The disturbance is also of shorter duration than that caused by a slow-moving front. If the warm air is relatively stable, overcast skies and precipitation may occur for some distance ahead of the front, and the heaviest precipitation may occur ahead of the surface cold front. If the warm air is moist and conditionally unstable, scattered showers and thunderstorms form just ahead of the cold front. The weather usually clears rapidly behind a fast-moving cold front, with colder temperatures and gusty, turbulent surface winds following the frontal passage.

Under some conditions, a line of showers and thunderstorms is formed from 50 to 300 miles ahead of, and roughly parallel to, a cold front. This is called a squall line. The weather associated with squall lines is often more severe than that associated with the subsequent cold front. After the passage of the squall line, the temperature, wind, and pressure usually revert to conditions similar to those present before the squall line approached. Occasionally, the showers and thunderstorms are scattered along the squall line so that some areas experience strong, gusty winds without any precipitation.

Dry cold fronts often cause very severe fire weather in many sections. Dry cold-front passages may occur in any region, but they are a major problem in the Southeast. Cold fronts tend to be drier farther away from the low-pressure center with which they are associated. Thus, a cold front associated with a Low passing eastward across Southern Canada or the Northern States may be very dry as it passes through the Southeast. In addition, the polar air mass following the cold front may become quite unstable because of surface heating by the time it reaches the Southeast.

The combination of strong, gusty winds and dry, unstable air creates serious fire weather. The second of two cold fronts passing through the Southeast in rapid succession also tends to be dry. The warm air mass ahead of the first cold front may be moist and produce precipitation, but the air mass between the first and second fronts usually will not have had time to acquire much moisture. Therefore, the second cold-front passage may be dry and will be the more serious from the fire-control standpoint.

The dry, trailing ends of cold fronts cause serious fire weather wherever they occur. Along the Pacific coast, the winds behind such cold fronts are, at times, from a northeasterly direction. This offshore direction means that the air flows from high elevations to low elevations and has foehn characteristics. The strong, shifting, gusty winds of the cold-front passage combine with the dry foehn wind to the rear of the front to produce a short-lived but extremely critical fire-weather condition.

The leading edge of an advancing warm air mass is called a warm front. The warm air is overtaking and replacing the cold air, but at the same time sliding up over the wedge of cold air. Warm fronts are flatter than cold fronts, having slopes ranging from 1/100 to 1/300. Because of this flatness, cloudiness and precipitation extend over a broad area ahead of the front, providing, of course, that there is sufficient moisture in the warm air.

Warm fronts are less distinct than cold fronts and more difficult to locate on weather maps. This is particularly true in rough terrain where high-elevation areas may extend up into the warm air before the warm front has been felt at lower elevation stations.

The first indication of the approach of warm, moist air in the upper levels ahead of the surface warm front may be very high, thin, cirrostratus clouds, which give the sky a milky appearance. These are followed by middle-level clouds, which darken and thicken as precipitation begins. This sequence may be interrupted by short clearing periods, but the appearance of successively lower cloud types indicates the steady approach of the warm front. Rains may precede the arrival of the surface warm front by as much as 300 miles. Rain falling through the cold air raises the humidity to the saturation level and causes the formation of low stratus clouds.

If the warm air above the warm front is moist and stable, the clouds that form are of the stratus type. The sequence is cirrus, cirrostratus, altostratus, and nimbostratus. Precipitation is a steady type and increases gradually with the approach of the surface front.

If the warm air is moist and conditionally unstable, altocumulus and cumulonimbus clouds and, frequently, thunderstorms will be embedded in the cloud masses that normally accompany a warm front.

The rate of movement of warm fronts is about half that of cold fronts. Winds are usually not as strong or gusty with the approach of fronts as with cold fronts. The shift in wind is generally from an easterly to a southerly direction as a warm front passes. After it passes, temperatures rise, precipitation usually stops, and clouds diminish or vanish completely.

From the standpoint of fire weather, warm fronts associated with moist air are a real benefit. The accompanying precipitation is widespread and long-lasting, and usually is sufficient to thoroughly moisten forest fuels, reducing the fire danger.

When the forces acting on two adjacent air masses are such that the frontal zone shows little movement, the front is called a stationary front. Surface winds on either side of the front tend to blow parallel to the front, but in opposite directions. Weather conditions occurring with a stationary front are variable; usually they are similar to those found with a warm front, though less intense. If the air is dry, there may be little cloudiness or precipitation. If the air is moist, there may be continuous precipitation with stable, warm air, or showers and thunderstorms with conditionally unstable, warm air. The precipitation area is likely to be broader than that associated with a cold front, but not as extensive as with a warm front.

Stationary fronts may quickly change back to moving fronts as a slight imbalance of forces acting on the air masses develops. A stationary front may oscillate back and forth, causing changing winds and weather conditions at a given location. It may become a cold or warm front, or a frontal wave may develop.

A frontal surface is similar to a water surface. A disturbance such as wind can cause the formation of waves on the water. If the wave moves toward the shoreline, it grows until it becomes top-heavy and breaks. Similarly, along frontal surfaces in the atmosphere a disturbance may form a wave. This disturbance may be a topographic irregularity, the influence of an upper-level trough, or a change in the wind field cause by local convection. Waves usually form on stationary fronts or slow-moving cold fronts, where winds on the two sides of the front are blowing parallel to the front with a strong shearing motion.

When a section of a front is disturbed, the warm air begins to flow up over and displace some of the cold air. Cold air to the rear of the disturbance displaces some of the warm air. Thus, one section of the front begins to act like a warm front, and the adjacent section like a cold front. This deformation is called a frontal wave.

The pressure at the peak of the frontal wave falls, and a low-pressure center with a counterclockwise (cyclonic) circulation is formed. If the pressure continues to fall, the wave may develop into a major cyclonic system. The Low and its frontal wave generally move in the direction of the wind flow in the warm air, which is usually toward the east or northeast.

As the system moves, the cold front moves faster than the warm front and eventually overtakes the warm front. The warm air is forced aloft between the cold air behind the cold front and the retreating cold air ahead of the warm front. The resulting combined front is called an occlusion or occluded front. This is the time of maximum intensity of the wave cyclone. The pressure becomes quite low in the occluded system with strong winds around the Low. Usually the system is accompanied by widespread cloudiness and precipitation. The heaviest precipitation occurs to the north of the low-pressure center.

As the occlusion continues to grow in length, the cyclonic circulation diminishes in intensity, the low-pressure center begins to fill, and the frontal movement slows down.

There are two types of occluded fronts a warm-front type and a cold-front type depending on whether the surface air ahead of the occlusion is warmer or colder than the air to the rear.

The cold-front type is predominant over most of the continent, especially the central and eastern regions. The weather and winds with the passage of a cold-front occlusion are similar to those with a cold front. Ahead of the occlusion, the weather and cloud sequence is much like that associated with warm fronts.

Most warm-front occlusions are found along the west coast. The air mass to the rear is warmer than the air mass ahead. Therefore, when the cold front overtakes the warm front, it rides up the warm-front surface and becomes an upper cold front.

The weather associated with a warm-front occlusion has characteristics of both warm-front and cold-front weather. The sequence of clouds and weather ahead of the occlusion is similar to that of a warm front. Cold-front weather occurs near the upper cold front. With moist and conditionally unstable air, thunderstorms may occur. At the surface, the passage of a warm-front occlusion is much like that of a warm front. The rainy season in the Pacific Northwest, British Columbia, and southeastern Alaska is dominated by a succession of warm front occlusions that move in from the Pacific.

Another type of upper cold front should be mentioned. Cold fronts approaching the Rocky Mountains from the west are forced to rise and cross over the mountains. Quite frequently in winter, a very cold air mass is located east of the mountains. Then, the cold front does not return to the surface, but rides aloft over the cold air as an upper cold front often accompanied by thundershowers. When such a front meets an mT air mass, and underrides it, a very unstable condition is produced that will result in numerous thunderstorms and, occasionally, tornadoes.