Aviation Weather

full text of the classic FAA guide

FRONTS

As air masses move out of their source regions, they come in contact with other air masses of different properties. The zone between two different air masses is a frontal zone or front. Across this zone, temperature, humidity and wind often change rapidly over short distances.

DISCONTINUITIES

When you pass through a front, the change from the properties of one air mass to those of the other is sometimes quite abrupt. Abrupt changes indicate a narrow frontal zone. At other times, the change of properties is very gradual indicating a broad and diffuse frontal zone.

Temperature

Temperature is one of the most easily recognized discontinuities across a front. At the surface, the passage of a front usually causes noticeable temperature change. When flying through a front, you note a significant change in temperature, especially at low altitudes. Remember that the temperature change, even when gradual, is faster and more pronounced than a change during a flight wholly within one air mass. Thus, for safety, obtain a new altimeter setting after flying through a front. Chapter 3 discussed the effect of a temperature change on the aircraft altimeter.

Dew Point

As you learned in Chapter 5, dew point temperature is a measure of the amount of water vapor in the air. Temperature—dew point spread is a measure of the degree of saturation. Dew point and temperature—dew point spread usually differ across a front. The difference helps identify the front and may give a clue to differences of cloudiness and/or fog.

Wind

Wind always changes across a front. Wind discontinuity may be in direction, in speed, or in both. Be alert for a wind shift when flying in the vicinity of a frontal surface; if the wind shift catches you unaware it can get you off course or even lost in a short time. The relatively sudden change in wind also creates wind shear, and you will study its significance in the next chapter, “Turbulence.”

Pressure

A front lies in a pressure trough, and pressure generally is higher in the cold air. Thus, when you cross a front directly into colder air, pressure usually rises abruptly. When you approach a front toward warm air, pressure generally falls until you cross the front and then remains steady or falls slightly in the warm air. However, pressure patterns vary widely across fronts, and your course may not be directly across a front. The important thing to remember is that when crossing a front, you will encounter a difference in the rate of pressure change; be especially alert in keeping your altimeter setting current.

TYPES OF FRONTS

The three principal types of fronts are the cold front, the warm front, and the stationary front.

Cold Front

The leading edge of an advancing cold air mass is a cold front. At the surface, cold air is overtaking and replacing warmer air. Cold fronts move at about the speed of the wind component perpendicular to the front just above the frictional layer. Figure 59 shows the vertical cross section of a cold front and the symbol depicting it on a surface weather chart. A shallow cold air mass or a slow moving cold front may have a frontal slope more like a warm front shown in figure 60.

FIGURE 59. Cross section of a cold front (above) with the weather map symbol (below). The symbol is a line with pointed barbs pointing in the direction of movement. If a map is in color, a blue line represents the cold front. The vertical scale is expanded in the top illustration to show the frontal slope. The frontal slope is steep near the leading edge as cold air replaces warm air. The solid heavy arrow shows movement of the front. Warm air may descend over the front as indicated by the dashed arrows; but more commonly, the cold air forces warm air upward over the frontal surface as shown by the solid arrows.

FIGURE 60. Cross section of a warm front (top) with the weather map symbol (bottom). The symbol is a line with rounded barbs pointing in the direction of movement. If a map is in color, a red line represents the warm front. Slope of a warm front generally is more shallow than slope of a cold front. Movement of a warm front shown by the heavy black arrow is slower than the wind in the warm air represented by the light solid arrows. The warm air gradually erodes the cold air.

Warm Front

The edge of an advancing warm air mass is a warm front—warmer air is overtaking and replacing colder air. Since the cold air is denser than the warm air, the cold air hugs the ground. The warm air slides up and over the cold air and lacks direct push on the cold air. Thus, the cold air is slow to retreat in advance of the warm air. This slowness of the cold air to retreat produces a frontal slope that is more gradual than the cold frontal slope as shown in figure 60. Consequently, warm fronts on the surface are seldom as well marked as cold fronts, and they usually move about half as fast when the general wind flow is the same in each case.

Stationary Fronts

When neither air mass is replacing the other, the front is stationary. Figure 61 shows a cross section of a stationary front and its symbol on a surface chart. The opposing forces exerted by adjacent air masses of different densities are such that the frontal surface between them shows little or no movement. In such cases, the surface winds tend to blow parallel to the frontal zone. Slope of a stationary front is normally shallow, although it may be steep depending on wind distribution and density difference.

FIGURE 61. Cross section of a stationary front (top) and its weather map symbol (bottom). The symbol is a line with alternating pointed and rounded barbs on opposite sides of the line, the pointed barbs pointing away from the cold air and the rounded barbs away from the warm air. If a map is in color, the symbol is a line of alternating red and blue segments. The front has little or no movement and winds are nearly parallel to the front. The symbol in the warm air is the tail of a wind arrow into the page. The symbol in the cold air is the point of a wind arrow out of the page. Slope of the front may vary considerably depending on wind and density differences across the front.

FRONTAL WAVES AND OCCLUSION

Frontal waves and cyclones (areas of low pressure) usually form on slow-moving cold fronts or on stationary fronts. The life cycle and movement of a cyclone is dictated to a great extent by the upper wind flow.

In the initial condition of frontal wave development in figure 62, the winds on both sides of the front are blowing parallel to the front (A). Small disturbances then may start a wavelike bend in the front (B).

FIGURE 62. The life cycle of a frontal wave.

If this tendency persists and the wave increases in size, a cyclonic (counterclockwise) circulation develops. One section of the front begins to move as a warm front, while the section next to it begins to move as a cold front (C). This deformation is a frontal wave.

The pressure at the peak of the frontal wave falls, and a low-pressure center forms. The cyclonic circulation becomes stronger, and the surface winds are now strong enough to move the fronts; the cold front moves faster than the warm front (D). When the cold front catches up with the warm front, the two of them occlude (close together). The result is an occluded front or, for brevity, an occlusion (E). This is the time of maximum intensity for the wave cyclone. Note that the symbol depicting the occlusion is a combination of the symbols for the warm and cold fronts.

As the occlusion continues to grow in length, the cyclonic circulation diminishes in intensity and the frontal movement slows down (F). Sometimes a new frontal wave begins to form on the long westward-trailing portion of the cold front (F,G), or a secondary low pressure system forms at the apex where the cold front and warm front come together to form the occlusion. In the final stage, the two fronts may have become a single stationary front again. The low center with its remnant of the occlusion is disappearing (G).

Figure 63 indicates a warm-front occlusion in vertical cross section. This type of occlusion occurs when the air is colder in advance of the warm front than behind the cold front, lifting the cold front aloft.

FIGURE 63. Cross section of a warm-front occlusion (top) and its weather map symbol (bottom). The symbol is a line with alternating pointed and rounded barbs on the same side of the line pointing in the direction of movement. Shown in color on a weather map, the line is purple. In the warm front occlusion, air under the cold front is not as cold as air ahead of the warm front; and when the cold front overtakes the warm front, the less cold air rides over the colder air. In a warm front occlusion, cool air replaces cold air at the surface.

Figure 64 indicates a cold-front occlusion in vertical cross section. This type of occlusion occurs when the air behind the cold front is colder than the air in advance of the warm front, lifting the warm front aloft.

FIGURE 64. Cross section of a cold-front occlusion. Its weather map symbol is the same as for a warm-front occlusion shown in Figure 63. In the cold-front occlusion, the coldest air is under the cold front. When it overtakes the warm front, it lifts the warm front aloft; and cold air replaces cool air at the surface.

NON-FRONTAL LOWS

Since fronts are boundaries between air masses of different properties, fronts are not associated with lows lying solely in a homogeneous air mass. Nonfrontal lows are infrequent east of the Rocky Mountains in midlatitudes but do occur occasionally during the warmer months. Small nonfrontal lows over the western mountains are common as is the semistationary thermal low in extreme Southwestern United States. Tropical lows are also nonfrontal.

FRONTOLYSIS AND FRONTOGENESIS

As adjacent air masses modify and as temperature and pressure differences equalize across a front, the front dissipates. This process, frontolysis, is illustrated in figure 65. Frontogenesis is the generation of a front. It occurs when a relatively sharp zone of transition develops over an area between two air masses which have densities gradually becoming more and more in contrast with each other. The necessary wind flow pattern develops at the same time. Figure 66 shows an example of frontogenesis with the symbol.

FIGURE 65. Frontolysis of a stationary front.

FIGURE 66. Frontogenesis of a stationary front.

FRONTAL WEATHER

In fronts, flying weather varies from virtually clear skies to extreme hazards including hail, turbulence, icing, low clouds, and poor visibility. Weather occurring with a front depends on (1) the amount of moisture available, (2) the degree of stability of the air that is forced upward, (3) the slope of the front, (4) the speed of frontal movement, and (5) the upper wind flow.

Sufficient moisture must be available for clouds to form, or there will be no clouds. As an inactive front comes into an area of moisture, clouds and precipitation may develop rapidly. A good example of this is a cold front moving eastward from the dry slopes of the Rocky Mountains into a tongue of moist air from the Gulf of Mexico over the Plains States. Thunderstorms may build rapidly and catch a pilot unaware.

The degree of stability of the lifted air determines whether cloudiness will be predominately stratiform or cumuliform. If the warm air overriding the front is stable, stratiform clouds develop. If the warm air is unstable, cumuliform clouds develop. Precipitation from stratiform clouds is usually steady as illustrated in figure 67 and there is little or no turbulence. Precipitation from cumuliform clouds is of a shower type as in figure 68, and the clouds are turbulent.

FIGURE 67. A cold front underrunning warm, moist, stable air. Clouds are stratified and precipitation continuous. Precipitation induces stratus in the cold air.

FIGURE 68. A cold front underrunning warm, moist, unstable air. Clouds are cumuliform with possible showers or thunderstorms near the surface position of the front. Convective clouds often develop in the warm air ahead of the front. The warm, wet ground behind the front generates low-level convection and fair weather cumulus in the cold air.

Shallow frontal surfaces tend to give extensive cloudiness with large precipitation areas (figure 69). Widespread precipitation associated with a gradual sloping front often causes low stratus and fog. In this case, the rain raises the humidity of the cold air to saturation. This and related effects may produce low ceiling and poor visibility over thousands of square miles. If temperature of the cold air near the surface is below freezing but the warmer air aloft is above freezing, precipitation falls as freezing rain or ice pellets; however, if temperature of the warmer air aloft is well below freezing, precipitation forms as snow.

FIGURE 69. A warm front with overrunning moist, stable air. Clouds are stratiform and widespread over the shallow front. Precipitation is continuous and induces widespread stratus in the cold air.

When the warm air overriding a shallow front is moist and unstable, the usual widespread cloud mass forms; but embedded in the cloud mass are altocumulus, cumulus, and even thunderstorms as in figures 70 and 71. These embedded storms are more common with warm and stationary fronts but may occur with a slow moving, shallow cold front. A good preflight briefing helps you to foresee the presence of these hidden thunderstorms. Radar also helps in this situation and is discussed in chapter 11.

FIGURE 70. A slow-moving cold front underrunning warm, moist, unstable air. Note that the front is more shallow than the fast-moving front shown in figure 68. Clouds are stratified with embedded cumulonimbus and thunderstorms. This type of frontal weather is especially hazardous since the individual thunderstorms are hidden and cannot be avoided unless the aircraft is equipped with airborne radar.

FIGURE 71. A warm front with overrunning warm, moist, unstable air. Weather, clouds, and hazards are similar to those described in figure 70 except that they generally are more widespread.

A fast moving, steep cold front forces upward motion of the warm air along its leading edge. If the warm air is moist, precipitation occurs immediately along the surface position of the front as shown in figure 72.

FIGURE 72. A fast moving cold front underrunning warm, moist, unstable air. Showers and thunderstorms develop along the surface position of the front.

Since an occluded front develops when a cold front overtakes a warm front, weather with an occluded front is a combination of both warm and cold frontal weather. Figures 73 and 74 show warm and cold occlusions and associated weather.

FIGURE 73. A warm front occlusion lifting warm, moist, unstable air. Note that the associated weather is complex and encompasses all types of weather associated with both the warm and cold fronts when air is moist and unstable.

FIGURE 74. A cold front occlusion lifting warm, moist, stable air. Associated weather encompasses types of weather associated with both warm and cold fronts when air is moist and stable.

A front may have little or no cloudiness associated with it. Dry fronts occur when the warm air aloft is flowing down the frontal slope or the air is so dry that any cloudiness that occurs is at high levels.

The upper wind flow dictates to a great extent the amount of cloudiness and rain accompanying a frontal system as well as movement of the front itself. Remember in chapter 4 we said that systems tend to move with the upper winds. When winds aloft blow across a front, it tends to move with the wind. When winds aloft parallel a front, the front moves slowly if at all. A deep, slow moving trough aloft forms extensive cloudiness and precipitation, while a rapid moving minor trough more often restricts weather to a rather narrow band. However, the latter often breeds severe, fast moving, turbulent spring weather.

INSTABILITY LINE

An instability line is a narrow, nonfrontal line or band of convective activity. If the activity is fully developed thunderstorms, figure 75, the line is a squall line (chapter 11, “Thunderstorms”). Instability lines form in moist unstable air. An instability line may develop far from any front. More often, it develops ahead of a cold front, and sometimes a series of these lines move out ahead of the front. A favored location for instability lines which frequently erupt into severe thunderstorms is a dew point front or dry line.

DEW POINT FRONT OR DRY LINE

During a considerable part of the year, dew point fronts are common in Western Texas and New Mexico northward over the Plains States. Moist air flowing north from the Gulf of Mexico abuts the dryer and therefore slightly denser air flowing from the southwest. Except for moisture differences, there is seldom any significant air mass contrast across this “Front”; and therefore, it is commonly called a “dry line.” Nighttime and early morning fog and low-level clouds often prevail on the moist side of the line while generally clear skies mark the dry side. In spring and early summer over Texas, Oklahoma, and Kansas, and for some distance eastward, the dry line is a favored spawning area for squall lines and tornadoes.

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