AIR MASSES AND FRONTS

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approximately 1:100 miles. Near the ground, the slope is often much steeper because of surface friction. Figure 4-31 illustrates the typical characteristics in the vertical structure of a slow-moving cold front. The lower half shows the typical upper airflow behind the front, and the upper half shows the accompanying surface weather. This is only one typical case. Many variations to this model can and do occur in nature. The slow-moving cold front is an active front because it has widespread frontal cloudiness and precipitation at and behind the front caused by actual frontal lifting. Surface Characteristics The pressure tendency associated with this type of frontal passage is indicated by either an unsteady or ALTITUDE FT 15,000 10,000 5,000 WIND FRONTAL INVERSION O 0 C WIND TEMP. CURVE SUBSIDENCE INVERSION AS COLD AIR 4-32 steady fall prior to frontal passage and then weak rises behind. Temperature and dew point drop sharply with the passage of a slow-moving cold front. The wind veers with the cold frontal passage and reaches its highest speed at the time of frontal passage. Isobars are usually curved anticyclonically in the cold air. This type of front usually moves at an average speed between 10 and 15 knots. Slow-moving cold fronts move with 100% of the wind component normal to the front. Weather The type of weather experienced with a slow-moving cold front is dependent upon the stability of the warm air mass. When the warm air mass is stable, POSSIBLE CUMULONIMBUS NIMBOSTRATUS ST ST POINT A CIRRO- STRATUS ALTOCUMULUS O 0 C PRECIP. AREA CIRRUS 200 150 100 50 0 50 100 MILES UPPER AIR TROUGH UPPER WIND FLOW AREA OF CLOUDY SKIES AND RAIN POINT A (SURFACE WIND FLOW) AG5f0431 Figure 4-31.—Typical vertical structure of a slow-moving cold front with upper windflow in back of the front.
a rather broad zone of altostratus and nimbostratus clouds accompany the front and extend several hundred miles behind the front. If the warm air is unstable (or conditionally unstable), thunderstorms and cumulonimbus clouds may develop within the cloud bank and may stretch for some 50 miles behind the surface front. These cumulonimbus clouds form within the warm air mass. In the cold air there may be some stratus or nimbostratus clouds formed by the evaporation of falling rain; but, generally, outside of the rain areas, there are relatively few low clouds. This is because of the descending motion of the cold air that sometimes produces a subsidence inversion some distance behind the front. The ceiling is generally low with the frontal passage, and gradual lifting is observed after passage. Visibility is poor in precipitation and may continue to be reduced for many hours after frontal passage as long as the precipitation occurs. When the cold air behind the front is moist and stable, a deck of stratus clouds and/or fog may persist for a number of hours after frontal passage. The type of precipitation observed is also dependent upon the stability and moisture conditions of the air masses. Upper Air Characteristics Upper air contours show a cyclonic flow and are usually parallel to the front as are the isotherms. The weather usually extends as far in back of the front as these features are parallel to it. When the orientation changes, this usually indicates the position of the associated upper air trough. (A trough is an elongated area of relatively low pressure.) The temperature inversion on this type of front is usually well marked. In the precipitation area the relative humidity is high in both air masses. Farther behind the front, subsidence may occur, giving a second inversion closer to the ground. The wind usually backs rapidly with height (on the order of some 60 to 70 degrees between 950 and 400 mb), and at 500 mb the wind direction is inclined at about 15 degrees to the front. The wind component normal to the front decreases slightly with height, and the component parallel to the front increases rapidly. On upper air charts, slow-moving cold fronts are characterized by a packing (concentration) of isotherms behind them. The more closely packed the isotherms and the more nearly they parallel the fronts, the stronger the front. 4-33 FAST-MOVING COLD FRONTS (INACTIVE COLD FRONT) The fast-moving cold front is a very steep front that has warm air near the surface being forced vigorously upward. At high levels, the warm air is descending downward along the frontal surface. This front has a slope of 1:40 to 1:80 miles and usually moves rapidly; 25 to 30 knots may be considered an average speed of movement. They move with 80 to 90 percent of the wind component normal to the front. As a result of these factors, there is a relatively narrow but often violent band of weather. Figure 4-32 shows a vertical cross section of a fast-moving cold front with resultant weather. Also indicated in the lower half of the diagram is the surface weather in advance of the front and the upper airflow above the front. If the warm air is moist and unstable, a line of thunderstorms frequently develops along this front. Sometimes, under these conditions, a line of strong convective activity is projected 50 to 200 miles ahead of the front and parallel to it. This may develop into a line of thunderstorms called a squall line. On the other hand, when the warm air is stable, an overcast layer of altostratus clouds and rain may extend over a large area ahead of the front. If the warm air is very dry, little or no cloudiness is associated with the front. The front depicted is a typical front with typical characteristics. The fast-moving cold front is considered an inactive front because lifting occurs only at and ahead of the front. The lifting is caused by descending air ahead of the front and only in part by the frontal surface. Surface Characteristics Pressure tendencies fall ahead of the front with sudden and strong rises after frontal passage. If a squall line lies some distance ahead of the front, there may be a strong rise associated with its passage and a shift in the wind. However, after the influence of the squall line has passed, winds back to southerly and pressures level off. The temperature falls in the warm air just ahead of the front. This is caused by the evaporation of falling precipitation. Rapid clearing and adiabatic warming just behind the front tend to keep the cold air temperature near that of the warm air. An abrupt temperature change usually occurs far behind the front near the center of the high-pressure center associated with the cold air mass. The dew point and wind
Page 1 and 2: Temperature, in the form of heating
Page 3 and 4: Characteristics of Air Masses The c
Page 5 and 6: WARM WARM AIR MASS MODIFICATION AIR
Page 7 and 8: THE PROCESS HOW IT HAPPENS RESULTS
Page 9 and 10: On the leeward side of the Great La
Page 11 and 12: 140 O 30 150 O mP 140 O mT 40 140 O
Page 13 and 14: FOG OR LOW STRATUS WARM OCEAN COLD
Page 15 and 16: 40 O 90 O 90 O Maritime Tropical (m
Page 17 and 18: 4-17 A B WINTER AIR MASSES CLOUDS C
Page 19 and 20: Maritime Tropical (mT) Air in Winte
Page 21 and 22: toward a warmer air mass. The coole
Page 23 and 24: Unstable Waves The unstable wave is
Page 25 and 26: temperatures and warm tropical air
Page 27 and 28: 75 60 45 30 15 0 15 30 45 60 180 15
Page 29 and 30: COLD FRONT COLD AIR From this it ca
Page 31: frontal intensity, is defined as th
Page 35 and 36: miles ahead of the front. These clo
Page 37 and 38: AG5f0435 Figure 4-35.—Typical iso
Page 39 and 40: Clearing usually occurs after the p
Page 41 and 42: eporting surface-pressure rises is
Page 43 and 44: A COOL AIR FLOW STRATOCUMULUS DISTA
Page 45 and 46: through it from the warm air mass a
Page 47 and 48: flow down the leeward side of the r
Page 49 and 50: When a cold front associated with a

AIR MASSES AND FRONTS

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