An analyzed Skew T, Log P Diagram can be used to determine if a front has passed a station, the strength of a front, the height of a front above a station, and frontal slope.

To determine whether a front is above a station, you must examine the temperature and dew point curves. The temperature lapse rate undergoes a change through the frontal zone. It may decrease at a slower rate, becoming more isothermal, or it may increase through the zone, producing an inversion. Figure 6-2-25 illustrates

lapse rate changes through frontal zones. Do not look for these changes above 400 millibars; more often than not, they are found below 500 millibars.

When there is a significant temperature contrast across a front, the front is classified as strong. Such fronts are marked by an inversion through the frontal zone. Cold fronts usually show a marked temperature inversion. When the temperature contrast across a front is small, it is classified as weak. Such fronts are marked by a temperature lapse rate that is slightly less steep through the frontal zone.

Cold fronts generally show a stronger inver-sion than warm fronts, and the inversion appears at successively higher levels as the front moves past a station. The reverse is true of warm fronts. Occluded fronts generally show a double inver-sion when they first form. Later, the temperature contrast across the occlusion lessens and the inversions are wiped out or they fuse.

When a front is accompanied by abundant cloudiness and precipitation, look for an increase in the dew point through the frontal zone (a dew point inversion). See figure 6-2-26,

When strong fronts are accompanied by little or no precipitation, it is usually due to subsidence occurring in the warm air. Subsidence (sinking air) causes warming and thereby strengthens inver-sions.

For weather activity to increase at a front, there must be a net upward motion of the warm air mass. Rising air currents bring about cooling,

condensation, and saturation. This results in clouds and eventually precipitation. Another determination that can be made using the data on a Skew T is the slope of a front. The slope can be determined when you know the distance to the surface front and the height of the front above the station of interest. The height of a front above a station is determined using the pressure-altitude curve. Normally, the height of the edge of the frontal zone adjacent to the warm air mass is determined. This is the FRONTAL SURFACE. For example, if a fast-moving cold

Figure 6-2-27.-Determination of frontal slope.

fronts surface position is 80 miles east of your station and if the height of the frontal surface, as determined from the Skew T, is 9,000 feet above your station, you can determine the slope of the front using simple mathematics. First, convert the height of the front above the surface (9,000 feet) into miles by dividing 9,000 feet by 5,280 feet (the equivalent of 1 mile). You should get 1.7 miles (rounded off). This is the height of the frontal surface 80 miles to the rear of the surface position. From the surface position, you now know that the front rises at the rate of 1.7 miles for each 80 miles of horizontal distance, or as expressed in ratio form, 1.7:80. The horizontal distance to the point where the frontal surface is 1 mile above the surface is 47 miles. This is determined using the formula shown in figure 6-2-27.