INSTABILITY INDICATIONS FROM THE WET-BULB CURVE

A layer of the atmosphere is potentially unstable if the potential wet-bulb temperature decreases with altitude. Potential instability refers to a layer that is lifted as a whole. The wet-bulb temperature may be found by lifting each individual point on the sounding dry adiabatically to saturation, and then back to its original level moist adiabatically, By connecting the points on a sounding, a wet-bulb curve can be constructed.

If the wet-bulb curve slopes to the right with increasing altitude, the potential wet-bulb temperature increases with height, and the layer is potentially stable.

If it slopes to the left with increasing height, more than the saturation adiabats, the layer is potentially unstable. If none of the potential curves intersect the sounding, thunderstorms are not likely to occur.

This method is a reasonably simple method for forecasting air-mass thunderstorms in the Eastern United States. It does require prediction of short-range changes in the vertical distribution of temperature and moisture.

. The first consideration is to eliminate those areas whose soundings disclosed one or more of the following moisture inadequacies:

. The Dewpoint depression is 13C or more at any level from 850 through 700 hPa.

. The Dewpoint depression sum is 28C or more at 700- and 600-hPa levels.

. The surface dewpoint is 60F or less at 0730 local with no substantial increase expected before early afternoon.

. There is a lapse rate of 21C or less from 850 to 500 hPa.

. There is a freezing level below 12,000 feet in an unstable cyclonic flow, producing only light showers.

. After eliminating all soundings that meet one or more of the previous six conditions, you should use the following parameters to make the forecast.

. The sum of the dewpoint depressions at 700 and 600 hPa in degrees C.

NOTE: The lapse rate is the difference in temperature between these two pressure levels. For example, if the temperature at 850 hpa was 15C and at 500hPa it was 10C, the difference would equal 25. These two computations are used as arguments for the graph in figure 5-5.

Figure 5-5.-Local area thunderstorm graph. Area "A" iS isolated thunderstorms, with a 12 to 1 chance of at least one rain gauge in dense network receiving rain. Area "B" is scattered thunderstorms, with a 4 to 1 chance of reported rain. Area "C" is no rain.

One further condition for the development of thunderstorms is the absence of large anticyclonic wind shear, which is measured at 850 hPa. No horizontal shear at this level may exceed 20 knots in 250 miles measured toward the low-pressure area from the sounding station. Figure 5-6 illustrates how this measurement is made.

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