During the passage of a thunderstorm, rapid and marked surface pressure variations generally occur. These variations usually occur in a particular sequence characterized as follows.

. An abrupt fall in surface pressure as the storm approaches.

. An abrupt rise in surface pressure associated with rain showers as the storm moves overhead (often associated with the first gust).

. A gradual return to normal surface pressure after thunderstorm passage, and the rain ceases.

Such pressure changes may result in significant pressure altitude errors on landing.

Of greater concern to the pilot are pressure altitude readings that are too high. If a pilot used an altimeter setting computed during the maximum pressure, and then landed after the pressure had fallen, the altimeter still could read 60 feet or more above the true altitude after landing.

Here is where you, as a forecaster, can make certain that timely and accurate altimeter settings are furnished to the tower for transmission to pilots during

Figure 5-2.-Cold dome of air beneath a thunderstorm cell in the mature stage.  Arrows represent deviation of windflow. Dashed lines indicate rainfall.

thunderstorm conditions. If the data are old and inaccurate, an aircraft mishap could result.

The standard method of forecasting air mass thunderstorms has long consisted primarily of an analysis of rawinsonde data with particular emphasis on the so-called "positive areas."

Many times conditions are favorable for thunderstorm development, with a large positive energy area showing up on the sounding, with no ensuing thunderstorm activity. At other times, thunderstorms may occur when they are not forecasted. Clearly, factors other than instability are important, and, at times, of overriding importance.

A number of thunderstorm forecasting methods have been developed, but many of these are beyond the scope of this manual. The forecasting of convective clouds by using variations of the parcel method are covered in this section. Further, a method for the prediction of these storms that enables the forecaster to arrive at a fairly accurate, reasonably objective forecast will be discussed.

For a more detailed discussion of the determinations of instability, stability, the convective condensation level (CCL), the level of free convection (LFC), and the lifting condensation level (LCL), refer to the AG2 TRAMAN, volume 2.

The temperature of a minute parcel of air is assumed to change adiabatically as the parcel is displaced vertically from its original position. If, after vertical displacement, the parcel has a higher virtual temperature than the surrounding atmosphere, the parcel is subjected to a positive buoyancy force and will be further accelerated upwards; conversely, if its virtual temperature has become lower than that of the surrounding air, the parcel will be denser, and eventuallyreturn to its initial or equilibrium position,

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