When using this method, it is assumed that the type lifting will be either orographic or frontal, Here we will be concerned with the Lifting Condensation Level (LCL) and the Level of Free Convection (LFC). The LCL is the height at which a parcel of air becomes saturated when it is lifted dry adiabatically. The LCL for a surface parcel is always found at, or below, the CCL. The LFC is the height at which a parcel of air is lifted dry adiabatically until saturated, and thereafter would first become warmer than the surrounding air. The parcel will then continue to rise freely above this level until it becomes colder than the surrounding air.

Figure 5-3.-Forecasting afternoon convective cloudiness.

Figure 5-4 shows the formation of clouds due to mechanical lifting. This figure shows the formation of a stratified layer of clouds above the LCL, and to the LFC. At the LFC and above, the clouds would be turbulent. The tops of the clouds extend beyond the top of the positive area due to overshooting, just as clouds formed due to heating. To determine the possibility of thunderstorms from this method, the following conditions should be met:

. The positive area must exceed the negative area; the greater the excess, the greater the possibility of thunderstorms,

. There must be sufficient lifting for the parcel to reach the LFC, The frontal slope or the orographic barriers can be used to determine how much lifting can be expected,

. The parcel must reach the ice crystal level (10C and below).

Figure 5-4.-Formation of clouds due to mechanical lifting.

. Even though the positive area does not exceed the negative area, cloudiness occurs after the parcel passes the LCL, and precipitation may occur after the parcel passes the ice crystal level.

One main advantage of this method is that it can be done quickly and with relative ease. A major disadvantage is that it assumes that the parcel does not change its environment, and that it overestimates or underestimates the stability and instability conditions.

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