Autoconvection

2-14 shows an example of analyzed positive and the negative energy areas as they would appear on a Skew-T, Log P diagram. Autoconvection AUTOCONVECTION is a condition started spontaneously by a layer of air when the lapse rate of temperature is such that density increases with elevation. For density to increase with altitude, the lapse rate must be equal to or exceed 3.42°C per 100 meters. (This is the AUTOCONVECTIVE LAPSE RATE.) An example of this condition is found to exist near the surface of the earth in a road mirage or a dust devil. These conditions occur over surfaces that are easily heated, such as the desert, open fields, etc.; they are usually found during periods of intense surface heating. Convection Stability and Instability In the discussion so far of convection stability and instability, PARCELS of air have been considered. Let us now examine LAYERS of air. A layer of air that is 2-20 500 600 700 850 1000 ISOBARS mb + + ⁺+ ⁺+_+⁺ ⁺+ ⁺+ ⁺+ UPPER NEGATIVE AREA (BLUE SHADING) EL POSITIVE AREA (RED SHADING) NEGATIVE AREA (BLUE SHADING) LFC LCL POINT A T T T T d d AG5f0214 Figure 2-14.—Example of positive and negative energy areas (mechanical lifting).