3. Neutral equilibrium - the lapse rate equals the dry adiabatic rate. Under these conditions, saturated air is unstable whereas unsaturated air is stable. This situation almost never occurs. 4. Unstable equilibrium - the lapse rate is greater (steeper) than the dry adiabatic rate for either saturated or unsaturated air. The air particle is warmer than its surroundings. Therefore, it continues to rise. This condition occurs less frequently than #1 & 2. It is restricted to the lower atmosphere (2-3 kilometers) in arid or semi-arid continental areas during periods of maximum heating of the surface. This condition produces local thunderstorms in summer, but doesn't produce any widespread storms. 5. Auto-convective equilibrium - the lapse rate exceeds -3.42°C./100 meters (-19°F./1000 feet). This is a rare occurrence. It results from a shallow layer of air above a strongly heated surface (sand, pavement, etc.). It is very local in extent. It produces an overturn of air without any outside source of energy. It represents extreme instability, and produces such things as "dust devils". Any layer of air of a particular thickness will expand to a greater thickness or be compressed to a lesser thickness. A sinking layer of air spreads horizontally, while a lifted layer of air is exposed to vertical stretching. A sinking and horizontally spreading layer increases air stability (the top of air layer is heated more than the bottom of the layer by adiabatic heating). Lifting air decreases stability, since the bottom of the layer