is called a negative lapse rate (inversion). If an air particle is warmer than its surroundings, its density will be less, so it will accelerate upward. If the particle is cooler than its surroundings, it will accelerate downward. The rate is determined by the difference in temp. of the air particle and its surroundings. All vertical motions in the atmosphere tend to be accelerated in the same direction because of the increasing temp. difference between the particle and its surroundings. At the wet adiabatic rate the difference is increased even more than at the dry adiabatic rate. Therefore, acceleration is greater in moist than in dry air. Air Stability Conditions 1. Stable equilibrium -- this occurs when the prevailing lapse rate is less than the wet adiabatic rate (thus, the air particle is always colder than its surroundings). This occurs when the ground surface is colder than the air above. This happens in winter over the continents in middle and high latitudes, and over the oceans in summer. Winds are free of vertical turbulence, so there is little or no precipitation. 2. Conditionally stable (or unstable) equilibrium -- the lapse rate is greater than the wet adiabatic rate, but less than the dry adiabatic rate. It is a stable condition for unsaturated air, but unstable for saturated air. It occurs when the ground surface is warmer than the air. It occurs mostly in low latitudes, over continents in summer and over the ocean in winter in mid-latitudes. This condition produces turbulent storms, with lots of precipitation when the air is near or at saturation.