## TU

Evaporation continues, and condensation begins.

Rate of evaporation equals rate of condensation or saturation.

Figure 11.15 During evaporation, water molecules escape from the surface of the liquid and enter the air as water vapor. During condensation, water molecules return to the liquid state. At equilibrium, evaporation and condensation continue, but the amount of water in the air and amount of water in the liquid form remain constant.

Section 2 • Properties of the Atmosphere 295

Figure 11.16 Condensation occurs at the lifted condensation level (LCL). Air above the LCL is saturated and thus cools more slowly than air below the LCL. Explain why air above the LCL cools more slowly than air below the LCL.

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A rising mass of air cools because the air pressure around it decreases as it rises, causing the air mass to expand. A rising air mass that does not exchange thermal energy with its surroundings will cool by about 10°C for every 1000 m it rises. This is called the dry adiabatic lapse rate—the rate at which unsaturated air will cool as it rises if no thermal energy is added or removed. If the air mass continues to rise, eventually it will cool to its condensation temperature. The height at which condensation occurs is called the lifted condensation level (LCL).

The rate at which saturated air cools is called the moist adiabatic lapse rate. This rate ranges from about 4°C/1000 m in very warm air to almost 9°C/1000 m in very cold air. This rate is slower than the dry adiabatic rate because water vapor in the air is condensing as the air rises and is releasing latent heat, as shown in Figure 11.16.