260 Applications and Investigations in Earth Science Because air pressure drops rapidly with increasing altitude above Earth's surface, rising air passes through regions ot successively lower pressure. Therefore, ascending air expands and cools adiabatically. Rising unsaturated air cools at the constant rate of 10°C per 1000 meters (1°C per 100 m). called the dry adiabatic rate. The dry adiabatic rate also applies to descending air, which is compressed and warmed. Once rising air reaches the dew-point temperature and water vapor condenses to form cloud droplets, latent heat that was stored in the water vapor is liberated. The heat released during condensation slows the rate of cooling of the air. Rising saturated air continues to cool by expansion, but at a lesser rate of about 5°C per 1000 meters (0.5°C per 100 m)-the wet adiabaticC rate. Figure 15.9 summarizes this cloud-forming process. Take a minute to carefully examine this figure before beginning Activity 15.8. Height (u) -8°C 000s Wet adiabatic rate (temperature of rising air drops at 5"C/1000 meters) 000 Condensation Y level 0008 12°C Dry adiabatic rate (temperature of rising air drops at 0007 000L Figure 15.9 Adiabatic cooling and cloud formation. Rising air cools at the dry adiabatic rate of 10°C per 1000 meters, until the air reaches the dew-point temperature and condensation (cloud formation) begins. As air continues to rise, the latent heat released by condensation reduces the rate of cooling. The wet adiabatic rate is therefore always less than the dry adiabatic rate. (siajau 0001/0.0 32 C ACTIVITY 15.8 Adiabatic Processes and Cloud Formation Figure 15.10 shows air that begins with a temperature of 25°C and a relative humidity of 50 percent flowing from the ocean over a coastal mountain range. Assume that the dew-point temperature remains constant in dry air (relative humidity less than 100 percent). When the air parcel becomes saturated, the temperature and dew-point temperature become equal. As the air continues to ascend, the temperature and dew-point temperature will decrease at the wet adiabatic rate, but the dew-point temperature does not change as the air parcel descends. (Completing Figure 15.10 may be helpful.) 1. Use Table 15,2 to determine the saturation mixing ratio, water-vapor content, and dew-point temperature of the air at sea level in Figure 15.10. Saturation mixing ratio: g/kg of air 01 Water-vapor content: g/kg of air Dew-point temperature: °C CI 2. Is the air at sea level saturated or unsaturated? unsaturated w 0008 Temp. Dew pt. Dew pt. 2000m Temp. Dew pt. Temp. F. Dew pt. w 0001 Temp. Temp. Dew pt. Temp, at Point A = 25°C Relative humidity at Point A = 50% Dry adiabatic rate = 10°C/1000 meters Wet adiabatic rate = 5°C/1000 meters Dew pt. Sea level A Figure 15.10 Adiabatic processes and cloud formation associated with air being lifted over a mountain barrier.