Chapter 5 Atmospheric Moisture

1. Chapter 5 Atmospheric Moisture

2. Phase transitions water substance The process whereby molecules break free of liquid water is known as evaporation. The opposite process is condensation, wherein water vapor molecules become a liquid. The change of phase directly from ice to water vapor, without passing into the liquid phase, is called sublimation. The reverse process (from water vapor to ice) is called deposition.

3. Equilibrium vapour-liquid Consider a hypothetical jar containing pure water with a flat surface and an overlying volume that initially contains no water vapor (a). As evaporation begins, water vapor starts to accumulate above the surface of the liquid. With increasing water vapor content, the condensation rate likewise increases (b). Eventually, the amount of water vapor above the surface is enough for the rates of condensation and evaporation to become equal. The resulting equilibrium state is called saturation (c).

4. Measures of humidity Humidity refers to the amount of water vapor in the air. The part of the total atmospheric pressure due to water vapor is referred to as the vapor pressure. The vapor pressure of a volume of air depends on both the temperature and the density of water vapor molecules. The saturation vapor pressure isan expression of the maximum water vapor that can exist. The saturation vapor pressure depends only on temperature.

5. Measures of humidity Absolute humidity is the density of water vapor, expressed as the number of grams of water vapor contained in a cubic meter of air. Specific humidityexpresses the mass of water vapor existing in a given mass of air. Saturation specific humidity is the maximum specific humidity that can exist and is directly analogous to the saturation vapor pressure. The mixing ratio is a measure of the mass of water vapor relative to the mass of the other gases of the atmosphere. The maximum possible mixing ratio is called the saturation mixing ratio.

7. Measures of humidity Relative humidity, RH, relates the amount of water vapor in the air to the maximum possible at the current temperature. RH = (specific humidity/saturation specific humidity) X 100% More water vapor can exist in warm air than in cold air, so relative humidity depends on both the actual moisture content and the air temperature. If the air temperature increases, more water vapor can exist, and the ratio of the amount of water vapor in the air relative to saturation decreases.

8. In (a), the temperature of 14°C has a saturation specific humidity of 10 grams of water vapor per kilogram of air. If the actual specific humidity is 6 grams per kilogram, the relative humidity is 60 percent. In (b), the specific humidity is still 6 grams per kilogram, but the higher temperature results in a greater saturation specific humidity. The relative humidity is less than in (a), even though the density of water vapor is the same.

9. Measures of humidity The dew point is the temperature to which the air must be cooled to become saturated and is an expression of water vapor content. In (a), the temperature exceeds the dew point and the air is unsaturated. When the air temperature is lowered so that the saturation specific humidity is the same as the actual specific humidity (b), the air temperature and dew point are equal. Further cooling (c) leads to an equal reduction in the air temperature and dew point so that they remain equal to each other. Exercise 1 Ch 4 (ed3)

11. Diabatic process A diabatic process is one in which energy is added to or removed from a system, such as air that is warmed by conduction when in contact with a warm surface or air that passes over a cool surface and loses energy by conduction. First law of thermodynamics: dH = p dα + cv dT External heat exchange = work on the surroundings + increase internal energy

12. Adiabatic process An adiabatic process is one in which no energy is added to or removed from a system, such as air that does not exchange heat with its surroundings . First law of thermodynamics: 0 = p dα + cv dT -p dα = cv dT

13. Dry adiabatic lapse rate DALR Processes in which temperature changes but no heat is added to or removed from a substance are said to be adiabatic. The rate at which a rising parcel of unsaturated air cools, called the dry adiabatic lapse rate(DALR), is very nearly 1.0 °C/100 m.

14. Lifting to saturation If a parcel of air rises high enough and cools sufficiently, expansion lowers its temperature to the dew or frost point, and condensation or deposition commences. The altitude at which this occurs is known as the lifting condensation level (LCL).

15. Saturated adiabatic lapse rate SALR The rate at which saturated air cools is the saturated adiabatic lapse rate(SALR), which is about 0.5 °C/100 m. This is slower than the dry adiabatic lapse rate due to release of latent heat

16. Unlike the DALR, the SALR is not a constant value. If saturated air cools from 30 °C to 25 °C (a 5° decrease), the specific humidity decreases from 27.7 grams of water vapor per kilogram of air to 20.4. A 5 °C drop in temperature from 5 °C to 0 °C lowers the specific humidity only 1.7 grams for each kilogram of air. This brings about less warming to offset the cooling by expansion, as well as a greater saturated adiabatic lapse rate.

17. Environmental lapse rate ELR The environmental lapse rate (ELR), applies to the vertical change in temperature through still air. A balloon rising through air with an ELR of 0.5 °C/100 m passes through air whose temperature decreases from 10 °C at the surface, to 9.5 °C at 100 m, and 9.0 °C at 200 m. The air within the balloon cools at the dry adiabatic lapse rate of 1.0 °C/100 m, faster than the ELR, and therefore attains a temperature of 8 °C at the 200-m level.