CHAPTER #4
Atmospheric Moisture





Hydrological cycle: a continuous transfer of water among terrestrial, oceanic, and atmospheric reservoirs. Within the atmosphere, water exists in all three forms: i) water vapor, ii) liquid (i.e. cloud droplets, raindrops), and iii) solid (i.e. ice crystals). Within the usual range of temperature and pressure, all three phases of water coexist (equilibrium). Water molecules continuously change their phases. On the average, the residence time of a water molecule is about 10 days.

The total amount of water within the atmosphere is very small. In fact, if all water were removed from the atmosphere as rain and distributed over the globe, the water would have only about 2.5 cm (1 in.) depth on the Earth's surface.

Evaporation (Condensation): a process by which water changes phase from a liquid (vapor) to a vapor (liquid).

Transpiration: a process by which water absorbed by plant roots eventually escapes as vapor though the surface of green leaves. On land, transpiration is often more important than direct evaporation from the surfaces of lakes, streams, and the soil.

Evaportranspiration: direct evaporation + transpiration



Sublimation (Deposition): a process by which water changes phase from a solid (vapor) to a vapor (solid).

Precipitation: drizzle, rain, snow, ice pellets, and hail; a process by which major portion of atmospheric water returns to the Earth's surface.

Figure 4.1, page #94 (Ahrens)


Global water budget: the balance sheet for the inputs and outputs of water to and from the various global reservoirs. Precipitation and evaporation are the two major components of the global water budget. Precipitation over land exceeds evaporation annually and vice versa is true over the oceans. The net gain (loss) of water over land (oceans) is balanced with a net flow of water from land to sea. Precipitation falling on land evaporates, infiltrates the ground, or run off as rivers and streams. The ratio of infiltrating the ground to running off depends on the intensity of precipitation and on the vegetation, topography, and the physical properties of the surface.


Dalton's law: The total pressure of a mixture of gases equals the sum of the pressure exerted by each constituent of gas.

Vapor pressure : the portion of total air pressure exerted by water vapor in a sample of air. The maximum possible vapor pressure is about 4% of the mean sea level pressure, or about 40 mb. Assuming that the water vapor is 1% of the atmospheric composition at the sea level, the vapor pressure, on the average, is about 10 mb.

Mixing ratio: the mass of water vapor (in grams) per mass of dry air (in kgs). Mixing ratio (w) can be expressed as: w = A e / (p - e), where e is vapor pressure, p is the atmospheric pressure, and A is a constant (= 0.622).

Specific humidity: the ratio of the mass of water vapor (in grams) to the ratio of the mass of moist air (in kgs). Specific humidity (q) can be expressed as: q = A e / p. It usually differs from the mixing ratio not more than 2%. Both specific humidity and mixing ratio decreases with increasing latitude.

Figure 4.9, page #94 (Ahrens)

Absolute humidity: the mass water vapor (in grams) per unit volume of dry air (in cubic meters). It is also known as vapor density of water.


Saturation: an atmospheric condition which occurs when the maximum amount of water vapor is available to keep moist air in equilibrium with a surface of pure water or ice. It represents the maximum amount of water vapor that air can hold at observed temperature and pressure.

Saturation vapor pressure: the vapor pressure of the air saturated with respect to water or ice at observed temperature.

Saturation mixing ratio: the mixing ratio of the air saturated with respect to water or ice at observed temperature and pressure.

Both saturation vapor pressure and saturation mixing ratio increases with increasing air temperature such that both double for about every 11C (20F) rise in air temperature.

Figure 4.10, page #99 (Ahrens)

Table 1, page #106 (Ahrens)


Relative humidity: the ratio of vapor pressure (mixing ratio) to the saturation vapor pressure (saturation mixing ratio). It is expressed as a percentage and reaches 100% when the air is saturated with respect to water (ice). At a given vapor pressure (or mixing ratio), relative humidity with respect to ice is higher than that with respect to water.

For unsaturated air, relative humidity is inversely proportional to the temperature. The mean annual water vapor of the troposphere is about the same over the desert area of the southwestern United States as it is over the Great Lakes, however, the relative humidity is significantly different in the two locations. In the absence of air mass advection, and variation in vapor pressure (or mixing ratio), the relative humidity reaches its maximum when the air temperature is minimum and vice versa.

Figure 4.12, page #101 (Ahrens)


Supersaturation: an atmospheric condition occurs when the relative humidity exceeds its equilibrium value (100%). The relative humidity of the unsaturated air increases when water vapor is added to the air (increasing the vapor pressure) and/or the air is cooled (lowering the saturation vapor pressure).

Adiabatic process: the process in which no heat exchange is allowed between the air parcel and its environment. That is, during its ascent or descent within the atmosphere, an air parcel is neither cooled or heated by radiation, conduction, mixing, or phase changes of water.

During an adiabatic process, the total molecular kinetic energy of the mixture of gases composing air parcel, internal energy (heat), does not change, DQ = 0.

The change in internal energy of an air parcel is proportional to the heat flow into or out of air parcel and/or the work done into or by the air parcel during compression or expansion of air, DQ =m Cv DT + P DV, where Cv is the specific heat at constant volume (= 718 J/kg - K). On can also use Cp, specific heat at constant pressure (= 1005 J/kg - K) when DQ = mCp DT - V DP is employed.


Expansional cooling: an adiabatic process in which an air parcels temperature drops that accompanied by a reduction in pressure during its ascent within the atmosphere.

Compressional warming: an adiabatic process in which an air parcels temperature rises that accompanied by an increase in pressure during its descent within the atmosphere.

If the air parcel is unsaturated, its relative humidity increases during expansional cooling and decreases during compressional warming.

Dry adiabatic lapse rate: a measure of temperature change of unsaturated air as it moves vertically within the atmosphere, DT/Dz = 10 C / 1000 m (5.5 F / 1000 ft.).

Moist adiabatic lapse rate: a measure of temperature change of saturated air as it moves vertically within the atmosphere. The moist adiabatic lapse rate ranges from about 4 C / 1000 m (2.2 F / 1000 ft.) for a very warm saturated air to almost 9 C / 1000 m (5 F /1000 ft.) for a very cold saturated air. On the average, DT/Dz is about 6 C / 1000 m (3.3 F / 1000 ft.).


The range in moist adiabatic lapse rate depends on the amount of latent heat released during condensation or deposition. The latent heat is the heat released or needed during the phase change of water. It is primarily function of air temperature and also air pressure. Latent heat of evaporation (condensation) ranges between 540 to 600 cal/gr, and latent heat of melting (freezing) is 80 cal/gr.

Lifting condensation level (LCL): an altitude to which air is lifted following dry adiabatic lapse rate and above which air is lifted following moist adiabatic lapse rate.



Hygrometer: the instrument used to monitor changes in relative humidity. There are two basic types of hydrometer: i) hair hygrometer, and ii) psychrometer. Hair hygrometer is less accurate than a psychrometer, but measures humidity directly.

Psychrometer consists of two mercury thermometers. The thermometers are alike except that one has a piece of cloth covering the bulb, wet-bulb thermometer. The cloth-covered thermometer is dipped in clean water, while the other, dry-bulb thermometer, is kept dry. Both thermometers are ventillated for a few minutes, either by whirling the instrument, sling psychrometer, or by drawing air pass it with an electric fan, aspirated psychrometer. Water evaporates from cloth and web-bulb termometers cools. The dry-bulb thermometer measures actual air temperature. Relative humidity is then obtained from the tables of dry-bulb temperature vs. the wet-bulb depression that is the difference between dry-bulb and wet-bulb temperatures.

Figure 4.22, page #110 (Ahrens)


Hygrograph: a hair hygrometer that records the continuous trace of relative humidity with time. The human or horse hair increases by 2.5% as the relative humidity increases from 0% to 100%.

Figure 4.23, page #110 (Ahrens)

Electrical hygrometer: consists of a flat plate with a film of carbon. An electric current is sent across the plate. As the water vapor is absorbed, the electrical resistance of the carbon coating changes and is translated to the relative humidity. It is used in radiosondes. Infrared hygrometer measures the amount of infrared energy absorbed by water vapor in a sample of air.

Dew-point and frost-point hygrometer measures the dew point or frost point temperature by cooling the surface of a mirror until condensation occurs. It is used in ASOS. Dew cell determines the amount of water vapor in the air by measuring the vapor pressure.


Heat index (Apparent temperature index): the perceived temperature to the human body based on both air temperature and the amount of moisture present in the air. Heat index becomes a factor for human health when the apparent temperature exceeds 80 F (27 C).

Figure 4.21, page #108 (Ahrens)