Humidity

The atmospheric transport and redistribution of water are the common elements of climate of the first kind and the atmospheric branch of the hydrologic cycle. Water occurs in the atmosphere as a solid, liquid, and vapor, but the vapor phase is the dominant form of atmospheric water. Water vapor is the atmospheric constituent with the greatest effect on atmospheric dynamics and thermodynamics and the atmospheric radiation balance. However, the atmospheric water vapor concentration varies enormously in the troposphere on small spatial and temporal scales. This important variable is a challenge to measure, but knowledge of the distribution of atmospheric water vapor is of fundamental importance to weather and climate, atmospheric radiation studies, and the global hydrological cycle.

Direct measurement of the water vapor content of the atmosphere requires instrument arrays found in laboratories rather than at ground-based observation sites. For routine observations, determining the water vapor content of the atmosphere is achieved using indirect measurement techniques that are complemented by expressing the water vapor content of the atmosphere as humidity.

3.6.1 Humidity expressions

There are several ways to express humidity, and each is controlled to some extent by air temperature. Absolute humidity is the weight ofwater vapor per unit volume of air and is expressed in units of grams of water vapor per cubic meter of air. Absolute humidity is a measure of the actual amount of water vapor in the air, but it is sensitive to changes in both air temperature and atmospheric pressure. Specific humidity is the weight of water vapor in the air per unit weight of air. Specific humidity is not sensitive to volume changes in the air, and it is expressed in units of grams of water vapor per kilogram of air. Relative humidity is a more easily achieved measure of water vapor in the air, but it is a measure of water vapor relative to ambient air temperature. It expresses the actual amount of water vapor in the air compared to the total amount of water vapor that can exist in the air at its current temperature. Relative humidity (RH) is determined using water vapor partial pressure and saturation vapor pressure and is expressed as a percentage as shown by

where ea is the actual vapor pressure in hPa and es is the saturation vapor pressure at the observed temperature in hPa. Other water vapor indicators include dew point temperature, saturation deficit, and the mixing ratio. The relationship among these indicators is found in standard textbooks.

3.6.2 Humiditymeasurement

Humidity is measured with devices known generically as hygrometers or as a hygrograph if fitted with a recording device. Hygrometers employ several different measuring methods and instrument designs that are based on the hygroscopic quality of material, on the longitudinal change of human or animal hairs, on cooling caused by evaporation of water, or on the change of electrical resistance (WMO, 1996). The earliest hygrometers used human hair that stretches as it absorbs moisture. The bundle of human hair is linked mechanically to a pointer that is calibrated to record relative humidity.

The instrument commonly found in a weather station is based on temperature differences and is called a psychrometer. A standard mercury or electrical thermometer indicating ambient air temperature is paired with a second mercury or electrical thermometer whose bulb is covered with a wick or sleeve saturated with distilled water. The thermometers are positioned in a ventilated space. Water is evaporated from the wick into the air until equilibrium between the wick and the air is reached. The temperature of the wet-bulb thermometer is lowered by an amount determined by the evaporative cooling. The equilibrium temperature indicated by the wet-bulb thermometer is known as the wet-bulb temperature. The difference between the dry-bulb and wet-bulb temperatures is the wet-bulb depression (DeFelice, 1998). For the wet-bulb thermometer at equilibrium ea = es - AP(T - Tw) (3.5)

where eais the actual vapor pressure in hPa, esis the saturation vapor pressure at TwinhPa, A is a constant of proportionality with a value of 6.6 x 10-4(1 +1.15 x 10-3 Tw), P is air pressure in hPa, T is the dry-bulb temperature in °C, and Tw is the wet-bulb temperature in °C. An alternative to Equation 3.5 for determining ea is to use a psychrometric chart that permits T, Tw, ea, the dew point temperature, relative humidity, and the mixing ratio to be determined when any two are known (Linacre and Geerts, 1997).

When Tw is known, es can be determined from standard tables or graphs based on the Clausius-Clapeyron relationship. The form of the Clausius-Clapeyron equation familiar in atmospheric science and applied to water vapor in the presence of air is

68 Measuring hydroclimate atmospheric components 80

re 3

68 Measuring hydroclimate atmospheric components 80

re 3

Fig. 3.4. Variation of saturation vapor pressure with air temperature.

Fig. 3.4. Variation of saturation vapor pressure with air temperature.

where Lv is the latent heat of vaporization (2.453 x 106Jkg_1), Rv is the gas constant for water vapor (4.615 x 102Wkg_1K_1), and es and T are defined previously (Andrews, 2000). Figure 3.4 shows the relationship between temperature and saturation vapor pressure defined by Equation 3.6. The slope of the curve changes at an exponential rate, and the curve decreases at a different rate at temperatures below 0 °C depending on whether the medium is liquid or ice. These characteristics are important for determining saturation of the atmosphere and precipitation formation and are described in Chapter 4.

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