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close this bookNatural Energy and Vernacular Architecture: Principles and Examples with Reference to Hot Arid Climates (UNU, 1986, 172 pages)
close this folderPart 1. Man, natural environment, and architecture
close this folder2. Architectural thermodynamics and human comfort in hot climates
View the document(introductory text...)
View the documentTemperature
View the documentThermal conduction and resistance
View the documentRadiation
View the documentThermal convection
View the documentAtmospheric pressure
View the documentWater vapor
View the documentCooling by evaporation
View the documentThermal gain
View the documentThermal loss
View the documentDynamic thermal equilibrium
View the documentHeat-regulating mechanisms of the human body
View the documentMeasurement of conditions of human comfort

Water vapor

At temperatures throughout the climatic range of the normal human habitat, water can exist as solid ice, liquid water, and gaseous water vapor. At the freezing point, ice and water can exist together. Above this temperature ice is completely converted to water, and below it, only ice exists. However, regardless of whether the water is solid or liquid, the air above it contains a certain amount of water vapor.

Generally speaking, the permanent gases in the air produce the pressure indicated by a barometer. However, if water is present at the base of the column of air, that water partially evaporates (becomes water vapor) and contributes to the atmospheric pressure. This share depends on the temperature. Air containing the maximum possible amount of water vapor for its temperature is said to be saturated. The temperature at which condensation begins in a mixture of air and water is termed the dew point.

There are several ways to express the relation between humidity and temperature. The amount of water vapor that a volume of air can support at saturation can be expressed as grams or grains of vapor per volume of air, or as the portion of the total atmospheric pressure that the water vapor contributes. Similarly, the water-vapor content of unsaturated air can always be expressed as the portion of the total pressure that the water vapor contributes, called the vapor pressure, or as the amount of atmospheric water vapor in grams per m³ or grains per ft³. These values can also be determined with respect to the dew point, which is the temperature to which air must be reduced, without altering its barometric pressure, to reach saturation. In this way, the watervapor content of air at a given temperature can be expressed as the ratio of the portion of the total atmospheric pressure contributed by water vapor to the portion necessary to cause saturation at that air temperature. This ratio, most often expressed as a percentage, is called the relative humidity.

Appendix 1 gives the values of water-vapor density and pressure for saturated water vapor over the range of temperatures from -1034 °C (1493 °F).

A given volume of water vapor is lighter than the same volume of air at the same temperature and pressure. In the atmosphere, therefore, saturated air is lighter than dry air of the same temperature and pressure. When water evaporates, the vapor simply rises into the air. If this process occurs in open air where there is freedom of motion, the water vapor can displace the equivalent volume of dry air without affecting the atmospheric pressure. Near water surfaces, therefore, rising water vapor is continuously replaced by dry air, which in its turn dampens and rises into the air. This water vapor eventually reaches a certain height, condenses on the floating particles always present in air, and becomes visible as clouds.

The processes involved in weather phenomena are not so simple. Such factors as heat, radiation, pressure, and wind interact to establish relative balances in the atmosphere, resulting in the constant recycling of water by evaporation, cloud formation, cloud motion, and precipitation.

Water vapor and temperature, pressure, and air movement are very important to the study of the climate and the microclimate both outside and inside buildings. They are key to an understanding of the formation of clouds, rain, dew, frost, and nearly all other meteorological phenomena. The behavior of water vapor must be understood to comprehend the physical and physiological processes of cooling by evaporation-the phenomenon upon which thermal comfort in hot climates largely depends. If air in a room is saturated with water vapor and its temperature decreases, then some water vapor will condense, leaving in the air only the amount that can be accommodated at the new temperature. However, if the air temperature rises, the air can accommodate additional water vapor and is called "dry air." This air can be described as "thirsty" until its temperature falls or it encounters water from which it can absorb vapor.

In winter, a dry feeling in the throat can result when moisture from the human body evaporates in a room overheated by a stove. A heated kettle of evaporating water can reestablish the moisture content of the air, corresponding to its increased temperature. The same feeling of dryness occurs in hot weather when evaporation of perspiration is necessary to lower body temperature. Here a parched throat indicates the need to drink water to maintain the supply of perspiration.

When air temperature drops below the saturation point, water collects in droplets on the dust particles always floating in the air. Or, if the air is in contact with a sufficiently cold surface, water vapor will condense on that surface. Thus water condenses on cold walls just as on a drinking glass containing a liquid cooled by ice. Similarly, when an amount of water vapor exceeding the saturation limit is introduced into air in an enclosed space, the excess vapor will condense, as on a bathroom mirror in winter or on the inner surfaces of the windows of a closed automobile with many people.