Humidity

Humidity is a parameter representing the amount of water vapor in the air. This is expressed in numerous ways: relative humidity, mixing ratio, specific humidity, and absolute humidity. Relative humidity is the most commonly used term when we refer to humidity in the field of geography and in our daily lives. Relative humidity is the ratio of the water vapor content that is actually present in the air to the water vapor content in the saturated air at a prescribed temperature, and its values are given as a percentage. As a definition, relative humidity is expressed as the ratio of the actual vapor pressure (e, the partial pressure of water vapor) to the equilibrium or saturation vapor pressure (es, the vapor pressure of the saturated air) and is given as RH = e/es × 100.

Alternatively, we are able to consider the parameter as the ratio of the actual mixing ratio (q; described later) to the saturation mixing ratio (qs). A condition in which air is completely saturated has 100% relative humidity. Air with relative humidity higher than 100% is called supersaturated. Relative humidity varies with water vapor content and/or temperature. In other words, relative humidity varies with temperature even when the total amount of water vapor does not change. This is because the saturation vapor pressure depends on temperature, whereas actual vapor pressure does not.

Relative humidity has an inverse relationship to the diurnal variation of air temperature near the ground surface, and therefore, its value becomes smaller as temperature increases. In urban areas, the surface air temperature is generally higher, and lesser evapotranspiration occurs than in its surrounding areas. As a result, relative humidity has a tendency to be lower in urban areas. Relative humidity can be measured in the field using instruments such as sling hygrometers.

Other parameters that represent humidity—the mixing ratio, specific humidity, and absolute humidity—are defined from a different perspective to relative humidity as these depend on water vapor content but are independent of temperature.

The mixing ratio (q) is the ratio of the mass of water vapor to the mass of dry air, whereas specific humidity (s) is represented by the ratio of the mass of water vapor to the total mass of air. Consequently, specific humidity has almost equivalent values to the mixing ratio. These variables are presented here in kilograms per kilogram but are usually expressed in grams per kilogram. The mixing ratio (q) and specific humidity (s) are both functions of vapor pressure (e) and total pressure (p). Hence, these variables are given as q = 0.622e/(p − 0.378e) and s = 0.622e/p. On the other hand, absolute humidity gives the water vapor density in the air and is expressed in kilograms per cubic meter.

Besides these parameters, other variables such as dew-point temperature and dew-point depression (temperature/dew-point spread) also represent the amount of water vapor in the air. These variables are generally used in the fields of weather forecasts and atmospheric sciences rather than in the field of geography.