Humidity: The Role of Water Vapor in Weather and Dew Point Formation

Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The average concentration of water vapor in the atmosphere is 0.48%

The dew point of a given body of air is the temperature to which it must be cooled to become saturated with water vapor. This temperature depends on the pressure and water content of the air. When the air is cooled below the dew point, its moisture capacity is reduced and airborne water vapor will condense to form liquid water known as dew. When this occurs through the air’s contact with a colder surface, dew will form on that surface. The dew point is affected by the air’s humidity. The more moisture the air contains, the higher its dew point.

Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air.

Humidity is measured using instruments such as hygrometers. These devices can be based on various principles, including the use of hair tension, electrical resistance, or condensation.

Three primary measurements of humidity are widely employed:

Absolute, Relative, and Specific.

Absolute humidity is expressed as either mass of water vapor per volume of moist air (in grams per cubic meter) or as mass of water vapor per mass of dry air (usually in grams per kilogram).

Relative humidity, often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature.

Specific humidity is the ratio of water vapor mass to total moist air parcel mass.

Vapor pressure (denoted e): It is the partial pressure of water vapor molecules in the atmosphere. Partial pressure is a term in thermodynamics of gas mixtures (in our case – air). We can break down the air pressure into the pressure each of its individual gas constituents would exert, had all the others been removed. The pressure in an air parcel is the sum of the partial pressures of all the constituents. The smaller the concentration of a gas in the mixture, the lower its partial pressure.

However, since molecules of different constituents have different mass, the partial pressure is not directly proportional to the molecular concentration.

The concept of vapor pressure is important for understanding the processes of evaporation and saturation. If we hold a parcel of air still over flat water surface, water molecules will escape the surface and start mixing with the other gases in the air parcel. This is evaporation – it can happen even if the liquid is not at its boiling temperature. Evaporation can only go on until the maximum amount of water vapor that air can hold is reached. At this point, the pressure that the water molecules exert as they are trying to escape the liquid is equaled by the partial pressure of water in the air parcel, called the saturation vapor pressure. Saturation is a process of equilibrium where water molecules cross back and forth across the boundary between water and air, maintaining a fixed concentration in the air. The saturation vapor pressure is a function of temperature.

Relative humidity: It is the ratio of actual vapor pressure to saturation vapor pressure (expressed as % if multiplied by 100). This is a common way to indicate air humidity. Because perspiration plays a very important function in maintaining body temperature, relative humidity figures into consideration of the degree of comfort we have when following our daily activities.

Mixing ratio: It is the mass of variable atmospheric constituent in grams per kilogram of air. Here it is the mass of water vapor in grams per kilogram of air. This is the most common way to indicate air humidity in scientific applications. At the Earth’s surface, mixing ratio varies from ~18 g kg^-1 in the tropics to less than 2 g kg^-1 near the poles.