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Water vapor can also be indirect evidence supporting the presence of extraterrestrial liquid water in the case of some planetary mass objects. Water vapor, which reacts to temperature changes, is referred to as a 'feedback', because it amplifies the effect of forces that initially cause the warming. Therefore, it is a greenhouse gas. [2]
The function of a vapor barrier is to retard the migration of water vapor. A vapor barrier is not typically intended to retard the migration of air. This is the function of air barriers. [8] Air is mixed with water vapor. When air moves from location to location due to an air pressure difference, the vapor moves with it.
Using the classical definition of advection, moisture advection is defined as: =in which V is the horizontal wind vector, and is the density of water vapor. However, water vapor content is usually measured in terms of mixing ratio (mass fraction) in reanalyses or dew point (temperature to partial vapor pressure saturation, i.e. relative humidity to 100%) in operational forecasting.
The Bergeron process notes that the saturation vapor pressure of water, or how much water vapor a given volume can contain, depends on what the vapor is interacting with. Specifically, the saturation vapor pressure with respect to ice is lower than the saturation vapor pressure with respect to water.
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. A related parameter is the dew point. The amount of water vapor needed to achieve saturation increases as the temperature increases.
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance. It is a measure of the permeability for vapor barriers. There are many industries where moisture control is critical.
Ventilation in a structure is also needed for removing water vapor produced by respiration, burning, and cooking, and for removing odors. If water vapor is permitted to accumulate, it may damage the structure, insulation, or finishes. [citation needed] When operating, an air conditioner usually removes excess moisture from the air.
In construction, a building or structure is waterproofed with the use of membranes and coatings to protect contents and structural integrity. The waterproofing of the building envelope in construction specifications is listed under 07 - Thermal and Moisture Protection within MasterFormat 2004, by the Construction Specifications Institute, and includes roofing and waterproofing materials.