LYDON OVEN UNIVERSITY
Sophomore Curriculum

Water In Air

HUMIDITY: The presence of gaseous water in air.

ABSOLUTE HUMIDITY: The mass of water vapor per unit volume of air. In US measure, this is usually described in grains per cubic foot of dry air. (A grain is 1/7000 of a pound avoirdupois, or 0.065 grams.)

RELATIVE HUMIDITY: The ratio of gaseous water in air of a given temperature and pressure expressed as a percent of the maximum gaseous water that air of that temperature and pressure can carry. Given a fixed temperature and atmospheric pressure, the ratio may be either the vapor pressure of the given mixture compared to the vapor pressure of saturated air, or the mass of the water vapor in the mixture compared to the mass of saturated vapor in air. Water vapor is considered “saturated” when either compression or cooling will cause condensation or precipitation of the water. See also “dew point temperature”.

PSYCHROMETRY: The measurement of water content in air. (Broadly: The physical science of air and water mixtures) The word arises from measurements obtained using a psychrometer. A psychrometer is an instrument for measuring the dry bulb and wet bulb temperatures simultaneously.

DRY BULB TEMPERATURE: This is the temperature of air commonly detected by a thermometer. This temperature indicates only that “sensible” heat available in the dry, or non-water mass of an air and water mixture. It does not reflect the total heat of the mixture.

WET BULB TEMPERATURE: This is the temperature of water evaporation in air, or that temperature which a thin film of water covering a thermometer bulb will assume as it is cooled by evaporation. (Hence the Greek word “psychros”, or cold, is the root of the word “psychrometer”.) The wet bulb temperature is a measure of the total heat in an air and water mixture.

DEW POINT TEMPERATURE: This is the dry bulb temperature at which air becomes saturated with water and condensation begins.

VAPORIZATION: The change from a liquid state to a gaseous state in one of two forms: Evaporation or Boiling. Evaporation is vaporization occurring at the free wetted surface of a liquid at temperatures below the boiling point temperature. Boiling is a more active vaporization that takes place within the internal volume of a liquid with its gaseous form rising to the surface of the liquid. The conditions for vaporization are generally considered an increase in the temperature of the liquid or a decrease of the pressure surrounding the liquid – or both.

VAPOR PRESSURE: The pressure exerted by a vapor that is in contact with and in equilibrium with its liquid. Also that portion of atmospheric pressure attributable to the amount of water vapor. When the vapor pressure of a liquid exceeds the pressure of the atmosphere, the liquid boils.

Next Class:  Evaporation Factors