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Humidity


Notes:

Before I discuss the digital forecast tool I want to make sure we all have a common definition of Humidity.

All air has some moisture in it (known as the Water Vapor Content). Evaporation and Condensation of this moisture are occurring all the time. Warm temperatures support a higher rate of evaporation than cold temperatures and so it is popularly (but incorrectly) said that warm air holds more moisture. (The air has nothing to do with it).

The saturation point is the point where the rates of Evaporation and Condensation are equal. This depends mostly on the temperature.

The term Relative Humidity refers to the how much moisture is currently in the air at the current temperature as percentage of the amount it would take for the air to become saturated at that temperature.

Dew Point is the temperature where the current water vapor content will reach the point where it is saturated. At this point the evaporation rate equals the condensation rate(I.e. dew will form). It is also a crude measurement of the absolute amount of moisture in the air. Dew will be observed on surfaces before the dew point is actually reached.

The amount of moisture in the air has a number of important effects beyond just making the air seem more comfortable. The moisture content changes the way a parcel of air cools or heats. Moist night air can be more steady. It changes the point where clouds form.

It is also important to remember that air is free to cool without reducing the current water vapor content . This is reflected by the relative humidity increasing. Large temperature differences are common during the California summer.

By the Way: The correct explanation of why clouds form: Water molecules are constantly coursing back and forth between phases (another word for the three states: vapor, liquid, and solid). If more molecules are leaving a liquid surface than arriving, there is a net evaporation; if more arrive than leave, a net condensation. It is these relative flows of molecules which determine whether a cloud forms or evaporates, not some imaginary holding capacity that nitrogen or oxygen have for water vapor. Evaporation increases with temperature, not because the holding capacity of the air changes, but because the more energetic molecules can evaporate more readily. (from Bad Meteorology http://www.ems.psu.edu/~fraser/Bad/BadClouds.html)

Formal definitions (From American Meteorological Society (http://amsglossary.allenpress.com/glossary)

relative humidity—The ratio of the vapor pressure to the saturation vapor pressure with respect to water. This quantity is alternatively defined by the World Meteorological Organization as the ratio of the mixing ratio to the saturation mixing ratio. These two definitions yield almost identical numerical values. Relative humidity is usually expressed in percent and can be computed from psychrometric data. Unless specified otherwise, relative humidity is reported with respect to water rather than ice because most hygrometers are sensitive to relative humidity with respect to water even at subfreezing temperatures, and because the air can easily become supersaturated with respect to ice, which would require three digits in coded messages for relative humidity with respect to ice.           Retallach, B. J., 1974: Physical Meteorology, p. 83.


dewpoint—(Or dewpoint temperature.) The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur. When this temperature is below 0C, it is sometimes called the frost point. The dewpoint may alternatively be defined as the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor. Isobaric heating or cooling of an air parcel does not alter the value of that parcel's dewpoint, as long as no vapor is added or removed. Therefore, the dewpoint is a conservative property of air with respect to such processes. However, the dewpoint is nonconservative with respect to vertical adiabatic motions of air in the atmosphere. The dewpoint of ascending moist air decreases at a rate only about one-fifth as great as the dry-adiabatic lapse rate. The dewpoint can be measured directly by several kinds of dewpoint hygrometers or it can be deduced indirectly from psychrometers or devices that measure the water vapor density or mixing ratio. See dewpoint formula.

saturation vapor pressure—1. The vapor pressure of a system, at a given temperature, for which the vapor of a substance is in equilibrium with a plane surface of that substance's pure liquid or solid phase; that is, the vapor pressure of a system that has attained saturation but not supersaturation. The saturation vapor pressure of any pure substance, with respect to a specified parent phase, is an intrinsic property of that substance, and is a function of temperature alone. For water vapor, the saturation pressure over supercooled liquid differs appreciably from that over ice. Compare equilibrium vapor pressure, vapor tension; see ClausiusClapeyron equation. 2. Same as equilibrium vapor pressure.

saturation—The condition in which vapor pressure is equal to the equilibrium vapor pressure over a plane surface of pure liquid water, or sometimes ice.

equilibrium vapor pressure—The pressure of a vapor in equilibrium with its condensed phase (liquid or solid). This equilibrium is dynamic in that evaporation and condensation are both occurring but are in equilibrium. It is a function of temperature only and is nearly independent of the existence or density of other gases in a system. It may, however, be dependent on the shape of the surface of the condensed phase. More generally, the equilibrium vapor pressure with respect to small droplets containing soluble impurities may be quite different from the saturation vapor pressure.