The atmosphere and the Weather

Water Vapor in the Atmosphere

Humidity

Humidity refers to any one of a number of ways of specifying the amount of water vapor in the air.

Suppose we enclose a volume of air in an imaginary, thin, elastic container - a parcel. The most common ways to express atmospheric water vapor content in this parcel of air are:

water vapor density or absolute humidity:

comparison of the weight (mass) of the water vapor with the volume of air in the parcel

specific humidity

comparison of the weight (mass) of the water vapor in the parcel with the total weight (mass) of all the air parcel

mixing ratio

comparison of the weight (mass) of the water vapor in the parcel with the weight (mass) of dry air

Humidity can also be expressed in terms of the water vapor pressure --> the push (force) that the water vapor molecules are exerting against the inside walls of the parcel.

Actual vapor pressure indicates the air's total water vapor content, whereas saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature.

Temperature-dependence of the saturation vapor pressure. Cooling of an under-saturated air mass (at a constant pressure) will produce saturated or supersaturated conditions.

Relative Humidity

Most commonly used way of describing atmospheric moisture, and most misunderstood.

Relative humidity -

ratio of the amount of water vapor actually in the air compared to the maximum amount of water vapor required for saturation at a particular temperature (and pressure)

		water vapor content
Relative humidity	=
		water vapor capacity

For example, air with a 50 percent relative humidity actually contains one-half the amount required for saturation.

Air with a 100 percent relative humidity is said to be saturated because it is filled to capacity with water vapor.

Dew point

The dew point temperature represents the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur.

In other words it is the temperature to which a parcel of air must cool before condensation exceeds evaporation.

The difference between air temperature and dew point temperature can indicate whether the relative humidity is low or high.

• when air and dew point temperature are far apart, the relative humidity is low
• when the are close to the same value, the relative humidity is high
• when air and dew point temperature are the same, the air is saturated and the relative humidity is 100 percent.

The dew point temperature is used to determine the start date of the Tucson summer monsoon. A monsoon day is defined as an average of 24 hourly dewpoint values that is 55°F or greater. The start of the monsoon is defined as three consecutive days of the daily dewpoint average being 55°F or greater.

Evaporation and Transpiration

The amount of water that actually evaporates from a given water surface in a given period depends on the following factors:

• Vapor pressure on the water surface. This factor depends on the water temperature. The higher the water temperature, the greater the surface water pressure. When water temperature is higher than the air temperature, evaporation will always take place.
• Vapor pressure in the air. The greater the vapor pressure of the air, the less evaporation there will be. The rate of evaporation varies directly with the difference between vapor pressure of the water surface and the water vapor pressure of the air.
• Wind. Air movement is usually turbulent, with moist air removed from near the water surface and replaced by dry air from above. Evaporation thus varies directly with the velocity of the wind. The higher the wind velocity, the more evaporation there is.

Measuring humidity

The common instrument used to obtain dew point and relative humidity is a psychrometer.

Instruments that measure humidity are commonly called hygrometers. One type -- called the hair hygrometer -- uses human (or horse) hair to measure relative humidity.

Sling psychrometer	Hair hygrometer

Finally, lets check out the current dew point temperature and relative humidity in the Univ. of Arizona campus.

Also, here are some US map of temperature, dew-point temperature and relative humidity.