Relative Humidity

Topics: Water vapor, Relative humidity, Vapor pressure Pages: 8 (1746 words) Published: February 21, 2013
Temperature and Humidity LabName__ __________________

This activity will look at two of the most fundamental and important elements of weather: temperature and humidity.


1. The students will discover the need for measuring instruments to quantify weather observations. 2. The students will learn how to use a sling psychrometer to determine temperature and relative humidity values. 3. The students will explain and understand the difference between absolute and relative humidity and the relationships between temperature and atmospheric humidity.


Atmospheric Temperature is one of the most frequently considered of the many meteorological elements. This influence of air temperature on the growth and well-being of earth’s life forms is paramount. Temperature is basically a measure of the molecular kinetic energy of a substance. (Or how fast the molecules are moving). Therefore reference points upon which compare molecular activities or temperatures are needed. The freezing point and boiling points of water are used as 32OF or 0OCis the freezing point and 212OF or 100OC is the boiling point.

Water is a basic necessity for the life forms here on Earth. At a given moment, only about 1/100,000 part of the earth’s supply of water is found in the atmosphere, yet it is this atmospheric water that brings moisture to the surface of the earth and makes it habitable. In the hydrologic cycle, water is constantly being moved from the earth’s surface into the troposphere where it gives rise to clouds and precipitation, thereby bringing the water back to the surface again.

Humidity is defined as the amount of water vapor in the atmosphere. It is measured in several ways. In one method, the mass of the water vapor is measured per unit volume, usually a cubic meter (m3). This measure of humidity is referred to as absolute humidity and reported as grams H2O/m3.

The humidity may also be expressed as a mixing ratio. The mixing ratio represents the mass of water vapor in the atmosphere measured in grams per kilogram mass (g/kg) of dry air.

Specific humidity, yet another way to measure and report humidity, is essentially the same as the mixing ratio, differing only in that the sample mass of air includes the mass of H2O vapor contained within it.

While each of the methods just described to report humidity involve measuring the mass of H2O vapor within a sample unit of air, there is yet another way to measure the amount of water vapor in the atmosphere that utilizes a different approach. The concept of partial vapor pressure breaks out the amount of the total atmospheric pressure attributable solely to H2O vapor. For instance if the atmospheric pressure is 1000 millibars, and H2O vapor represents 3% of the mass of the air (which is manifested as air pressure) we could describe the amount of H2O vapor present by reporting the actual vapor pressure as 30 millibars.

Relative humidity, the most widely reported measure of humidity, is not a direct measure of the amount of water vapor in the atmosphere, but rather the ratio of the air's water vapor content to its water vapor capacity at a given temperature. The capacity is the amount of H2O vapor needed to produce saturation at that particular temperature.

The various formulae for relative humidity are:

amount of water vapor in atmosphere (per unit) X 100 water vapor capacity of atmosphere (per unit)


mixing ratioX 100
saturation mixing ratio


actual vapor pressure X 100
saturation vapor pressure

Humidity can be expressed in several ways. One measure of humidity is vapor pressure. Every gas existing in air is contributing to part of the pressure being exerted by the air. The vapor pressure is the part of the total atmospheric pressure being exerted by the water vapor in the air. Therefore, it is a...
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