Physical ScienceModule 2

Air and Relative Humidity

Water/Air

Under usual Earth conditions, water exists in any of three physical states:

solid (ice, snow) liquid (which we typically call water) gas (humidity, water vapor [steam])

Humidity

According to meteorologists, humidity can be Absolute or Relative

Giving us the terms absolute humidity and relative humidity

In common usage, the adjective is often dropped, and the speaker (or writer) just says "humidity," expecting us to know of which type they are speaking

Even some television and radio weathercasters, including those formally trained in meteorology, often do not clarify the difference when reporting the weather conditions

Humidity

Absolute humidity is defined as the ratio of the mass of water vapor contained per volume of moist air. 

Relative humidity, technically, is the ratio between the partial pressure of water in the air and the maximum possible vapor pressure of water at a particular temperature.

Relative humidity – is “Relative” to us…it’s what we feel and what the meteorologist discusses on the news channel

Relative Humidity

It is dimensionlessRelative humidity is usually what the

media announcers mean when they say "humidity," and it is useful in determining conditions for human comfort.

But, it can be confusing because its value varies with air temperature.

Relative Humidity

For example, the morning may have a relative humidity of 78%, which by afternoon drops to 53% as the air temperature rises. (Note: the absolute humidity for that day remained essentially constant, but the vapor pressure of water increased with the temperature).

Relative Humidity

Similarly in winter, the outdoor relative humidity may be 63%, but when outdoor air permeates our warm homes and offices, the relative humidity level may drop to 35% or lower. (In this example, the absolute humidity is quite low in the outdoor air, but the vapor pressure of water at cold temperatures is also low, thus, the outdoor air is more humid, relatively speaking.)

Short Movie on Humidity

http://glencoe.mcgraw-hill.com/sites/dl/free/0078600472/161752/00038308.html