Humidity

Related Topics:
Humidity_Control

Humidity is the concentration of water vapor in the air. The concentration can be expressed as absolute humidity, specific humidity, or relative humidity. A device used to measure humidity is called a hygrometer. A humidistat is used to regulate the humidity of a building with a de-humidifier. These can be analogous to a thermometer and thermostat for climate control. Due to the changing partial pressure of water vapor in air as temperature changes, the water content of air at sea level can get as high as 3% at 30 °C (86 °F), and no more than about 0.5% at 0 °C (32 °F).

Absolute humidity

Absolute humidity is a method of expressing the amount of water vapor by using the ratio of the mass of the water vapor to the volume of the air. Absolute humidity is expressed as a ratio of kilograms of water vapor,

m_w

, per cubic meter of air,

v_a

AH = {m_w \over v_a}

As such, its value changes as the air pressure changes.

Specific humidity

Specific humidity or the mixing ratio is a method of expressing the amount of aqueous vapor in air by using a ratio of water vapor to dry air. Specific humidity is expressed as a ratio of kilograms of water vapor,

m_w

, per kilogram of air,

m_a

That ratio can be given as:

SH = {m_w \over m_a}

Partial pressure of water vapor and air can also be used to express the ratio.

Relative humidity

Relative humidity is the ratio of the current vapor pressure of water in any gas (especially air), known as the absolute vapor pressure (AVP), to the equilibrium vapor pressure or saturation vapor pressure (SVP), at which the gas is called saturated at the current temperature, expressed as a percentage.

Formulaically expressed as:

RH = {AVP \over SVP}

Equivalently, it is the ratio of the current mass of water per volume of gas and the mass per volume of a saturated gas.

The numerators of these ratios are the two ways of expressing absolute humidity. The following graph compares dew point (maximum humidity in red) to 50% relative humidity (green line halfway between zero and the dew point across the range of temperatures).

A gas in this context is referred to as saturated when the vapor pressure of water is at the equilibrium vapor pressure for water vapor; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It also corresponds to the possibilility of dew or fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of droplets of liquid. (Even though these droplets may be so small as to fall imperceptibly slowly through the mixed gas we call air, this behavior is too different from that of water vapor to reflect it in the same scale. This explains the restriction of relative-humidity discussions to 100% and below.)

The statement that relative humidity can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber, created by Charles T. R. Wilson for nuclear physics experiments, which uses an extremely brief state of supersaturation to accomplish its function.

Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by preventing the evaporation of perspiration from the skin. This effect is calculated in a heat index table.

Why humidity can be less than 100% when it's raining

Humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens when the air rises and cools. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.

Humidity and air density

Humid air is lighter, or less dense, than dry air. This is due to the fact that a molecule of water weighs less than a Nitrogen or Oxygen molecule (since air is composed mostly of the latter two).

Scientists have known humid air is lighter than dry air for a long time. The first was Isaac Newton, who, in 1717, stated that humid air is less dense than dry air in his book Optics.

To see why humid air is less dense than dry air, one must consider one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple terms, he found that a fixed volume of gas at a given temperature and pressure would always contain the same number of molecules regardless of what kind of gas it is. This is known as the ideal gas law.

Consider a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which each have an atomic weight of 28. Another 21% of the air is oxygen, with each molecule having an atomic weight of 32. The final one percent is a mixture of other gases, which is negligable. Molecules are free to move in and out of this cubic foot of air. What Avogadro discovered leads to the conclusion that if water vapor molecules are added to the cubic foot of air, some of the nitrogen and oxygen molecules would leave because the total number of molecules in our cubic foot of air stays the same. The water molecules that replace nitrogen or oxygen have an atomic weight of 18. This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, the density decreases.

This may seem counterintuitive as water is commonly percieved to be much heavier than air. It is true that liquid water is heavier, or more dense, than air. However, the water that makes the air humid is not liquid, it is water vapor, which is lighter than nitrogen or oxygen gas.

Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. But, humid air is lighter than dry air at the same temperature and pressure.

Effects on human body

Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot lose its heat, resulting in a condition called hyperpyrexia. With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain, and other internal organs. Physical strength declines and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected. This resulting condition is called heat stroke or hyperthermia.

Recommendations for comfort

Humans and animals control their body temperature by sweating. The US Environmental Protection Agency cites the ASHRAE Standard 55-1992 Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%, with below 50% preferred to control dust mites. At high humidity sweating is less effective so we feel hotter. Dry air feels colder at room temperature leading to discomfort, lower productivity and demands for more heating. When relative humidity is ideal, temperatures in buildings can be lowered without causing discomfort to people in them.

Popular culture

First spoken by Warren Hymer in the 1939 movie Mr. Moto on Danger Island, the expression "It's not the heat, it's the humidity" refers to unpleasantly muggy weather; that is, the fact that humid air can be significantly less comfortable than drier air at the same temperature.

References

United States Environmental Protection Agency, "IAQ in Large Buildings". Retrieved Jan. 9, 2006.

External links

Glossary definition of absolute humidity - National Science Digital Library
Glossary definition of psychrometric tables - National Snow and Ice Data Center
Glossary definition of specific humidity - National Snow and Ice Data Center

Atmospheric thermodynamics | Physical quantity | Psychrometrics

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