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  Humidity
What Is Moisture ?
Condensed or diffused water / liquid content in the air / substance is known as moisture.
 

What Is Humidity ?
Water vapor content in the air is known as humidity.
Several ways to express the humidity of air
 
Absolute Humidity
 It expresses the water vapor content of the air using the mass of water vapor contained in a given volume of air.  It may be measured in grams of vapor / cubic meter of air. A problem with using absolute humidity is that an air parcel changes volume as the ambient temperature and pressure change.  This means that the absolute humidity changes when the volume changes, even though the mass of water vapor has not changed.
 
Specific Humidity
 It measures the water vapor content of the air using the mass of the water vapor for a given mass of air.  It may be measured in grams of water vapor per kilogram of air.  The kilogram of air measured includes the water vapor present (compare this to mixing ratio, below).  Unlike absolute humidity, specific humidity does not change as the air parcel expands or is compressed.
Mixing Ratio 
It measures the water vapor content of air using the mass of water vapor for a given mass of dry air. It may be measured in grams of water vapor per kilogram of dry air.
 
Notice The Difference Between Mixing Ratio And Specific Humidity:
Specific Humidity includes the water vapor in the air in the denominator, while mixing ratio measures water vapor per mass of dry air.  Since water vapor comprises only a few percent of the mass of air, the values for specific humidity and mixing ratio are very close for a given parcel of air.  Mixing ratio is not affected by changes in pressure and temperature.  This is a commonly used measure by meteorologists.  At a temperature of 20 0C, at average sea level pressure, the saturation mixing ratio is 14 grams of water vapor per kilogram of dry air.
 
Vapor Pressure
It measures the water vapor content of the air using the partial pressure of the water vapor in the air.  (Pressure may be expressed using a variety of units: pascals, millibars, pounds per square inch, among others). The gases in the atmosphere exert a certain amount of pressure (about 1013 millibars / 1.013 bar at sea level).  The contribution by water vapor is rather small, since water vapor only makes up a few percent of the total mass of a parcel of air.  The vapor pressure of the water in the air at sea level, at a temperature of 20 0C, is 24 mb at saturation.
 
Saturated Vapour Pressure
The process of evaporation in a closed container will proceed until there are as many molecules returning to the liquid as there are escaping.  At this point the vapor is said to be saturated, and the pressure of that vapor (usually expressed in mm Hg) is called the saturated vapor pressure.Most of these measures of humidity cannot be easily determined directly.  It is actually easier to calculate relative humidity.
 
Relative Humidity
It is a ratio that compares the amount of water vapor in the air with the amount of water vapor that would be present in the air at saturation.  It can also be stated as the ratio of the actual mixing ratio to the saturation mixing ratio.  Relative humidity is given as a percentage: the amount of water vapor is expressed as a percentage of amount of water vapor at saturation.  If 20 grams of water vapor were present in each kilogram of dry air, and the air would be saturated with 40 grams of water vapor per kilogram of dry air, the relative humidity would be 20/40=50%.
 

Dew Point Temperature (Td)
It is the temperature at which the water vapor pressure in gas mixture equals the saturation water vapor pressure and is usually expressed in °F or °C.  Any plane surface in a given environment which is colder than the dew point temperature of the air or gas surrounding it will accumulate a dew or frost layer.
 
Frost Point (Td)
Below 0 °C, frost will exist on plane surfaces.  It should be noted that the saturation water vapor pressure is different over ice than water.
 
Supercooled Water
Below 0 °C and typically as cold as -20 °C, water can exist on smooth plane surfaces in liquid form rather than ice crystals.  This can lead to some ambiguity when utilizing chilled mirror hygrometers. Different types of sensors with different measuring principles are commonly used for measurement of humidity:
 
Dry And Wet-bulb Instrument Or Psychrometer
A simple and yet ingenious method to determine air humidity is an instrument that contains two thermometers, also called bulbs: one measures the normal air temperature while the other is continuously wet and measures the wet-bulb temperature.  The wet-bulb is covered by a wick that is hanging in a small reservoir with clean water.   The wick sucks water and keeps the wet-bulb continuously wet.  A built-in fan maintains a constant air flow along the two bulbs.  The air flow absorbs water from the wick, which cools the wet-bulb (evaporative cooling).  The more dry the air flow is, the more water will be absorbed from the wick, and the more the temperature of the wet-bulb will drop.  Under rather dry conditions, the wet-bulb is a few degrees cooler than the dry-bulb.
 
Electronic Humidity Sensor Or Capacitive Sensor
Electronic sensors, mostly capacitive sensors, are usually relatively cheap and easy to install and maintain.  There exists a relationship between capacitance and the humidity to be measured.  Capacitance is transferred into a signal that is converted by the computer to RH or another humidity reading.  They have a major disadvantage: they easily become unreliable particularly at high humidity levels.  They are usually good when new, but they quickly deteriorate under rough conditions like changing humidity and spraying. In tests, most electronic devices, even the more expensive ones, became inaccurate within 3 to 9 months after purchase.  If you choose to use an electronic sensor for humidity control, it should be calibrated every month, particularly under wet conditions.  Keep in mind that calibration under wet conditions with another electronic device is very dubious.
 
 
Hair-hygrometer
Hair is made from keratin, a protein that is wound into a coil.  The turns of the coil are held together by a type of chemical bond called a hydrogen bond.  Hydrogen bonds break in the presence of water, allowing the coil to stretch and the hair to lengthen.  The bonds re-form when the hair dries, which allows people to style their hair simply by wetting it, shaping it, then drying it.
A hygrometer measures humidity or the amount of moisture in the air.  This is done by measuring the change in length of an organic fibre (e.g. human hair) brought about by the absorption of moisture.
The Hair Hygrometer works on the principle that human (and horse) hair changes its length in accordance with the relative humidity of the atmosphere and not the vapour pressure.  It is a fairly simple device but not very accurate.  It serves well as a household device, but as the hair ages, they lose their elasticity and eventually fail to register any change in humidity.  A hair hygrometer may be designed to activate a pen on a clock-driven drum.  Such an instrument, called a Hygrograph gives a continuous record of fluctuations in humidity over a given time period.
A mechanical hygrograph uses the same principle.  It takes a bunch of hair, and transmits the movement through mechanical means to an armature with a marking implement at one end.  This produces a line on a slowly rotating drum.  The readings can be plotted on graph paper and can also be stored in a Data Logger.
 
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Last Updated April 28, 2009
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