What is Humidity?? * Humidity is the state of atmosphere in relation to amount of water vapor it contains. Humidity is closely related to its temperature – higher the air temperature, more vapor the air can hold. * For this reason, saturation vapor pressure (ew) goes up with air temperature; i.e., as temperature goes up ew also goes up.
Significance of Humidity
The amount of water vapor in air effectively controls the weather condition by controlling evapotranspiration from land and water surfaces.
Evaporation rate is proportional to difference between saturated vapor pressure at water temperature (ew) and actual vapor pressure in air (ea).
EL = C (ew – ea),
Where, EL is lake evaporation rate, C is a constant of proportionality.
Causes of Humidity
Molecules of water having sufficient kinetic energy to overcome attractive forces tending to hold them within the body of liquid water are projected through the water surface into the air.
The process by which liquid water is converted into vapor is called vaporization or evaporation. Since the kinetic energy increases and surface tension decrease as temperature rises, evaporation rate increases with temperature.
Most of the atmospheric vapor is the product of evaporation from water surfaces. The direct transformation from ice to vapor, and vice versa, is called sublimation. The process by which vapor changes to the liquid or solid state is called condensation.
Properties of Water Vapor
The partial pressure exerted by water vapor is called vapor pressure (e). If all the water vapor in a closed container of moist air with an initial total pressure p were removed, then the final pressure p’ of dry air alone would be less than p.
Then, e = p – p’
When the maximum amount of water vapor for a given temperature is contained in a given space, the space is saturated with water vapor. The pressure exerted in a saturated space is called saturation vapor pressure (ew), which is the maximum vapor pressure possible at a given temperature. ew = f (air temperature)
The latent heat of vaporization is the amount of heat absorbed by a unit mass of a substance, without change in temperature, which passing from liquid to vapor state. A change from vapor state to liquid state releases equal amount of heat. Latent heat of vaporization ()

The latent heat of vaporization, , expresses the energy required to change a unit mass of water from liquid to water vapor in a constant pressure and constant temperature process. The value of the latent heat varies as a function of temperature. At a high temperature, less energy will be required than at lower temperatures. As varies only slightly over normal temperature ranges a single value of 2.45 MJ kg-1 is taken in the simplification of the FAO Penman-Monteith equation. This is the latent heat for an air temperature of about 20°C. Source: FAO

The heat of vaporization of water (Hv) varies with temperature, but can be determined accurately up to 40°C by
Hv = 2.50 – 0.00236 T
(Hv is in kilojoules per gram, and T is in degree Celsius) or by
Hv = 597.3 – 0.564 T
(Hv is in calories per gram, and T is in degree Celsius).

The latent heat of fusion for water is the amount of heat required to convert one gram of ice to liquid water at same temperature. When one gram of liquid water at 0°C freezes into ice at same temperature, the latent heat of fusion (0.337 kJ/g or ≈ 80 cal/g) is liberated.
The latent heat of sublimation for water is the amount of heat required to convert one gram of ice into vapor at same temperature without passing through intermediate liquid state. It is equal to the sum of the latent heat of vaporization and latent heat of fusion.
At 0°C the latent heat of sublimation for water is about 2.837 kJ/g (2.5 + 0.337). Direct condensation of vapor into ice at same temperature liberated an equivalent amount of heat ( ≈ 677 cal/g). The value of 677 comes from the addition of 597.3 and 80.
The specific gravity of water vapor is 0.622 times that of dry air at the same temperature and pressure. The density of water vapor ρv in grams per cubic centimeter is ρv = 6.22(eT*Rg)
Where T is the absolute temperature in Kelvin(K) and Rg, the specific gas constant for dry air, equals 2.87 x 103 cm2s-2K-1 when the vapor pressure e is in kilopascal.
The density of dry air ρd in grams per cubic centimeter is ρd = 10(pdT*Rg)
Where pd is the pressure in kilopascals.
The density of moist air is equal to the mass of water vapor plus the mass of dry air in unit volume of the mixture. If pa is the total pressure of the moist air, pa-e for pd gives pa=10paT*Rg*(1-0.378 epa )
This equation shows that moist air is lighter than dry air
Measures of Atmospheric Moisture * Vapor Pressure
One of the empirical equations used to calculate vapor pressure (e) is: e = ew – (0.000367) (5 p/9) (T – Tw) [1 + (5 Tw – 448)/14139]
Where,
* T and Tw are dry- and wet-bulb temperature (°C) of a psychrometer consisting of two thermometers, * ew is the saturation vapor pressure (mb) corresponding to Tw, and * p is the atmospheric pressure (mb).

* Atmospheric Pressure (P)
The atmospheric pressure, P, is the pressure exerted by the weight of the earth's atmosphere. Evaporation at high altitudes is promoted due to low atmospheric pressure as expressed in the psychrometric constant. The effect is, however, small and in the calculation procedures, the average value for a location is sufficient. A simplification of the ideal gas law, assuming 20°C for a standard atmosphere, can be employed to calculate P:

Where,
P atmospheric pressure [kPa], z elevation above sea level [m], * Dew Point
Dew point: It is the temperature at which the space becomes saturated when air is cooled under constant pressure and with constant water vapor content. It is the temperature having saturation vapor pressure ew = existing vapor pressure e. * Mixing Ratio
Mixing Ratio (wr): The mixing ratio is the mass of water vapor per unit mass of perfectly dry air in a humid mixture. wr=0.622 (epa-e )
(Mass of water vapor)(Mass of dry air) [g/kg]

* Depth of precipitable water: It is the amount of water vapor in a layer of air. * Absolute Humidity
Absolute Humidity: It is the mass of water vapor contained in a unit volume of air at any instant. ρw = 217 (e/T) where e is in mb and T is in °C. or (Mass of water vapor)(Volume of air) [g/m3]

* Specific Humidity
Specific Humidity (q): It is the mass of water vapor per unit mass of moist air. q = (622 e) / (p – 0.378 e) ≈ 622 e /p, where e = vapor pressure (mb) and p = total pressure of the moist air (mb).
(Mass of water vapor)(Mass of air) [g/kg] * Relative Humidity
It is the percentage ratio between the actual vapor pressure (e) and the saturation vapor pressure (ew) at the same temperature. The relative humidity is not a direct measure of moisture in air.
H = 100 (e/ew)
H = [(112 – 0.1 T + Td)/(112 + 0.9 T)]8
The relative humidity may also be defined as the percentage ratio between the amount of water vapor actually contained per unit volume and the amount of water vapor that it can hold at the same temperature when saturated.
Hygrometers
Psychrometer
Principle of measurement
When water or ice covers the bulb of a thermometer (wet-bulb), latent heat is removed from the surface of the bulb as the water evaporates, and the wet-bulb temperature becomes lower than the air (dry-bulb) temperature.
At a lower humidity, water evaporates more actively, so that the wet-bulb temperature lowerssharply. The aspirated psychrometer measures humidity by measuring the difference between the dry-bulb temperature and wet-bulb temperature
Psychrometric formula and Psychrometric table

When the air steadily flows around the wet-bulb, the wet-bulb temperature falls below the air temperature by water evaporation from the surface of the wet-bulb. When the heat flow moving into the wet-bulb from the ambient air has reached equilibrium with the latent heat flow removed from the wet-bulb by evaporation, the following equation, called the Sprung psychrometric formula, is derived with the Assuman type aspirated psychrometer, e= ew- (A/755) p (t-tw) ..................................equation(1)
Where:
A/755: Psychrometer constant, A is 0.50 when the wet-bulb is not frozen and 0.44 when it is frozen. e: Vapor pressure hPa ew: Saturation vapor pressure ,hPa p: Atmospheric pressure hPa t: Dry-bulb temperature ℃ tw: Wet-bulb temperature ℃

Vapor pressure is calculated with this equation (1). Table 3.1 and Table 3.2 show the saturation vapor pressure for water and ice as a function of temperature.
The second term on the right side of the equation (1) is calculated a function of p and (t-tw), which is tabulated as the vapor pressure table in Table 3.3 and Table 3.4 for the unfrozen and frozen wet-bulb

Calculations of vapor pressure, dewpoint temperature, and relative humidity
The vapor pressure, dewpoint temperature, and relative humidity are calculated from the measurement with the aspirated psychrometer using the tables described above.

Calculation of vapor pressure
1) Make correction of the instrumental error of the dry-and wet-bulb thermometer.
2) Using Table 3.1 or 3.2, determine the value of the saturation vapor pressure for water (ew) or ice (ei) as a function of the wet-bulb thermometer temperature (tw).

Calculation of vapor pressure(continuation)
3) Using Table 3.3 and 3.4 calculate the second term on the right side of the equation (1) as a function of atmospheric pressure (p) and (t-tw), where t is the dry-bulb thermometer temperature.
4) The vapor pressure e is obtained by making a subtraction between the above two values.

Determination of dewpoint temperature
The dewpoint temperature is determined as a function of the vapor pressure in Table 3.1. If the vapor pressure is too low to find out in Table 3.1, calculate the dewpoint pressure by interpolation to 1/100. If the vapor pressure is equal to or less than 0.05 hPa, consider the dewpoint temperature to be less than –50℃

Calculation of relative humidity
Determine the saturation vapor pressure (ew), as a function of (tw) and then calculate the ratio of the vapor pressure (e) to (ew). If the vapor pressure is determined to be a minus value, consider the relative humidity to be zero.

Hair hygrometer
Principle of measurement and structure
The hair hygrometer uses the characteristic of the hair that its length expands or shrinks response to the relative humidity. the dimensions of various organic materials vary with their moisture content. A humidity change takes an effect on the moisture content in such materials. The length of human hair from which liquid are removed increases by 2 to 2.5% when relative humidity changes by 0 to 100%. Different types of human hair show different changes in length. However, there is still a relationship between the length of hair and relative humidity.
The hair hygrograph is a hair hygrometer to which a clock-driven drum is installed to record humidity no a recording chart

Chilled-mirror dewpoint hygrometer
Structure and composition sensor (mirror)
The basic structure of the sensor unit for a chilled-mirror dewpoint hygrometer is shown in Figure 3.7.
Sample air is drawn to the metallic mirror surface through piping to determine the dewpoint temperature. As the mirror cools, condensation forms when its surface temperature falls below the dewpoint temperature, but evaporates and disappears at higher temperatures. The temperature of the metallic mirror when condensation forms is measured using a platinum resistance thermometer, and the result is taken as the dewpoint temperature. Condensation conditions are monitored using a photo-detector with the reflection of a light-emitting diode (LED) on the mirror. Irradiated light is scattered when condensation is present, and the amount of reflected light changes with the mirror’s surface condition. A peltier element is used to control the mirror’s temperature..

Geographic Distribution of Humidity
Atmospheric moisture tends to decrease with increasing latitude, but relative humidity, being an inverse function of temperature, tends to increase. Atmospheric moisture is greatest over oceans and decreases with distance inland. It also decreases with the elevation and is greater over vegetation than over barren soil.

Time Variations in Humidity
Like temperature, atmospheric water vapor is at a maximum in winter and at a maximum in summer. The seasonal variation is much less over oceans and coastal areas and is at a minimum over tropical seas. Unlike actual water vapor content, relative humidity is at a minimum in summer and at a maximum winter
The diurnal variation of atmospheric moisture is normally small, except where land and sea breezes bring air of differing characteristics. Near the ground surface, condensation of dew at night and reevaporation during the day may result in a minimum moisture content near sunrise and a maximum by noon. Relative humidity, of course, behaves in a manner opposite to that of temperature, being at a maximum in the early morning and at a minimum in the afternoon