Relative humidiy ( humidity)

Relative humidity

The ratio = the vapor pressure / the saturation vapor pressure

The ratio = the mixing ratio / the saturation mixing ratio.

These two definitions yield almost identical numerical values.

Relative humidity is usually expressed in percent and can be computed from psychrometric data.Unless specified otherwise, relative humidity is reported with respect to waterrather than ice because most hygrometers are sensitive to relative humidity with respect to water even at subfreezing temperatures, and because the air can easily become supersaturated with respect to ice, which would require three digits in coded messages for relative humidity with respect to ice.

mixing ratio

The ratio of the mass of a variable atmospheric constituent to the mass of dry air.

If not otherwise indicated, the term normally refers to water vapor. For many purposes, the mixing ratio may be approximated by the specific humidity. Either r or w is commonly used to symbolize water vapor mixing ratio, with r used for thermodynamic terms in this glossary. In terms of the pressure p and vapor pressure e, the mixing ratio r is

saturation mixing ratio

the value of the mixing ratio of saturated air at the given temperature and pressure.

specific humidity

In a system of moist air, the ratio of the mass of water vapor to the total mass of the system.

The specific humidity q is related to the mixing ratio  r_v by

dewpoint (Or dewpoint temperature.)

The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur.

When this temperature is below 0°C, it is sometimes called the frost point.

The dewpoint may alternatively be defined as the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor.

Isobaric heating or cooling of an air parcel does not alter the value of that parcel's dewpoint, as long as no vapor is added or removed.

Therefore, the dewpoint is a conservative property of air with respect to such processes.

However, the dewpoint is nonconservative with respect to vertical adiabatic motions of air in the atmosphere.

The dewpoint of ascending moist air decreases at a rate only about one-fifth as great as the dry-adiabatic lapse rate.

The dewpoint can be measured directly by several kinds of dewpoint hygrometers or it can be deduced indirectly from psychrometers or devices that measure the water vapor density or mixing ratio.

The Dry Bulb, Wet Bulb and Dew Point temperatures are important to determine the state of humid air. The knowledge of only two of these values is enough to determine the state - including the content of water vapor and the sensible and latent energy (enthalpy).

Dry Bulb Temperature - T_db

The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature.
The Dry Bulb Temperature refers basically to the ambient air temperature. It is called "Dry Bulb" because the air temperature is indicated by a thermometer not affected by the moisture of the air.
Dry-bulb temperature - T_db, can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture. The temperature is usually given in degrees Celsius (^oC) or degrees Fahrenheit (^oF). The SI unit is Kelvin (K). Zero Kelvin equals to -273^oC.
The dry-bulb temperature is an indicator of heat content and is shown along the bottom axis of the psychrometric chart. Constant dry bulb temperatures appear as vertical lines in the psychrometric chart.

Wet Bulb Temperature - T_wb

The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.
Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.
The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapor.
The wet bulb temperature is always lower than the dry bulb temperature but will be identical with 100% relative humidity (the air is at the saturation line).
Combining the dry bulb and wet bulb temperature in a psychrometric diagram or Mollier chart, gives the state of the humid air. Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in the Psychrometric Chart.

Dew Point Temperature - T_dp

The Dew Point is the temperature at which water vapor starts to condense out of the air (the temperature at which air becomes completely saturated). Above this temperature(dew point temp)  the moisture will stay in the air.

• if the dew-point temperature is close to the dry air temperature -  the relative humidity is high
• if the dew point is well below the dry air temperature - the relative humidity is low

If moisture condenses on a cold bottle taken from the refrigerator, the dew-point temperature of the air is above the temperature in the refrigerator.
The Dew Point temperature can be measured by filling a metal can with water and some ice cubes. Stir by a thermometer and watch the outside of the can. When the vapor in the air starts to condensate on the outside of the can, the temperature on the thermometer is pretty close to the dew point of the actual air.
The Dew Point is given by the saturation line in the psychrometric chart.

Dew Point Temperature Charts

Dew point temperatures from dry and wet bulb temperatures are indicated in the charts below.

Dew Point Temperature Charts

Dew Point Temperature Chart in degrees Celcius

Dew Point Temperature Chart in degrees Fahrenheit