# Study of Moisture content in air - Pychrometry

by Dasa . 27 No 2021

The study of moisture content in the air is Psychrometry.
Dry air is the mixture of various gases but without moisture. Dry air does not exist in nature, and the air always contains some water vapor.

The water vapor is less dense than dry air. So the air takes more water vapor, the overall density of the air reduces, and pressure reduces.

Humidity Density Pressure

The air takes more moisture from the surroundings with an increase in temperature. So the moisture carrying capacity of air increases with an increase in temperature.

Temperature Air Moisture carrying capacity Humidity

The above is true when there are a surplus amount of water bodies nearby. But when there is no water body nearby, no moisture addition is happening. So

Temperature Air Moisture carrying capacity Humidity

In summer, coastal areas will have high humid weather & lands away from sea or lakes will have dry, hot weather.

In winter, the air will have less moisture content as the moisture carrying capacity of air is less. So the humidity in the air will start condensing into water.

The density of air is 0.0765 lb/ft­³ @ STP
The density of air is 0.075 lb/ft³ @ 14.696 psi & 70° F.

STP - Standard Temperature and Pressure - 0°C/273.15°K/32°F & 1atm/1.01325bar/ 14.696psi

Click here amount of moisture content, to know the method to calculate the moisture content in the air.

## Dry bulb temperature DBT

Dry-bulb temperature is the air temperature measured by a dry-bulb(a standard thermometer) that is freely exposed to the air and away from radiation and moisture.

## Wet-bulb temperature WBT

Wet-bulb temperature is the air temperature measured by a wet-bulb( cover the tip of the thermometer bulb with wetted cloth-muslin sleeve) that is freely exposed to the air and away from radiation and moisture.

## Wet-bulb depression

The difference in temperature between the dry bulb and the wet bulb is called Wet-bulb depression.

Low wet-bulb depression indicates higher humidity.
When,
75° F DBT & 68° F WBT will have 70.3% RH and
75° F DBT & 72° F WBT will have 86.74% RH

## Dew Point temperature DPT

It is the air temperature at which condensation of moisture begins when the air is cooled.