Relative humidity and dew point

Humidity

• It is an indicator of the amount of water vapor in air
• Its is divided into absolute and relative humidity.

Absolute Humidity:

• Mass of water vapor present in unit volume of air is called absolute humidity.

Relative Humidity:

• Ratio of mass of water vapor present in certain volume of air and mass of water vapor required to saturated it at same condition.
• $\therefore RelativeHumidity=\frac{m}{M}\times 100$ where m= mass of water vapor present and M is mass of water vapor required to saturated.
• Is is unit less quantity.

Dew point

• It is the temperature at which the water vapor present in air is just sufficient to saturated it.
• $\therefore$The vapor pressure at room temperature is equal to the saturated vapor pressure at dew point
• Mass of water vapor present in atmosphere is directly proportional to saturated vapor pressure (svp).
• $\therefore \frac{m}{M}=\frac{p}{P}$where, p is svp at dew point and P is SVP at room temperature

Note:

RH = $\frac{m}{M}\times 100$%

The negative slope of the fusion curve indicates that the melting point of ice decrease with increase of pressure