Dew Point & Relative Humidity

Dew Point & Relative Humidity

How to use a Psychrometer

Water in the Atmosphere

Humidity: measure amount of water vapor stuck between molecules in atmosphere; air’s ability to hold water depends on the

air temp The hotter the air, the more water the air

can hold Relative humidity: amount of water vapor

(%) present compared to total amount air can hold at a given temperature If relative humidity is 100% then

precipitation is occurring Dew Point – temperature at which RH = 100%

If dew point = actual temperature then precipitation is occurring

The “Dry Bulb” Don’t let it fool

you. It is just a thermometer.

measures air temperature

The “Wet Bulb” wet “sock” tied

to bulb (called ‘wick’)

cools as water evaporates

A Dry Day… lots of moisture

evaporates wet bulb will be

much cooler than dry bulb

8°C

20°C

Difference between wet bulb & dry bulb is 12 °C.

20°C14°C12°C

A Humid Day… little moisture

will evaporate wet bulb will be

slightly cooler than dry bulb

14°C

20°C

Difference between wet bulb & dry bulb is 6 °C.

Finding Dew Point and Relative Humidity Warning #1: Be sure to READ the

correct chart: DPT or RH Warning #2: Dew Point Temperature IS

NOT “Difference between wet bulb and dry bulb”.

Warning #3: The wet bulb temp IS NOT the DPT.

Dry-bulb temperature is your air temperature.

20 °C

Subtract (the difference) between the dry bulb and wet bulb

14°C20°C

20-14=6

8°C20°C

20-8=12

Put it all together

Relative Humidity = 11%

10°C14°C

14-10=4

Relative Humidity = 60%

10°C14°C

14-10=4

Dew Point = 6°C

The Dew Point Chart works the same way

Relative Humidity (%)

Dew Point (˚C)