HUMIDITY HUMIDITY AND AND

DEW POINTDEW POINT

Humidity amount

of water vapor in the air

Specific Humidity ACTUAL

amount of water vapor in the air at a given time and placeUnits - g of

water vapor / Kg of air

Saturated AirAt a steady temperature the

amount of water vapor is at it’s peak (maximum)

Rate of condensation = Rate of evaporation

• Dependent on air temperature• Higher the air temp. - greater

the amount of water vapor in the air

• Water vapor in air doubles every 10°C

Dew Point The temperature at

which air is cooled to become saturated and condensation begins

More water vapor in the air

Less the air has to cool

Higher the Dew Point

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Relative Humidity Comparison of the specific humidity and

the maximum amount of water vapor that the air can hold at a given temperature and pressureUNIT - %

Relative humidity = specific humidity X 100% maximum capacity

100% Rel. Hum. – saturated air0% Rel. Hum. – dry air

Psychrometer Instrument to

measure humidity

Dry bulb – measures air temp.

Wet bulb – has a water soak wick around the bulb – water evaporates – cools the air

Evaporation is a cooling process

Psychrometer

Dry air - greater evaporation - greater cooling - greater the difference in the 2

temperatures High humidity – no evaporation (saturated

air)- temperatures of the 2

bulbs will be the same or very close

Evaporation question

Why do you feel so hot and sticky on hot, humid days in the summertime?

Answer

High humidity means air has a lot of water vapor in it. Your body sweats but the water has no where to go so it stays on your body and you feel sticky and aren’t cooled by the evaporation of the sweat.