1. HUMIDITY AND DEW POINT

2. Humidity
amount of water vapor in the air

3. Specific Humidity Saturated Air
ACTUAL amount of water vapor in the air at a given time and place
Units - g of water vapor / Kg of air
Saturated Air
At 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

4. 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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5. Relative Humidity
Comparison of the specific humidity and the maximum amount of water vapor that the air can hold at a given temperature and pressure
UNIT - %
Relative humidity = specific humidity X 100%
maximum capacity
100% Rel. Hum. – saturated air
0% Rel. Hum. – dry air

6. 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

7. Evaporation is a cooling process

8. 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

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

10. 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.