Dew Point & Relative Humidity

1. Dew Point & Relative Humidity How to use a Psychrometer

2. 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

3. The “Dry Bulb” • Don’t let it fool you. It is just a thermometer. • measures air temperature

4. The “Wet Bulb” • wet “sock” tied to bulb (called ‘wick’) • cools as water evaporates

5. 14°C 20°C 12°C 8°C A Dry Day… • lots of moisture evaporates • wet bulb will be much cooler than dry bulb 20°C Difference between wet bulb & dry bulb is 12 °C.

6. A Humid Day… • little moisture will evaporate • wet bulb will be slightly cooler than dry bulb 20°C 14°C Difference between wet bulb & dry bulb is 6 °C.

7. 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.

8. Dry-bulb temperature is your air temperature. 20 °C

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

10. 20°C 14°C 20-14=6

11. Put it all together 20°C 8°C 20-8=12 Relative Humidity = 11%

12. 14°C 10°C 14-10=4 Relative Humidity = 60%

13. The Dew Point Chart works the same way 14°C 10°C 14-10=4 Dew Point = 6°C

14. Relative Humidity (%)

15. Dew Point (˚C)