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

3. Water in the Atmosphere Humidity: measure amount of water vapor stuck between molecules in atmosphere; Humidity: measure amount of water vapor stuck between molecules in atmosphere; air’s ability to hold water depends on the air temp air’s ability to hold water depends on the air temp The hotter the air, the more water the air can hold 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 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 If relative humidity is 100% then precipitation is occurring Dew Point – temperature at which RH = 100% Dew Point – temperature at which RH = 100% If dew point = actual temperature then precipitation is occurring If dew point = actual temperature then precipitation is occurring Humidity: measure amount of water vapor stuck between molecules in atmosphere; Humidity: measure amount of water vapor stuck between molecules in atmosphere; air’s ability to hold water depends on the air temp air’s ability to hold water depends on the air temp The hotter the air, the more water the air can hold 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 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 If relative humidity is 100% then precipitation is occurring Dew Point – temperature at which RH = 100% Dew Point – temperature at which RH = 100% If dew point = actual temperature then precipitation is occurring If dew point = actual temperature then precipitation is occurring

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

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

6. A Dry Day… lots of moisture evaporates lots of moisture evaporates wet bulb will be much cooler than dry bulb wet bulb will be much cooler than dry bulb 8°C 20°C Difference between wet bulb & dry bulb is 12 °C. 20°C14°C 12°C

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

8. Finding Dew Point and Relative Humidity Warning #1: Be sure to READ the correct chart: DPT or RH 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 #2: Dew Point Temperature IS NOT “Difference between wet bulb and dry bulb”. Warning #3: The wet bulb temp IS NOT the DPT. Warning #3: The wet bulb temp IS NOT the DPT.

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

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

11. 14°C 20°C 20-14=6

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

13. 10°C 14°C 14-10=4 Relative Humidity = 60%

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

15. Relative Humidity (%)Relative Humidity (%)

16. Dew Point (˚C)Dew Point (˚C)