1. DPT & Relative Humidity Charts
Handy Dandy Earth Science Reference Tables Page 747

2. Weight of Water Vapor in Air Relative Humidity =
: The ratio of the air’s actual water content to its potential water vapor content at a given temperature.
Relative Humidity
Weight of Water Vapor in Air
Relative Humidity =
Weight of Potential Water Vapor in Air at Saturation
Basically, relative humidity tells you how saturated the air is with water.

3. A psychrometer is used to measure relative humidity.
The dry bulb measures the air temperature
The wet bulb gives a measure of how unsaturated the air is—the evaporation of water from the wet bulb lowers the temperature reading (and the evaporation depends on the saturation of the air)

4. Dewpoint Temperature: The temperature the air would have to be cooled to saturate the air (causes water to condense

5. The “Dry Bulb” Don’t let it fool you. It is just a thermometer.
It measures the air temperature.
Duh!
20°C

6. The “Wet Bulb” Has a little wet booty tied to the bottom.
Gets cool when water evaporates.
12°C
Wet Booty

7. A Dry Day… A lot of moisture will evaporate.
The wet bulb will be a lot cooler than the dry bulb.
20°C
20°C
14°C
12°C
8°C
Difference between wet bulb & dry bulb is 12 °C.

8. A Humid Day… A little bit of moisture will evaporate.
The wet bulb will not be much cooler than the dry bulb.
20°C
14°C
Difference between wet bulb & dry bulb is 6 °C.

9. Page 747 & 750 of your Handy Dandy Earth Science Reference Tables
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.

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

11. 14 °C

12. 4 °C

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

14. 20°C
14°C
20-14=6

15. 18°C
16°C
18-16=2

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

17. 14°C
10°C
14-10=4
Relative Humidity = 60%

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

19. Try These. 58% 17 °C 86% 4 °C 57% 4 °C 91% 16 °C 100% 14 °C 45% -9 °C
Dry Bulb
Wet Bulb
R Humidity
DPT
26°C
20 °C
6 °C
5 °C
12 °C
8 °C
18 °C
17 °C
14 °C
0 °C
-3 °C
58%
17 °C
86%
4 °C
57%
4 °C
91%
16 °C
100%
14 °C
45%
-9 °C

20. More Difficult. 82% 18 °C 45.5% -.5 °C 100% 17 °C 20% -17 °C 69%
Dry Bulb
Wet Bulb
R Humidity
DPT
21°C
19°C
11°C
6 °C
17 °C
17°C
2°C
-3 °C
9°C
6.5 °C
70°F
60 °F
82%
18 °C
45.5%
-.5 °C
100%
17 °C
20%
-17 °C
69%
3.5 °C
15 °C
21 °C
52%
11 °C(50 °F)

22. The End