DPT & Relative DPT & Relative Humidity ChartsHumidity Charts

Handy Dandy Earth Handy Dandy Earth Science Reference Science Reference Tables Page 747Tables Page 747

: The ratio of the air’s actual water content to its potential water vapor content at a given temperature.

Relative Humidity

• Basically, relative humidity tells you how saturated the air is with water.

Relative Humidity =Weight of Potential Water Vapor in Air at Saturation

Weight of Water Vapor in Air

• 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)

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

The “Dry Bulb”The “Dry Bulb”

Don’t let it fool Don’t let it fool you. It is just a you. It is just a thermometer.thermometer.

It measures the It measures the air temperature.air temperature.

Duh!Duh!

20°C

The “Wet Bulb”The “Wet Bulb”

Has a little wet Has a little wet booty tied to booty tied to the bottom.the bottom.

Gets cool when Gets cool when water water evaporates.evaporates.

12°C

Wet Booty

A Dry Day…A Dry Day…

A lot of A lot of moisture will moisture will evaporate.evaporate.

The wet bulb The wet bulb will be a lot will be a lot cooler than the cooler than the dry bulb.dry bulb.

8°C

20°C

Difference between wet bulb & dry bulb is 12 °C.

20°C14°C12°C

A Humid Day…A Humid Day…

A little bit of A little bit of moisture will moisture will evaporate.evaporate.

The wet bulb The wet bulb will not be will not be much cooler much cooler than the dry than the dry bulb.bulb.

14°C

20°C

Difference between wet bulb & dry bulb is 6 °C.

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

Warning #1: Be sure to READ the Warning #1: Be sure to READ the correct chart: DPT or RHcorrect chart: DPT or RH

Warning #2: Dew Point Temperature IS Warning #2: Dew Point Temperature IS NOT “Difference between wet bulb and NOT “Difference between wet bulb and dry bulb”.dry bulb”.

Warning #3: The wet bulb temp IS NOT Warning #3: The wet bulb temp IS NOT the DPT.the DPT.

Dry-bulb temperature is your air temperature.

20 °C

14 °C

4 °C

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

14°C20°C

20-14=6

16°C18°C

18-16=2

8°C20°C

20-8=12

Put it all together

Relative Humidity = 11%

10°C14°C

14-10=4

Relative Humidity = 60%

10°C14°C

14-10=4

Dew Point = 6°C

The Dew Point Chart works the same way

Try These.Try These.

Dry BulbDry Bulb Wet BulbWet Bulb R R HumidityHumidity

DPTDPT

26°C26°C 20 °C20 °C

6 °C6 °C 5 °C5 °C

12 °C12 °C 8 °C8 °C

18 °C18 °C 17 °C17 °C

14 °C14 °C 14 °C14 °C

0 °C0 °C -3 °C-3 °C

58% 17 °C°C

86% 4 °C°C

57% 4 °C°C

91% 16 °C°C

100% 14 °C°C

45% -9 °C°C

More Difficult.More Difficult.

Dry BulbDry Bulb Wet BulbWet Bulb R R HumidityHumidity

DPTDPT

21°C21°C 19°C19°C

11°C11°C 6 °C6 °C

17 °C17 °C 17°C17°C

2°C2°C -3 °C-3 °C

9°C9°C 6.5 °C6.5 °C

70°F70°F 60 °F60 °F

82% 18 °C°C

45.5% -.5 °C°C

100% 17 °C°C

20% -17 °C°C

69% 3.5 °C°C

52% 11 °C°C(50 °F)(50 °F)21 °C°C 15 °C°C