ESRT Page 12 - Dewpoint & Relative Humidity

The instrument (right) is a psychrometer. It consists of two thermometers, one wet and the other dry. The 'dry bulb thermometer' measures air temperature. As water evaporates from the wet bulb it lowers the temperature of the wet bulb thermometer. The greater the difference in temperature between the two thermometers the dryer the air.

Why? Because the dryer the air the faster water evaporates and the faster water evaporates the more it cools the wet bulb.

Using the difference in temperature between the two thermometers and the charts on page 12 of the ESRT we can find the Relative Humidity and the Dewpoint of the air.

In the example (right) the dry bulb (air temperature) is 22C and the wet bulb is 16C. Using the chart below find the Relative Humidity.

IMPORTANT! Read questions carefully. Be sure you know what you are looking for and that you are using the correct chart.

1) In the 'Dry-Bulb Temperature' column (left) find the dry bulb temperature of 22C
2) In the (top) row marked 'Difference Between Wet-Bulb and Dry-Bulb Temperatures' find the difference in temperature between the dry bulb (22C) and the wet bulb (16C). To get the difference you must subtract. 22 - 16 = 6. The difference is 6C.
3) See where the 22 row intersects with the 6 column. They intersect at 53 which is the Relative Humidity expressed as a percent.

The relative humidity is 53%. The air is holding 53% of it's capacity.

 

Helpful Hints:

1) Be sure you are using the right chart. Don't use the Dewpoint chart to find Relative Humidity.

2) Understand what information you are given. If you are given wet and dry bulb temperatures you will have to subtract to get the difference, BUT if you are given the dry bulb temperature and the temperature difference you don't need to subtract!

3) Be sure the temperatures you are given are in C. If you are given temperatures in F you will have to convert (using the chart on ESRT page 13) first.

4) Don't be lazy. Use a straight edge, a piece of paper, the edge of a book, when finding where the the row and column intersect. It's easy to make a mistake!

5) Don't forget, the dry bulb temperature is just the air temperature.

Finding Dewpoint

Using the same example as before (dry bulb 22C and wet bulb 16C) find the dewpoint.

1) In the 'Dry-Bulb Temperature' column (left) find the dry bulb temperature of 22C
2) In the (top) row marked 'Difference Between Wet-Bulb and Dry-Bulb Temperatures' find the difference between 22C (dry bulb) and 16C (wet bulb). Just as before you must subtract to get the difference. 22 - 16 = 6. The difference is 6C.
3) See where the 22 row intersects with the 6 column. They intersect at 12. The dewpoint temperature is 12C

What does this mean?

If the air (now at 22C) was chilled to 12C then:
> The relative humidity would be 100%
> Condensation would occur. Clouds would form.
> Water would effectively stop evaporating.

Practice Questions

Understanding Relative Humidity & Dewpoint (powerpoint tutorial)

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