MEASURING RELATIVE HUMIDITY:
A PSYCHROMETER

WHAT DO YOU THINK?

Will a wet thermometer read a temperature that is the same as a dry one? _________________________________________________________________

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1. Select two thermometers that are reading the same temperature.

2. Wrap the paper towel around the bulb of one of the thermometers and hold it there with the rubber band.

3. Wet the paper towel in the cup of water. Wait 5 minutes.

4. There should be a difference in the temperatures of the two thermometers. Record the two temperatures and subtract them to get the difference.

5. Use the chart on the next page to estimate the relative humidity.

Dry Bulb Temperature ______________________

Wet Bulb Temperature______________________

Difference______________________

Relative Humidity______________________

	Dry Bulb Temperature in Degrees Fahrenheit
Difference Between Dry Bulb and Wet Bulb Temperatures in Degrees Fahrenheit	60	64	68	72	76	80	84	88
1	90	91	91	92	92	92	93	93
2	80	82	83	83	84	85	86	86
3	71	73	74	76	77	78	79	80
4	61	65	66	68	69	71	72	73
5	53	57	59	61	62	65	66	67
6	44	49	51	54	56	58	60	61
7	36	41	44	47	49	52	54	56
8	27	34	37	40	43	47	49	51
9	20	27	31	34	37	41	43	45

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Can you think of why the wet bulb temperature is less than the dry bulb temperature?

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