Guide to Weather
Weather Basics
Understanding Weather
Air Temperature
Air Pressure
Wind Speed & Direction
Humidity & Dew Point
Clouds & Visibility
Frequently Asked Questions

 Measuring the Relative Humidity & Dew Point Temperature

In order to measure the relative humidity and dew point temperature, you will need to get the dry-bulb temperature and the wet-bulb temperature. A dry bulb thermometer is used to measure the ambient temperature; a wet bulb thermometer is used to measure the lowest temperature in the ambient atmosphere in its natural state. The wet-bulb thermometer takes this reading by evaporating water from the wet muslin- or cotton-covered bulb of a thermometer.

A psychrometer consists of a dry bulb thermometer and a wet bulb thermometer mounted together in order to take simultaneous temperature readings. You can construct a wet-bulb thermometer by attaching a wet cotton ball with a rubber band to the bulb of a standard thermometer. You can construct a psychrometer by mounting a dry-bulb thermometer and a wet-bulb thermometer onto a thin board.

Measuring Dry-bulb and Wet-bulb Temperature

  1. Wet the cotton ball with water, then place it around the outside of the bulb at the bottom of the wet bulb thermometer. Be sure to stretch the cotton ball so it covers the bulb completely.
  2. The air around the wet bulb thermometer must be moving (like wind) in order to get the most accurate reading. If there is no wind, either fan the air around the psychrometer (if it is permanently mounted) using a flat, thin piece of paper or cardboard, or move the psychrometer through the air (if you are holding it in your hand) until the temperature stops falling.
    • The water in the cotton ball could evaporate. To get accurate readings at a later time, be sure to keep the cotton wet.
    • To avoid false readings, always remove the cotton ball from around the thermometer bulb before adding water to the cotton.
  3. After a few minutes, read the temperature on the wet and dry bulb thermometers.

Calculating Relative Humidity

To determine the relative humidity, look up those readings on the Relative Humidity Chart. The dry bulb temperature is listed vertically (down the left side) and the wet bulb temperature is listed horizontally (across the top). The intersection of those readings (where they meet) is the relative percent of humidity.

Calculating Dew Point

To determine the dew point temperature, subtract the wet bulb reading from the dry bulb reading to determine the "wet bulb depression." Then look up the wet bulb depression and dry bulb temperature readings on the Dew Point Temperature Chart. The wet bulb depression is listed horizontally (across the top) and the dry bulb temperature is listed vertically (down the left side). The intersection of those readings is the dew point temperature.

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