Ed Barnes, U.S. Water Conservation Laboratory
9 to 12
Earth Science, Meteorology
To better understand what relative humidity measures and what it is used for
Fan, 2 thermometers, 1 shoe lace
15 minutes to 1 hour, depending on the exercises used
Relative humidity is the ratio of the actual amount of moisture in the atmosphere to the amount of moisture the atmosphere can hold. Therefore, a relative humidity of 100% means the air can hold no more water (rain or dew is likely), and a relative humidity of 0% indicates there is no moisture in the atmosphere. Relative humidity is used by meteorologists to help predict the weather, by pathologists to predict disease development on plants, and by agricultural scientists to estimate evapotranspiration
Relative humidity, combined with air temperature, can be used to estimate the actual amount of moisture in the atmosphere, sometimes referred to as precipitable water. Water vapor acts as a green house gas by trapping infrared radiation reflected from the earth. This explains why desert temperatures can become much lower at night, as there is little moisture in the air to trap the heat.
Relative humidity can be determined from wet bulb and dry bulb temperatures. Dry bulb temperature is the actual air temperature, while wet bulb temperature can be determined by using a shoe lace to cover the bulb of a thermometer. Wet the shoe lace and use a fan to pass air over the thermometer. As the water evaporates, energy is used and the temperature should decrease. After the temperature has stopped decreasing, record this temperature as the "wet bulb" temperature.
You can use the Relative Humidity Table to determine relative humidity from your recorded temperatures or calculate it yourself using the Relative Humidity Equations.
1. Measure relative humidity in the morning and then at mid-day. Under most conditions, the relative humidity decreases from morning to noon as the air warms. Calculate precipitable water (w, cm) by:
where Ta is air temperature (°C) and RH is relative humidity (%). This value should remain fairly constant unless a weather front passes.
2. Measure the relative humidity in the middle of a well-watered grass field. As quickly as possible, measure the relative humidity in the middle of a dry, nonirrigated area. The relative humidity should be higher over the grass because of the transpiration of the plants.
3. Keep records of relative humidity and precipitable water for several months. You should notice that precipitable water changes with weather patterns and generally increases from winter to summer.
4. Use the equations on the back of this sheet to develop a computer program that will automatically calculate relative humidity and precipitable water, given wet and dry bulb temperatures.
Jensen, M.E., R.D. Burman, and R.G. Allen. 1990. Evapotranspiration and irrigation water requirements. ASCE Manuals and Reports on Engineering Practice No. 70.
Mims, Forrest. 1995. Engineer's mini-note book, environmental projects. Radio Shack, Fort Worth TX 76102.
Rosenberg, N.J., B.L. Blad, and S.B. Verma. 1990. Microclimate. Second edition. John Wiley & Sons, New York.