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How to Figure the Dew Point

The dew point is the temperature at which the water vapor in a quantity of air would begin condensing. By definition, the air must remain at a constant pressure while being measured. The temperature of the dew point is related to the relative humidity; at the dew point, the relative humidity is 100 percent. Use the value of the dew point to determine the point at which dew or clouds will form.

Things You'll Need

Thermometer
Hygrometer (Relative humidity gauge)
Calculator
Paper
Pencil

Instructions

- 1
  
  A dew point higher than 70 degrees Fahrenheit can make people feel uncomfortable.
  
  Read the relative humidity on the hygrometer, and record the reading as a percent. Read the temperature in either degrees Fahrenheit or Celsius, and record this temperature. Convert the temperature in Fahrenheit to Celsius by subtracting 32 and dividing the result by 1.80. Use the Celsius value for the calculations.
- 2
  Subtract the relative humidity in percent from 100. Divide the result by five. Subtract this result from the temperature in Celsius. This the approximate dew point temperature in Celsius. To convert back to Fahrenheit, multiply the answer by 1.8 then add 32.
- 3
  
  You will need a scientific calculator to compute the natural log of a number.
  
  Compute a closer value of the dew point by using the formula based on the thermodynamic properties of water vapor in air, also known as the psychrometric chart. Find the value of gamma (g) using the formula g = (a * T ) / (b + T) + ln(RH/100) , where ln is the natural log, constant a is 12.271, constant b is 237.7, T is temperature in Celsius and RH is percent relative humidity.
  
  Now compute the dew point (Td) using the formula Td = (b * g) / (a - g), where g, a and b are defined as above. This computation is valid for the range of temperature and relative humidity most encountered in nature.

Nature