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How to Calculate Frost Points Below 0 F

Many people notice that snowdrifts along the street in winter slowly disappear even at very cold temperatures. The snow in these drifts is changing directly from a solid (ice) to a gas (water vapor) without passing through an intermediate liquid state. This process is called sublimation. The same thing has happened, but in reverse, when you find frost on your car in the morning: water vapor has become ice without first becoming water. Frost point is the temperature at which this change from gas to solid takes place. With a little care, you can easily calculate frost points below 0 degrees F.

Things You'll Need

  • Internet access, a television or a home weather station
  • Scientific calculator
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Instructions

    • 1

      View an Internet weather site, a television weather program or a home weather station to learn the current temperature in degrees Celsius (C) and relative humidity (RH). For example, you access the Weather Underground and learn the temperature is -20 C (-4 degrees F) and the relative humidity is 37 percent.

    • 2

      Find the sum of 2.034080948 x 10^-8 and the product of 6.136820929 x 10^-11 and the current temperature using a scientific calculator. For example, 2.034080948 x 10^-8 + ((6.136820929 x 10^-11) x -20) = 1.91134453 x 10^-8.

    • 3

      Find the sum of 3.031240396 x 10^-6 and the product of your answer and the temperature using your calculator. For example 3.031240396 x 10^-6 + ((1.91134453 x 10^-8) x -20) = 2.64897149 x 10^-6.

    • 4

      Find the sum of 2.650648471 x 10^-4 and the product of your answer and the temperature using your calculator. For example (2.650648471 x 10^-4 + ((2.64897149 x 10^-6) x -20) = 2.120854173 x 10^-4.

    • 5

      Find the sum of 1.428945805 x 10^-2 and the product of your answer and the temperature using your calculator. For example (1.428945805 x 10^-2 + ((2.120854173 x 10^-4) x -20) = 0.0100477497.

    • 6

      Find the sum of 4.436518521 x 10^-1 and the product of your answer and the temperature using your calculator. For example (4.436518521 x 10^-1 + ((0.0100477497) x -20) = 0.2426968581.

    • 7

      Find the sum of 6.107799961 and the product of your answer and the temperature using your calculator. For example (6.107799961 + ((0.2426968581) x -20) = 1.253862799. Your current saturation vapor pressure in kilopascals (kPa) is 1.253862799 kPa.

    • 8

      Find the product of the current relative humidity and the quotient of the saturation vapor pressure and 100. For example, 37 x (1.253862799/100) = 0.4639292356. Your current vapor pressure is 0.4639292356 kPa.

    • 9

      Find the product of the natural log of the current vapor pressure and 243.12 using your calculator. For example, ln(0.4639292356) x 243.12 = -186.721812.

    • 10

      Find the difference of your answer and 440.1 using your calculator. For example, -186.721812 - 440.1 = -626.821812.

    • 11

      Find the quotient of your answer and the difference of 19.43 and the natural log of the vapor pressure using your calculator. For example, -626.821812/(19.43 - ln(0.4639292356)) = -31.03381971.

    • 12

      Find the sum of 32 and the product of 9/5 and your answer to find the frost point in Fahrenheit. For example, 32 + (9/5 x -31.03381971) = -23.86087548. The frost point under these conditions is approximately -23.8 F.


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