How to Convert Humidity Expressions With Computers & Calculators

Converting Humidity Expressions With an Online Calculator

• 1

  Find a site on the Internet that has a humidity convertor calculator. Online calculators have the ability to convert relative humidity to dew point, specific humidity, partial pressure and several other expressions.

• 2

  Enter the available data temperature, barometric pressure and humidity into the required field on the website calculator. If the temperature or barometric pressure data is in units other than what is required for the website calculator, convert the units to metric or Imperial/USA. Many website calculators are available for this task.

• 3

  Press the appropriate button on the calculator to get the conversion to the humidity expression needed. Additional conversion may be available on some online programs if you need to change back to English or metric units.

Calculation of Humidity Expressions Using a Downloadable Program

• 4

  Search the Internet for a humidity calculator program. Several are available online that are available for download and work on PCs and Apple computers. Some of the programs are freeware, such as Vaisala's program and some cost money. Download the program onto your computer and install it.

• 5

  Open the program on your computer. On Vaisala, open the "Basic" tab. Enter relative humidity and temperature data. Press "Calculate" to obtain the dew point.

• 6

  Convert measures of relative humidity to other measures on Vaisala using the "Advanced" tab. Conversions are available for several measures, including absolute humidity, mixing ratio and saturation vapor pressure. Conversions are also available to English and metric units with this program.

Manual Calculation of Saturation Vapor Water Pressure, Water Vapor Pressure, Relative Humidity and Dew Point

• 7

  Calculate saturation vapor water pressure (SVWP) by first noting the temperature in degrees Centigrade. Enter the temperature (T) value into the following equation, where SVWP = 6.1078 + T (0.437 +T (0.0143 + T (0.00265 + T (0.000003 +T (0.00000002 +T (0.00000000006 +T)))))). The result is expressed as hPa, or hectopascals.

• 8

  Calculate water vapor pressure (WVP) by first noting the temperature in degrees Centigrade. Enter that value in the formula: WVP = 6.112 e^(17.62 T/(243.12 + T)). Solve for the exponent of the value e first. The value for e is 2.718. The resulting value from the formula is WVP in hPa.

• 9

  Calculate relative humidity (RH) with the value for water vapor pressure. The formula is RH = (WVP/eT) x 100, where T is the temperature in degrees Centigrade and e = 2.718. There are no units for the value RH.

• 10

  Calculate the value for dew point temperature (Td) when given the value of relative humidity and temperature in degrees Centigrade. First, calculate the value of b, where b = (ln (RH / 100) + ((17.27 * T) / (237.3 + T))) / 17.27. Enter the value of b in the formula to finish the dew point temperature calculation, Td = (237.3 x b) / (1 - b). The value of the dew point temperature is in degrees Centigrade.