Relative Humidity and Dew Point

• Relative humidity can be expressed as the actual vapor pressure divided by the saturation vapor pressure times 100 percent.
• Actual vapor pressure is designated by "e".
• Saturation vapor pressure is designated by "es".
• The formula for relative humidity is RH = (e / es) * 100%.

Table 1: Saturation Vapor Pressure Over Water

• Table 1 provides saturation vapor pressure values for various air temperatures.
• At 27°C, air has a saturation vapor pressure of 35 mb.
• If air cools with no moisture change, the saturation vapor pressure decreases.
• As the saturation vapor pressure (es) lowers and approaches the actual vapor pressure (e), the relative humidity increases.
• If air cools to 21°C (70°F) and reaches saturation, the dew-point temperature is 21°C.
• At saturation, "e" and "es" are equal.

Using Table 1 to Determine Dew Point

• By knowing the actual vapor pressure in a room, Table 1 can be used to find the temperature at which air becomes saturated, thus determining the dew point.
• Conversely, knowing the dew point allows us to find the actual vapor pressure using Table 1.

Calculating Relative Humidity

• Table 1 can be used to obtain the saturation vapor pressure (es) and the actual vapor pressure (e) if the air temperature and dew point are known.
• Example: Air at 29°C with a dew point of 18°C.
  • At 29°C, es = 41mb (from Table 1).
  • At a dew point of 18°C, e = 21mb.
  • Therefore, RH = (21 / 41) * 100% = 51%.

Calculating Dew-Point Temperature

• If air temperature (27°C) and relative humidity (60%) are known, the dew-point temperature can be determined:
  • At 27°C, es = 35 mb (from Table 1).
  • 60% = e / 35
  • e = 21mb
• This actual vapor pressure of 21 mb corresponds to a dew-point temperature of 18°C.