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The water vapor capacity of air is directly related to, and limited by, its
temperature. The table below presents the water vapor capacity of a kilogram
of air at various temperatures. Use the table to answer the following questions.
Table 1 : Water vapor capacity of a kilogram of air at average sea level pressure.
Temperature (ºF)
Temperature (ºC)
Grams of water vapor per kg of air (g/kg)
- 40 - 40 0.
- 22 - 30 0. -4 - 20 0. 14 - 10 2 32 0 3. 41 5 5 50 10 7 59 15 10 68 20 14 77 25 20 86 30 26. 95 35 35 104 40 47
ES106 LAB PART A, ACTIVITY 2
Activity 2 : Relative Humidity and Dew Point Temperature
Relative humidity is the most common measurement used to describe water
vapor in the air. In general, it expresses how close the air is to reaching its
water vapor capacity. Relative humidity is the ratio of the air’s water vapor
content (amount actually in the air) to its water vapor capacity at a given
temperature, expressed as a percent. The general formula is:
Relative Humidity )x100%
watervaporcapacity
watervaporcontent
For example, the water vapor capacity of a kilogram of air at 25ºC would be 20
grams per kilogram. If the actual amount of water vapor in the air was 5
grams per kilogram (the water vapor content), the relative humidity would be
calculated as follows:
Relative humidity 100 25 % 20 /
g kg
g kg
- Use the Table 1 or Figure 1, and the general formula for relative humidity to
determine the relative humidity for various water vapor contents of air at identical temperatures. Fill out Table 2, below
Table 2: Change in relative humidity with no change in temperature
Air Temp (ºC) Water Vapor Content Water Vapor Capacity Relative Humidity 2 5ºC 2 g/kg g/kg %
2 5ºC 5 g/kg g/kg %
2 5ºC 7 g/kg g/kg %
- From Table 3, if the temperature of air remains constant, how does
adding water vapor affect the relative humidity?
- What effect will removing water vapor have on the relative humidity?
- Use Table 1 and the general formula for relative humidity to determine the
relative humidity for air at various temperatures with identical water vapor content. Fill out Table 3 below.
Table 3: Change in relative humidity with no change in water content
Air Temp (ºC) Water Vapor Content Water Vapor Capacity Relative Humidity 25ºC 2 g/kg g/kg %
15ºC 2 g/kg g/kg %
5ºC 2 g/kg g/kg %
- So, if the amount of water vapor in the air remains constant, what
effect will cooling or warming the air have on the relative humidity?
Refer to Table 1 for these questions
- What is the dew-point temperature of a kilogram of air that contains 14 grams of water vapor?
Dew-point temperature =
________________________ ºC
- What is the dew-point
temperature of a kilogram air that contains 5 grams of water vapor?
- What is the relative humidity of
the air with 5 grams of water if its temperature is 25ºC?
Dew-point temperature =
________________________ ºC
Relative humidity =
________________________ %
