Water Vapour Calculations

Quick Start

For barrier calcultions and for many other reasons, we often want to know how RH changes with temperature or how much water vapour is in the air at a given RH and temperature.

Here you can adjust T and %RH and find the Dew Point, Wet Bulb and Depression values, plus the absolute values of energy content and weight of water.

Water Vapour Calculations

T °C
Dew Point °C
Wet Bulb : Depr. °C
Energy kJ/kg
Abs. Humidity g/kg

At any given temperature, air can hold a maximum amount of water. If you try to add more water to that air it will fall out as dew or fog. At that point the air has reached 100% humidity. If you take all the moisture out then the air has 0% humidity. As you increase the temperature, the air can hold more water. So air that was 100% humid at 10 °C is only 53% humid at 20°C. So Relative Humidity is the amount of moisture in the air compared to the maximum amount it can hold.

If you have some air with some water in it and cool it, at some temperature it will reach 100% humidity and dew will start to form. This is the Dew Point of the original air. So air that happened to be 53% humid at 20°C will have a dew point of 10°C.

With modern technology it’s easy to measure the relative humidity and dew point. But before such instrumentation, calculations had to be performed indirectly. In this situation two thermometers were used. One was normal (called the Dry Bulb) and the other had a wet cloth around the bulb of the thermometer and you whirled the thermometer around allowing the water around the bulb to evaporate and cool the thermometer. This gave you the Wet Bulb temperature that was always lower than the dry bulb. The difference was called the Depression (shown here after the Wet Bulb value). If the air was highly humid, then not much water would evaporate from the wet bulb, so the depression would be small. If the air was very dry then lots of water would evaporate and the depression would be large. By knowing the dry bulb temperature and the depression you could calculate the relative humidity.

The Absolute Humidity (though that’s a term you never see) can be expressed as grams of water per kg of air, g/kg.

Finally, if your job is to remove moisture from the air then you need to know how much Energy to put in to remove that moisture. The more moisture, the more energy. But also the warmer the air, the more you have to cool it to start removing the moisture. Hence you need to know kJ/kg.