Saturation vapour pressure

The saturation vapour pressure (SVP) contains an equation for calculating (SVP), which clearly shows that the only variable is temperature. According to the ideal gas law, because of the gases are composed of molecules that have a mass, exert a pressure that can be measured. From our knowledge about gas exerting pressure, the mass of air around us is called atmospheric pressure and usually reported in weather forecasts in millibar (mbar). The traditional type aneroid barometer that you see hanging on walls uses this pressure to push on a diaphragm that is connected to a mechanism to give a reading.


Now I will show you the equation form:


*you must always consider the temperature

To calculate SVP online you can use weather.gov


If you wanted to increase pressure on something you would push harder, or if you want to fatten a piece of paper you could put a weight on top. Weight is mass acted upon in gravity. The pressure or ‘weight’ of a gas can be thought of in the same way, the only difference being that gas pushes in all directions at the same time.


Because we have such a large difference between atmospheric pressure and water vapour at the saturation point means that only a small fraction of water is required to saturate air. When we think about this we find it fantastic; our plant in the right distance from the sun to allow liquid water to exist; water has properties that means only a small amount can be held in the atmosphere relative to the amount of air.


The relationship of water and air drives our climate. Generally, on the global scale, the warm moist air above the sea at the equator moves towards the cooer poles, forms clouds that return the water by way of rain back to the sea. On a more local scale, evaporation from sea, lakes water is called “Hydrologic Cycle”.


Now you perhaps see a big consequence of global warming. As a temperature increases and pushes up the SVP, we are not short of water to supply the atmosphere and hence the warming in the media about climate change and to expert warmer, and more extreme weather. The sea will hold onto water more than fresh water because of the salt content.


Here is an equation that tells you everything:


VP: is the actual vapour pressure of water so relative humidity is the actual pressure of an amount of water relative to the pressure it would be at the saturation point.

Moving the terms around by simple algebra we can calculate the actual vapour pressure from knowing the temperature, which gives us the SVP and the relative humidity:

VP = SVP × %RH


Before the days of digital technology and calculators, in order to work out what was happening in the saturation where moisture was involved, people had to be familiar with Psychrometry. This is not to be confused with the entirely different and modern practice of Psychrometry.

There are to aspects of psychrometry that we have to mention. One brings in tow new terms that you will come across if you look into humidity in any detail. The other aspect involves the science behind what happens when going from one climatic condition to another.


The wet bulb and dry bulb

The terms wet bulb and dry bulb comes from the original, and reportedly, the most accurate method of measuring humidity. Tow thermometers are using the device, called a psycrometer where they are fixed side by side. One of the thermometers has a thin piece of muslin wrapper around the bulb, which is wet with pure (distilled or deionized) water, hence, the term wet.



The psycrometer is waved in the air which cause the water on the wet bulb evaporate. The amount of water that leaves the wet bulb is directly related to the humidity of the air. At saturation, no water will evaporate from the wet bulb, whereas in dry air, water vapour eaves the wet bulb very quickly.

Just as water evaporating from our bodies coos us down, each water molecule leaving the wet bulb takes a little of heat with it and cools the bulb of thermometer. After waving the psycrometer around for about twenty seconds, the reading from each of the tow thermometers is recorded. The dry bulb is the actual air temperature can be used out the SVP. The wet but is used to calculate the SV. From these tow parameters the %RH is easily calculated as shown before. 




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