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Air saturation

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Posted 30 December 2014 - 01:53 PM


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Hello everyone

This is my first post here and im not expert in meteorology so I apologize if the question look dumb to some of you.

I understand at high atitudes as the air become colder (ie at temp below -40C) the air become colder and as air the air become cold it wont be able to hold water vapor. My question does this mean that air is "saturated" so it cant hold more water? Or it has something to do with the ability of the cold air (the air isnt saturated)?

richard mann

Posted 31 December 2014 - 02:03 AM

richard mann

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.. Welcome "Wof".
With your question, probably the best place to start is with looking at the "Water Cycle", and more basic "evaporation", ... leading to "condensation". 

More basically, if air is cold enough, even at only for example, 10 ft. above warmer air with enough moisture in it to condense, i.e. become somewhat saturated, .. with that moisture's movement upward to meet with that colder air, it will condense.

Full saturation, leading to precipitation of some kind, occurs where and when some amount of moisture holding air, rises to whatever point it needs to elevation wise, to reach what's call the "Dew Point".



Posted 22 September 2016 - 10:06 PM


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Air can hold more water vapour when it is hotter. When the relative humidity is as high as 100% the vapour will start condensing out if the air cools. Here is an example of how relative humidity changes:

Suppose you have a given mass (parcel) of air that is heated. It expands and pushes the surrounding air out, but if you measure pressure of the surrounding air before and after you will find it remains the same (if you are quick enough to measure pressure before it rises significantly) and so does the pressure of the heated, though less dense air. The heated air is at the same pressure as the surrounding air otherwise air would rush in. The weight of all components of the heated parcel is the same as it was (including water vapour), but the density of all components is less. The partial pressure of nitrogen, water vapour, oxygen, etc remains the same as you heat the parcel and so does the total pressure (it remains at the atmospheric pressure). The relative humidity is defined as the partial pressure of the water vapour (which has remained the same) divided by the partial pressure of what the water vapour would be if the air were saturated with water vapour at that higher temperature. Well as you increase the temperature, the partial pressure of water vapour, if the air were saturated at that temperature, increases a lot, so relative humidity falls. But you have the same contents in the parcel that has expanded and, being hotter, it will rise to colder heights. You can prove all this by using the fact that the partial pressure of a gas= total pressure x mole fraction of the gas. If no gas escapes from the heated parcel, the mole fraction of each component (water vapour, etc) remains the same. When you proceed to a higher altitude the pressure does get less and you cannot assume the partial pressures of all components are the same - they are all less and you will have to use a new saturation pressure for water vapour (the temperature is less so the saturation pressure is less) and a new partial pressure for the water vapour.


Posted 03 October 2016 - 10:34 PM


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The water vapors evaporate in the atmosphere and form the clouds These water vapors are also present in the air in which animals, birds and humans breath.


The air has a maximum capacity of how much water it can hold. When the water vapor in the air reaches the maximum capacity, the air is considered to be saturated. Because no more water vapor can be held by the air, the extra water vapors become condensed into liquid water near the Earth's surface. The condensed water as a result of air saturation is what we refer to as dew.


Temperature affects that how much water the air can hold. Warm air  can carry more water vapors and cold air can carry less water vapors. That is why air can get so much more humidity in the heat. Humidity is the measurement of how much water is in the air.

Claire Anderson


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