It is an observed fact that presence of moisture in atmospheric air brings down the atmospheric pressure. What is the mechanism for this process and scientific reasons associated with this fact?
Scientists from weather,atmospheric science, environment and meteorology can give a clear picture of this phenomenon.
In a quasi geostrophic approximation, surface air convergence, is associated with atmospheric low pressure systems. This convergence, helps to increase the presence of water vapor in the air.
Surface low pressure systems are usually accompanied by air divergence at higher atmospheric levels, which are responsible of lower air pressure at lower levels (surface).
Mr.Ribera , thank you for your answer, please elaborate the cause of low pressure in presence of moisture and avoid all approximations. It is a well known fact and about 50% general people without any science back ground/knowledge know this atmospheric behaviour, because rain and storms are associated with low atmospheric pressures. Also state the source of moisture entering the atmosphere.
As I understand Mr. Ribera's comment, the humidity does not cause low pressure, but humidity is a consequence of the low pressure field. On the other hand, at the same temperature, the density of air saturated with humidity is lower than density of dry air, possibly causing the humid air to rise in the atmosphere and to leave a low pressure field in its wake (neglecting the temperature differences, which I shouldn't.)
Surface atmospheric low pressure is associated with ascending air. Tha air gets colder as it goes higher and this cooling increases the relative humidity of the air. If the air gets cold enough, it saturates and condensation/precipitation begins.
The low pressure is not related to the presence of water vapor. It is because the air column over one area loses mass/air.
A bulk of humid air or saturated air contains a large number and volume of water vapor which by nature as per ambient conditions are in condensing state. On condensing, the bulk of air with moisture considered for this observation, occupies less space on part condensation that produces some vacuum which is low pressure in the domain. Thank you, Mr.Ingo Riess for your answer. Moisture associated comes from blowing air from high pressure zone or evaporative surface.
Please refer the link http://superbeefy.com/what-is-the-relationship-between-air-pressure-and-humidity/
I understand the link information as below :
Air has many different kinds of molecules: nitrogen, oxygen, and water vapor, for example. Nitrogen molecules are heavier than oxygen molecules and oxygen molecules are heavier than water vapour molecules. [p.s. Need some confirmation]
Nitrogen and oxygen molecules collide with other molecules forcefully, whereas the lighter water vapor’s collisions are less powerful. The water vapor molecules don’t travel far after a collision, so they collide more frequently.
Therefore, as humidity increases (more water vapor in the air), air pressure decreases, and as humidity decreases, air pressure increases.
Saravana, although the discussion may drift a little, in a gas mixture momentum is transferred between molecules. When a heavier N2 hits lighter a H2O, momentum is transferred, causing the water to travel much faster. In average, the kinetic energy (or momentum?) of each molecule should be rather similar.
BTW: relative mass H=1, O=16, H2O=18, N2=28, O2=32
Dear Ingo Riess and Balkrishnan, please remember after collision of a N2 molecule with N2; 2N2 is not forming but in case of H2O ; 2H2O is forming in ambient conditions, what we call it as condensation. This condensing process is causing vacuum and this is low pressure.
Dear Mr.D.B.Jani, you have correctly understood the fact of nature. But on asking a weather scientist, he will describe the process in a lengthy manner like: horizontal/vertical divergence and other similar terms. Hence condensation of moisture present in atmosphere is a prime cause of low atmospheric pressure, in addition to rise of hot air.
The surface heating causes surface pressure to become low due to which the surrounding air converges towards the low( as per the rule of pressure gradient force). If the surrounding air contains moisture it will be brought towards the low. So the low pressure brings the moisture.
Otherwise it can also happen that the moisture formed is nothing but water vapor which is a greenhouse gas. So increase in water vapor may increase the temperature just above the surface in atmosphere and facilitating the low to sustain.
Need discussion on this..
Dear Annu, thank you for your answer, you can think in that manner there is no wrong with your assumptions, but more correct is condensing nature of moisture that create vacuum due to more volume of moisture becomes/occupies less volume on condensing with decrease in partial pressure of moisture portion in the atmospheric air is responsible for low atmospheric pressure.
We can think of this system like this. In the atmospheric boundary layer there is an abundance of moisture compared to the free atmosphere which lies above. The presence of moisture lowers the density of the air so basically this type of an arrangement corresponds to lower density air sitting below the higher density air. This is an inherently unstable configuration so the buoyancy force sets in to drive the convection. If there is a cyclonic circulation present in the free atmosphere, then the moist air will start rising above (an anticyclonic circulation on the other hand will suppress the rising) and in order to fulfill the continuity air from the surrounding will start rushing in to the void created. Thus the rising of the moist air creates void which corresponds to a low pressure forming.
I hope this answer helps.
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