Cloud formation and pressure?

Question

When clouds form, water vapor (a gas) in the air nucleates around small solid particles (forming liquid water droplets).

If the pressure is lower (e.g. at the top of a mountain) water boils at a lower temperature; This suggest that low pressure conditions favor water existing as a gas rather than as a liquid. If this is true, why do clouds tend to form under low pressure conditions (remembering that cloud formation requires water to change from its gaseous state to a liquid state)?

Answer 1

Pressure versus Temperature and cloud formation

Author:      pioneerlearners

As we are teaching the formation of clouds we have been faced with a difficult

scenario.  One source explains that as air pressure decreases temperature also

decreases.  Another source explains that as temperature rises air pressure

decreases.  Could you help us to explain this to our fifth grade science

students?  A general overview of the relationship between air pressure and

temperature would also be appreciated.

Response #:  1 of 1

Author:      allsopp

Air pressure decreases with height because as you move up through the

atmosphere, there is less and less air above you pushing down.  Because

pressure decreases with height, air expands as it rises.  When the air

expands, it uses up energy by pushing the surrounding air outward.  The

molecules in the air lose energy and slow down.  So air cools as it rises and

warms as it sinks.  Rising air cools at a rate of 5.5 degrees F for every 1000

feet.  How does this affect cloud formation?  Warm air can hold more water

vapor (gas) than cold air.  When warm moist air rises, it cools.  Eventually

it cools to the point where it is saturated with water vapor.  At this point

the water vapor begins to condense out as tiny droplets of liquid water . . .

clouds form.  If the air sinks, it warms and the liquid water evaporates

causing clouds to dissipate.

Air moves out away from the center of high pressure.  Air must come down from

above to replace the air moving away from the high.  Thus there is a general

sinking or subsidence of air in a high pressure area.  High pressure generally

brings dry fair weather.

Air is pushed toward a low pressure center.  The converging air is forced

upward.  Rising air (if there is sufficient water vapor) will result in clouds

and possibly precipitation.  Low pressure is generally associated with cloudy

or stormy weather.

from:

http://www.newton.dep.anl.gov/newton/ask...

ADDITIONAL INFO

Quoted from the link below:

"the temperature of a system can be changed by changing the pressure" and

"high pressure causes high temperature"

we can now say that:

as the pressure increases, the temperature also increases

vice versa

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Atmospheric pressure decreases with increase in altitude. This is because the number of air molecules i.e. air density decreases at higher altitudes.

Answer 2

The low  pressure plays an important part in bringing down the boiling point of water at higher altitudes.But, in the case of cloud formation,low temperature(rather than low pressure) at higher altitudes plays an important part.That is the reason.More details follow.

As you go up,temperature generally decreases;so when a parcel of air rises due to buoyancy,the temperature decreases and becomes equal to dew point temperature at a certain height.So,water vapour condenses into water droplets which constitute a cloud.Low pressure helps only to bring down the temperature rapidly by expansion of the air parcel at higher levels.

Pressure also decreases as you go up.This helps to bring down the boiling point of liquid at higher levels as pressure is proportional to temperature. Less pressure means less temperature which again means less boiling temperature.