# Pressure and rainning

1. May 22, 2004

### darkar

I just read a sentence from a book, It says that when the atm pressure is low, it will be raining. Is this true? If true, why the pressure drop before raining?

2. May 23, 2004

### arildno

Now, I'm no meteorologist, but the following should be of relevance:

1. Why does the rain fall down?
Clearly, the atmospheric pressure is unable to counteract the force of gravity acting upon the raindrops
(That is, the bouyancy force is less than the gravity force)

2. What causes the bouyancy force to become less than the force of gravity?
Since basically, balance between bouyancy force and gravity force requires balancing air density and water vapour density, one should expect the balance to be broken if the water vapour density increases.

3. If the water vapour condenses (for example becoming a fluid/solid rather than a gas),
its density should increase, and hence, the water will fall down.

4. Possibly, one mechanism of condensing the vapour would be to change the temperature.

5. However, Important:
This is speculation from my side, the actual mechanisms are probably more complicated than this.
4.

3. May 23, 2004

### Janus

Staff Emeritus
Because of uneven heating and other effects, our atmosphere has both low and high pressure areas. The tendancy is for the air to even out these differences and thus you get air constantly moving from the high pressure areas to the low pressure areas (this creates local winds).

Another fact is that air at high pressure can hold more moisture than low pressure air can. So when this humid high pressure air moves into the low pressure air, its pressure drops, it cannot not hold as much moisture and the excess condenses out and falls as rain. The greater the difference between the low and high pressure areas, the stronger the winds and the more moist air that will pour over and the more stormy and wet weather you get. This is why a barometer can be a good indicator as to what kind of weather to expect.

4. May 23, 2004

### Bystander

At constant temperature, a given mass of air holds LESS water at "high" pressure than at "low" ("high" and "low" indicating earth surface meteorological extremes). Water vapor capacity is, to first order, a function of the volume and temperature of the air mass. As an air mass is expanded adiabatically it cools enough to reduce its temperature to less than that of the dew point temperature for its water content as ONE mechanism to produce rain --- there are other rain/precipitation mechanisms that do NOT involve pressure drops.