A330 asked " Question that confuses me is: Dewpoint is when it rains when temp gets lowered to dew point. but what makes the dew point change, if it's pressure then i thought pressure is temperature. like it rains when it's boiling hot"
Nobody else answered. I am not an expert, but I think:
(a) Dew point is not when it rains. Dew point is when tiny droplets form, as air with some moisture in it gets cooled down. The tiny droplets may stay like that as fog, coalesce on solids such a grass blades and form dew, or (at height) coalesce as raindrops or hail.
(b) Dew point changes with humidity. "Dry" air has little moisture and a very low dew point. Dry air which passes over water, e.g. coming from the USA across the Atlantic, picks up water vapour on the way and reaches higher humidity.
Water in moist air stays in the form of water vapour unless the temperature drops, in which case the relative humidity rises. When RH reaches 100 percent, i.e. air temp = dew point temp, droplets form. One example is air reaching Welsh hills, gets pushed up, and forms clouds. If conditions are right, droplets coalesce as raindrops and it makes Welsh rain. The air, having lost some moisture, gets lower (absolute) humidity. RH is still 100 percent while the air is as high as it got. When it descends on the downwind side of the hills, temperature rises, water content/humidity stays at the post-rain level, but dew point is now lower - so England gets less rain than Wales in prevailing west winds. Over England, the medium RH air gets cooled in thermals as it rises, typically forming cumulus clouds, but is much less likely to rain unless, e.g., cu gets very tall - hence cu nim = rain + hail.
Chris N.
Last edited by chrisN; 23rd June 2006 at 17:36.