young@hoqax.UUCP (HUH) (08/27/87)
There is something about relative humidity that I've been curious about for a while. I am hoping that someone on the net could help me understand this. From what I remember from my high school chemistry class, the relative humidity (for a given location and time) is the ratio: actual pressure of water vapor dissolved in the atmosphere ---------------------------------------------------------- vapor pressure of water at the same temperature where the vapor pressure is the equilibrium pressure of the vapor with its liquid at a given temperature. My question is, why is the relative humidity not always 100%? Shouldn't the liquid water on the surface of the earth (which there seems to be a plenty of) evaporate into the atmosphere and eventually reach an equilibrium, thereby making the actual pressure of water vapor equal to the vapor pressure of water? (Think about a sealed jar partially filled with water.) Can the concept of 'vapor pressure' be used in such a 'global' sense? What other factors are there that influence the humidity? Thanks in advance for any explanation. Young Huh
platt@emory.uucp (Dan Platt) (08/28/87)
In article <1067@hoqax.UUCP> young@hoqax.UUCP (HUH) writes: > >There is something about relative humidity that I've been curious about for >a while. I am hoping that someone on the net could help me understand this. > >My question is, why is the relative humidity not always 100%? > >Shouldn't the liquid water on the surface of the earth (which there seems to be >a plenty of) evaporate into the atmosphere and eventually reach an equilibrium, >thereby making the actual pressure of water vapor equal to the vapor pressure of >water? In answer to this, there are lots of things that happen to air as it moves around. Air moving over water may have a high vapor pressure of water. If the air rises and cools, the water condenses and it rains. If this air sinks, or is warmed, then the relative humidity of the air drops. Around low pressure zones, the air rushes into the middle to fill up the low pressure, and the air rises in the center (generally) which is one of many reasons why low pressure zones are so often spawning storms. On the other hand, high pressure zones are composed of air moving away from a high pressure area, with falling air replacing the air that moved away. This air has a tendency of being dry (unless it's bringing air from over water like the Bermuda high does in Georgia -- then the air is humid, and the heating of this air by the ground causes enough lifting to make local thunderstorms, but little cooling). On yet a grander scale, there are bands called Hadley cells in which the air rises, travels along a toroidal shape (like a doughnut), such as what happens in the tropics. The hot air from the Equator rises, drops the moisture, moves north (or south), sinks around the tropic lines, making a really dry air mass which goes south (or north) taking what water it can find back to the equator. This is why there are jungles around the equator, but most of the tropic circles run through deserts (like the Sahara, Gobi, Australia, Kalahari, etc). The reason for all of this mixing and turbulence, and the resulting failure for the atmosphere to be in equilibrium with the water is that the presence of the water vapor allows a great release of energy as the water condenses (latent heat of vaporization). This allows for the formation of heat engines (called Huricanes) in the tropics as an extreme case, but also explaines why the atmosphere turns over so violently around thunder storms. There's a lot of energy represented in the lifting of vapor laden air -- which is released upon condensation. Dan
drw@cullvax.UUCP (Dale Worley) (08/30/87)
young@hoqax.UUCP (HUH) writes: > My question is, why is the relative humidity not always 100%? > > Shouldn't the liquid water on the surface of the earth (which there seems to be > a plenty of) evaporate into the atmosphere and eventually reach an equilibrium, > thereby making the actual pressure of water vapor equal to the vapor pressure of > water? You are completely correct, except for the fact that the Earth's atmosphere is not in equilibrium. It is a monstrous heat engine driven by solar radiation. If the Sun went out, not only would the R.H. go to 100%, but all the winds would stop, etc. Dale -- Dale Worley Cullinet Software ARPA: cullvax!drw@eddie.mit.edu UUCP: ...!seismo!harvard!mit-eddie!cullvax!drw Apollo was the doorway to the stars - next time we should open it. Disclaimer: Don't sue me, sue my company - they have more money.