Humidity

Although it occurs indiscriminately all over the globe, humidity in its raw state is a tropical phenomenon. It prefers places with nearby bodies of water and can most often be observed in flagrante delicto, as it were, in the summer. In these regions and at that time of year it is often offered as proof that:

A) There is no god.

B) If there is, he really doesn't like us.

It is thus reviled because it does such nasty things. The walls of unairconditioned rooms in the tropics, in summer will start to bead with condensation which can give things a nasty case of mildew. If you didn't put a clothes pin on your big bag of potato chips, then they will get soggy. As destructive to life, limb and property as these are, they are not the worst humidity has to offer.

Humidity's primary demoralizing effect is caused by the fact that it impedes your sweat from doing its job. The human body requires your sweat to evaporate in order to remove kinetic energy and lower its temperature. But if the sweat cannot evaporate (because the air is replete with humidity) then it has no choice but to trickle down your leg under your pantaloons in an alarming manner. The upshot of this is that, due to trickling sweat, your socks will reach a state of humidity rivalling that of the air.

I for one, would advocate government regulation of humidity for the good of all citizens. Any humidity exceeding 70 percent should be brought into custody and prosecuted. Police would be issued barometers and any air that doesn't comply should be shot.

Let's take that one step at a time. Firstly, understand this: There is a certain amount of water vapor in the air. That amount ranges from zero (see: winter in New England) to a maximum value. When that maximum value is reached, the air is said to be saturated with water. Any more water vapor, and the whole of earth's atmosphere would just sort of THWOMP into water (water-nine, anyone?). Actually that last part isn't true. Just look up super saturation for that. Keep in mind, also, that the saturation point varies depending on several factors, such as temperature and pressure.
Now that we've set a minimum and maximum amount of water vapor that can be in the air, we have a scale on which to measure relative humidity. Just compare the actual humidity measurement to the maximum humidity, and you'll get your ratio or percentage. It's basically just a Holy-fuck-how-much-worse-can-it-get measurement.

Specific Humidity: In a mixture of water vapor and air, the weight of water vapor per unit weight of dry air. Also known as humidity ratio.

Instead of a min-max scale, as above, this is simply a comparison of the weight of the two components of "air": the water vapor and the actual "dry air". Let's say you have a metric ton of everyday, over-the-counter air. A portion of that - say 1/4 - is actually composed of water vapor. The other 3/4 is "dry air". Weigh the water, weigh the air (good luck), and there's your nice ratio.

Definitions gleefully provided by About.com

Hu*mid"i*ty (?), n. [Cf. F. humidit'e.]

Moisture; dampness; a moderate degree of wetness, which is perceptible to the eye or touch; -- used especially of the atmosphere, or of anything which has absorbed moisture from the atmosphere, as clothing.

⇒ In hygrometrical reports (as of the United States Signal Service) complete saturation of the air is designated by Humidity 100, and its partial saturation by smaller numbers.

© Webster 1913.