Dalton's Law of Partial Pressures - New Look
Why do we seem "winded" on a hot, humid Summer day? Why can't we cool down? Good questions. At Standard Temperature and Pressure, at Sea Level, one Atmosphere consists of nitrogen, oxygen, carbon dioxide, water vapor and trace gases. If you took the partial pressures of each gas they would exert their pressures independently of each other but, in total, would add up to 760 mm Hg at STP.
Each of these gases make up a specific percentage of the whole atmosphere. This doesn't change. Nitrogen 79% , oxygen 20% and all the rest 1%. If water vapor partial pressure increases the partial pressures of the other gases is reduced to reflect the presence of water vapor in the atmosphere. The slight reduction in oxygen's partial pressure is enough to "tire" us a bit.
In dry climates the humidity is low. In sunny South Carolina the humidity is quite high. The "Comfort Index" indicates conditions are very uncomfortable. Why is that?
Upon evaporation of water (changing from a liquid state to a gaseous state) the evaporation could use up 580 +- calories per gram of water evaporated. In total, that is a "cooling effect" that makes you feel good and normal body temperature is a good thing to control. Think heat prostration and heat stroke.
If the water vapor content is high, evaporation ceases or decreases. You can't convert water to water vapor to create the cooling effect.
Notice, all of these changes are taking place at sea level, with slight adjustments for the elevation.
