How mdoes relative humidity affect us?

Answer 1

The most apparent way is in our comfort (assuming "us" means humans, and not something bigger, like the planet, or all its inhabitants). As humidity rises, our body's ability to stay cool via perspiration decreases, because the perspiration is less likely to evaporate, or takes longer to do so. (The evaporation is what causes the cooling.) The reason is that the air is already saturated with moisture, or becoming so.

When relative humidy is low (meaning the air is somewhat dry, but more accurately that it could hold substantially more water at its current temperature) we may feel our skin drying, or feel colder than the actual temperature would suggest. Our nasal passages, throats and lungs tend to dry out and make breathing more difficult or cause coughing or sore throat.

An indoor relative humidity of roughly 40 to 50% allows us to stay comfortable in both summer and winter without using as much air conditioning or heating as we would otherwise have to, which saves us money and causes less pollution due to electricity generation and/or fuel processing.

According to Therma-stor (linked below), "maintaining relative humidity below 50% prevents dust mite infestations, mold and mildew growth, and inhibits bacteria [in the home]." Mold also causes a musty odor.

Besides comfort and health, low relative humidity can cause plants to dry out sooner, and shocks from increased static electricity to become more common. Wood furniture and doors may slide less smoothly in high humidity since they contain more water, and expand slightly -- or become warped -- increasing friction on the moving parts.

On a larger scale, relative humidity (the amount of water vapor in the air compared to how much it could hold at that temperature) greatly affects the weather. As the USA Today article points out, if there were no water in the air there would be no clouds, no precipitation of any familiar kind, no lightning and no fog. "And, without all of this water in all of its forms, Earth's life, if there were any at all, but be as hard to find as life on Mars is to find even signs of."