When I used to live in South Florida, I noticed that even a small amount of physical exertion would make me sweat a lot, and I had problems breathing. It was so hot, and having a shower did not seem to help. As soon as I came out of the shower I would be hot, sweating and sticky again. The body of my van, especially the lower parts, started rusting, and some parts were chipping off. The problem was humidity. High humidity can be very hazardous, more so "when combined with high temperature...high humidity disrupts the body's ability to cool itself, which may lead to a heat stroke. Exceptionally high humidity can also trigger asthma symptoms. Therefore, people with heart problems or asthma are advised to be extremely careful during such conditions." I eventually had to give up my love of beaches and move from South Florida to the Mid-West where I now breathe fresh country air with ease.
Humidity, in simply terms, is water vapor in the atmosphere. The measure of the water vapor in the air is called relative humidity. The warmer the air, the more water vapor it can hold. In humid places, we feel hot uncomfortable because "the body tries to cool itself through evaporation of moisture on the skin. But when the air is humid, evaporating moisture becomes more difficult and the actual cooling effect is much smaller."
Humidity is caused by a number of things. First, there is evaporation, which is "when a liquid converts into a gaseous state". Evaporation can occur from oceans, rivers, and lakes. Therefore, places like deserts, where there is little or no water, have almost zero or very little evaporation. " Solar energy from the sun is the key driver for evaporation. By adding heat to a body of water, its temperature will increase and some water will evaporate. Because solar energy is more intense at the equator, there tends to be more water vapor in the atmosphere near the equator. It is of course necessary to have a body of water in order to have evaporation; this is why there is a lot of water vapor over the oceans, but less over land." About 90 % of water vapor is from evaporation.
The second cause of humidity is transpiration, which is "s the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf surfaces are dotted with openings called stoma." About 10 % of water vapor is from transpiration.
Humidity is also caused by the evaporation of sweat from humans and other animals, and also by respiration of humans and other animals.
In terms of relative humidity, the most humid cities in the world are those not very far from the equator and coasts.