Humidity is the concentration of water vapor present in the air or atmosphere. Water vapor is just water in a gaseous state, which is produced by evaporation, transpiration from plants and the sublimation of ice.
Humid weather can feel hot, muggy, wet, smoggy, or steamy. This is because water vapor is a greenhouse gas, meaning that heat radiated from Earth’s surface is absorbed by water vapor molecules in the lower atmosphere. Water molecules then reradiate that heat out in all directions. High temperatures can therefore feel much hotter if they are coupled with high humidity levels.
Most temperature measurements are taken using ‘dry-bulb’ thermometers. Dry-bulb thermometer can accurately measure air temperature, but do not take humidity into account. In other words, dry-bulb thermometer readings do not provide much information about how temperatures actually feel to people on the ground. So-called wet-bulb temperature measurements, however, gauges both air temperature and total humidity. The wet-bulb temperature is measured using a thermometer wrapped in a wet fabric, which more closely represents the human body’s ability to cool down.
Weather reports commonly use what’s known as “relative humidity”. Relative humidity refers to the percentage of moisture that can be retained by the atmosphere at a given temperature without the water converting into a liquid (condensation). For example, at a temperature of 70°F, the air can hold approximately .51 grams of moisture per cubic foot maximum. If there are .51 grams of water vapor per cubic foot in the air at 70°F, the relative humidity would be 100%. If the temperature is at 70°F, and there are roughly .25grams of water vapor in the air per cubic foot, then the relative humidity would be about 50%.
Generally, cold air is dryer and holds less moisture than warm air. This is especially evident at the Earth’s poles (Antarctica and parts of the Arctic), which are considered deserts. According to the Nasa’s Earth Observatory website, most of the Earth’s water vapor was concentrated along the equator from 2002 to 2022. High temperatures combined with humidity levels, then, are mostly likely to be felt in Brazil, Colombia, Ecuador, Democratic Republic of the Congo, Gabon, Indonesia, Kenya, Kiribati, Maldives, Republic of the Congo, Somalia, Sao Tome and Principe, and Uganda which are the thirteen countries that the equator pass through.