Wanderlust World Wanderlust World Relative humidity is a ratio between the amount of water vapor in the air and the amount of water vapor the air can hold, therefore it is always expressed as a percentage. To find relative humidity, the partial pressure of water vapor (at a given location) must be divided by the saturation vapor pressure of water at the location's temperature. The resulting number must then be multiplied by 100. When relative humidity reaches 100 percent, the air is fully saturated with water vapor and the dew point is reached.
The dew point temperature is the temperature at which air can no longer hold all of its water vapor in a gaseous form. High humidity percentages indicate the dew point is close to the outside air temperature. When the dew point is reached, water vapor (gas) condenses to a liquid and is released in the form of precipitation.
In the summer months, a rise in humidity can feel like a rise in apparent temperature. This phenomenon occurs because high humidity hinders the process of sweat evaporation from the skin, which in turn limits the body's ability to cool itself. According to the Heat Index, a relative humidity of 90 percent at 80 degrees Fahrenheit would make it feel as if it were 88 degrees Fahrenheit. High humidity can easily lead to heat exhaustion and heatstroke.
The Earth is estimated to have about 326 million cubic miles of water. This number includes liquid, ice and gaseous forms of water. Only 3,100 cubic miles of water is estimated to be present in the atmosphere as water vapor at any given time. Although this is a small portion of the 326 million cubic miles of water, it impacts the Earth's weather dramatically. Without water vapor and humidity present in the air, there would be neither clouds nor precipitation on the planet.