Understanding Humidity and the Heat Index

Understanding humidity and the heat index starts by learning that the heat index was first developed in 1978 to refer to a combination of air temperature and humidity that affects the inhabitants of an environment. The heat index attempts to draw comparisons between the actual weather and the human perception of how hot it feels within a given environment.

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Natasha McLachlan is a writer who currently lives in Southern California. She is an alumna of California College of the Arts, where she obtained her B.A. in Writing and Literature. Her current work revolves around insurance guides and informational articles. She truly enjoys helping others learn more about everyday, practical matters through her work.

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Laura Walker graduated college with a BS in Criminal Justice with a minor in Political Science. She married her husband and began working in the family insurance business in 2005. She became a licensed agent and wrote P&C business focusing on personal lines insurance for 10 years. Laura serviced existing business and wrote new business. She now uses her insurance background to help educate...

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Reviewed by Laura Walker
Former Licensed Agent

UPDATED: Jul 16, 2021

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George Winterling first developed the heat index in 1978. By 1979 the United States National Weather Service had adopted the index that was developed from the work done by Robert G. Steadman. It is similar to the wind chill index in the assumptions it makes it regards to body mass, clothing, activity, etc. From the United States to Canada the heat index varies slightly. The base dew point used to calculate the heat index is different in the two countries. In Canada the base dew point is 45F and in the United States the base dew point is 57F.

Heat Index

The heat index (HI), also known as humiture, refers to a combination of air temperature and humidity that affects the inhabitants of an environment. The heat index attempts to draw comparisons to the human perception of how hot it feels within a given environment. This is a concept known as “felt air temperature.” The heat index affects the human body’s ability to cool itself through perspiration, which causes it to retain more than it normally would in a drier climate. The heat index can be formulated based on the overall subjectivity of how an object feels in accordance with the room temperature and humidity of the environment in which it currently resides.


Physicists define heat as transferable energy by thermal interaction. Heat flows interchangeably between two systems of higher and lower temperatures. When these systems merge, they exchange energy through the microscopic interactions of their constituents. If both systems remain at different temperatures, then the heat flows spontaneously from hotter to cooler temperatures, resulting in the hotter energy cooling at variable rates. This irreversible process, also known as thermal equilibrium, continues until both temperatures stabilize to equal proportions or until the net flow of energy reaches zero.


Humidity refers to one of several measurements of water vapor and other constituents suspended in the air. The content of this mixture is called absolute humidity. Relative humidity refers to the water vapor content expressed in meteorological forecasts and humidistats, a monitoring device that measures the amount of humidity in a given environment. In fact, meteorologists assert that high relative humidity increases the likelihood of precipitation, dew, and fog. Meteorologists determine the humidity rate by analyzing the data collected from global satellites. In addition, they can predict the weather several days in advance. Relative humidity reduces the human body’s ability to cool through perspiration. Therefore, humans have become reliant on air conditioning systems to cool their body in areas that have become saturated with relative humidity levels. Without air conditioning to cool the human body, people may become susceptible to certain respiratory problems, especially if the individual already suffers from medical conditions affecting the lungs. Other sufferers may experience a variety of symptoms, including dizziness, faintness, fatigue, dehydration, numbness, loss of concentration, loss of motor skills, and more. Air conditioning reduces the humidity and temperature levels during the summer. During the winter, air conditioning and heating units regulate the relative humidity to deter it from falling below a certain level, which may cause various symptoms of discomfort, such as dry skin, dry eyes, chapped lips, and excessive thirst. People should take extra care to ensure that their skin, hair, and eyes remain moist during the winter months.

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