ROCHESTER, NY (WROC) – Air temperature can be a tricky thing, especially when there are multiple variables in the air that have an effect on temperature and the way it feels. Having an air temperature under dry conditions is one story, but feeling that same temperature in a much more humid atmosphere can make the actual temperature feel a whole lot hotter and more uncomfortable than it actually is. This ‘real-feel’ to the air found especially during the “heat” of summer is known as the heat index. 

The heat index is defined as how hot the air actually feels like when factoring in both air temperature and relative humidity. It is also known as the apparent temperature, which is the temperature perceived by humans. The heat index can be calculated using either dew point, or relative humidity values compared with the actual air temperature.

Fun Fact: The index was developed by Robert G. Steadman in 1979 after various studies and analysis. 

The NWS then performed further analysis to develop an equation that would predict heat index values given the current air temperature and relative humidity. Below is a chart of some these values:

Photo courtesy of NOAA

The heat index heavily relies on how much moisture there is in the air.  For example, an air temperature at 85℉ with a dew point of 70℉, or 63% relative humidity will actually feel like 90℉. This new value is considered to be the heat index. However, if you have an air temperature at 85℉ with a dew point of 60℉ and relative humidity of 45%, it will still feel like 85℉. The heat index can even be lower than the temperature with a low enough relative humidity. You can calculate the heat index yourself by clicking here. Try it!

When your body gets hot, its natural response is to sweat. If there’s a high amount of moisture in the air, it’s harder for the sweat on your skin to evaporate back into the atmosphere. This makes it difficult to regulate your body temperature especially in hot and humid and can take a toll on your body. Below are some of the effects of intense heat:

Photo courtesy of NOAA

Things to note:

Excessive Heat Advisories are issued when the heat index reaches 105℉ to 104℉ here in WNY within the next 12 to 24 hours. 

An Excessive Heat Watch is issued when there is a potential for the heat index value to reach or exceed 105℉ within the next 24 to 48 hours.

An Excessive Heat Warning is issued when the heat index value is expected to reach or exceed 105℉ within the next 12 to 24 hours.