Behind the Forecast: How the heat index is calculated

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

Science Behind the Forecast: Heat Index

LOUISVILLE, Ky. (WAVE) - With temperatures soaring into the 90s and high humidity in the forecast, meteorologists (including myself) have been bombarding viewers with talk of the heat index. So what is the heat index anyway?

To explain how the heat index works, it's important to note how humidity keeps the body from cooling efficiently. Our bodies cool themselves by sweating. This sweat evaporates, taking the heat away from our bodies. When humidity is high it lowers the amount of evaporation that can occur on our skin. The excess moisture remains on our skin, negating the cooling necessary to deal with the heat.

Remember warm air can hold much more moisture than cold air. This is why the air can feel soupier when the temperatures are higher.

The National Weather Service defines the heat index as "how hot it feels when the relative humidity is factored in with the air temperature." The relative humidity is "a dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated." Relative humidity on its own does not actually dictate how much moisture is in the air.

The heat index equation is a bit complicated. Here it is.

HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH

  • HI is the heat index in degrees Fahrenheit
  • T represents the temperature in degrees Fahrenheit
  • RH is the relative humidity percentage

There are several adjustments made to this equation depending on the relative humidity and temperature ranges.

Confusing right? Well, that’s why the National Weather Service has this handy chart.

So if the air temperature is 95° and the humidity is 70% it will feel like 105°. At a temperature of 88° with relative humidity at 100%, the heat index would be 121°.

We have Excessive Heat Warnings out for our entire area this weekend. These are issued within 12 hours of dangerous heat. Heat indices must be 105°+ for at least two days with nighttime temperatures above 75° for them to be issued.

An important thing to note is that the heat index is based on what it feels like in the shade. Direct sunlight can add up to 15 degrees to the heat index.

The record heat index for Louisville was set back on July 2nd, 1980. This is when we saw a heat index of 120°.

While breezes typically have a cooling effect, strong winds and hot air can lead to even more dangerous conditions. This is because the wind whips the sweat away from our bodies before evaporation can cool us down.

Heat is one of the leading weather-related killers in the United States, according to the National Weather Service. As of July 18, 9 toddlers have died in hot cars.

A recent study found that the number of days with a heat index over 100° may double across the United States by 2050. On average in Kentucky, we see about six days each year with a heat index of higher than 100°. That could rise to 45 days by 2050 if climate change continues on its current track. In Indiana, there are four days on average each year with a heat index above 100°. That could increase to 34 days per year by 2050 for the Hoosier state.

At the end of the day, it is important to use precautions when temperatures and humidity on high. Below are some symptoms of heatstroke and heat exhaustion to keep an eye out for during our heatwave.

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