What is the Heat Index?

There has been a lot of talk surrounding the heat that we had today and the heat we will have on Saturday. After all, it is the warmest air we've seen in 2016 thus far. However, the effects of this mini heatwave expand beyond simply taking a gander at the thermometer. When measuring the severity of heat, there is sometimes another component that needs to be analyzed to understand how truly brutal the heat will be on the human body. That is what we call humidity. This can be measured as relatively humidity or a dew point temperature. Both can be used to determine what the air will feel like when you step outside. 

In terms of dew points, it is pleasant when they are below 55°. Once you hit the 55-65° range it begins to feel humid. Anything above 70° is essentially oppressive, or very muggy. When you have dew points (or relative humidity values) that are very high, it makes the actual air temperature simply a number. That is because to grasp what it actually feels like to your skin, you must account for not only the temperature outside, but the humidity value as well. So if you have a high of 100° but very low dew points (like in the desert), it will likely not feel that bad. However, if you have a temperature in the upper 80s with dew points near 70°, it will feel sticky, uncomfortable, and very hot outside. That is the price we pay for living in the Midwest!

You can have a heat index with a temperature as low as roughly 80° if the humidity is high enough. The heat index can be upwards of 30-40° above what the actual air temperature is! And it is these values that become dangerous and potentially life-threatening if you spend significant amounts of time outdoors without using appropriate precautions.
Heat exhaustion, heat cramps, heat stroke, and other heat-related illnesses are all possibly outcomes of spending too much time in excessive heat. And while we won't meet the criteria to reach a "Heat Advisory" level on Saturday, our heat index values will be in the upper 90s to near 100°. That is hot enough to cause heat-related illness, especially considering this is our first significant heatwave of the year.
Our bodies have not acclimated to the heat quite yet, so make sure you stay hydrated, wear light colored clothing, and avoid direct sunlight if possible on Saturday!