The “Heat Index” will be above 100 degrees the next couple days! But what does this actually mean?

Officially in Fond du Lac Saturday, the heat index hit 102 degrees. This came with an air temperature of 91 degrees and a dewpoint of 75, which made the “relative” humidity about 60%.

What does this all mean? The heat index is similar to a wind chill temperature. It is not the actual temperature. It is however what the temperature “FEELS like” on our skin. Its an equation that factors the air temperature and the amount of moisture in the air. So a temperature of 90 but with a 40% relative humidity would only equate to a heat index of 91. The heat index is directly related to the moisture in the air (dew point temperature) because the amount of sweat that builds up on your body is what makes you FEEL hotter than what the temperature is. So if the relative humidity is lower, your sweat will evaporate faster which is our body’s way of cooling you off. If the sweat can’t evaporate as fast as it forms, you feel hotter. The more moisture in the air, the slower the sweat evaporates from our body, and the hotter you feel = higher heat index.

Saturday was a bit of an exception to all of this because it was windy. Wind is not included in the equation to achieve the heat index so if it is windy, that will aid in helping to evaporate some of our sweat faster, so it wasn’t as bad as a day with the same temperature and relative humidity, but with no wind.

On a related note, you may wonder what “Relative Humidity” actually is. Relative humidity is how much moisture is in the air compared to how much moisture the air can hold before condensing (Turning to cloud). It is NOT a measure of the actual moisture in the air. This is why its called relative. Its the amount of moisture, relative to the temperature. This is why a relative humidity of 70% in summer would feel miserable but a relative humidity of 80% in winter could still be bone dry.

The dewpoint compared to the temperature is what gives us a relative humidity.

In summer, a temperature of 80 degrees with a dewpoint of 40 degrees gives us a relative humidity of 22%. That would be considered dry air. However a temperature of 80 degrees with a dewpoint of 70 degrees would give us a relative humidity of 77%. That would feel pretty muggy. The higher the dewpoint, the more muggy the air feels. The closer the dewpoint is to the air temperature, the higher the relative humidity, however higher relative humidity doesn’t mean higher moisture.

In winter we could have a temperature of 20 degrees with a dewpoint of 15 degrees. Because the dewpoint is close to the temperature, the relative humidity will be high. In this case 73%. But that air would still be dry because a dewpoint of only 15 degrees is dry air. BUT… RELATIVE to a temperature of 20 degrees, its a high percentage of moisture for that temperature. Thus the term RELATIVE humidity.

And one other side note… The dewpoint temperature will always be less than or equal to the air temperature. The closer the dewpoint gets to the air temperature, the more saturated the air becomes. Once the dewpoint meets the air temperature, the air becomes completely saturated and we get fog. So if you know the temperature is 60 degrees and it’s foggy, you would be safe to assume the dewpoint is 60 degrees as well.