Weather 101: What is the Heat Index and Why do we Care?

Weather 101

Weather 101: We will be discussing what the heat index is and why we should care, especially in the middle of summer when we are talking about both heat and high humidity.

The exact definition of Heat Index from the National Weather service is as follows: The heat index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature (what it would feel like without any moisture in the air).

We must first discuss why there is such a concern when the humidity levels start to increase. When it gets hot, your body’s natural response is to sweat. This is the mechanism used to cool your body, when there isn’t much humidity in the air, the air is dry, this allows the moisture on your body to evaporate rather easily and quickly which cools the body. However, even just a little bit of humidity can put a damper on this effect. Higher humidity values will limit the evaporation of the sweat produced by your body which makes you feel more uncomfortable and can also cause exhaustion sooner.

To find the heat index temperature the National Weather service has a heat index chart, for example, if the air temperature outside is 80 degrees and you have a relative humidity of 75%, the air would actually feel more like 84 degrees.

The red areas toward the top of the chart indicate temperatures that would be extremely dangerous to anyone working outside in the heat or who would be enjoying leisurely activities for a long period of time.

If the heat index for the Albany viewing area is forecast to reach 95°-99° for two or more consecutive hours the National Weather Service would issue a Heat Advisory. This is issued when DANGEROUS HEAT conditions are expected.

If the heat index for the Albany viewing area is forecast to reach 100°-105°+ for two or more consecutive hours then the National Weather Service would issue an Excessive Heat Warning. This is only issued when EXTREMELY DANGEROUS HEAT conditions are expected.

Think of a hot summer day, much like the ones we’ve been experiencing recently with temperatures in the afternoon reaching into the mid to upper 80’s. If the humidity remains low on an 86° day then there would be little to no HEAT INDEX value. However, if the humidity begins to approach 75% then we would be talking about a heat index value in the upper 90’s to near 100°, this is when you would need to use extreme caution as heat exhaustion could set in rather quickly.

Now, we don’t often get too many days in the summer with air temperatures in the low to mid 90’s, but of course, it can happen and below is another chart showing you what the heat index would be for varying humidity levels if the air temperature were to be 92°. At 75% humidity the Heat Index or real feel temperature is all the way to 116°, this is the danger zone and days of that magnitude you should limit your exposure outside.

According to the National Weather Service, Heat Index values are devised for shady, light wind conditions, so that means that if you were to be exposed to full sun the heat index value can be up to 15° higher than the values you see on the charts above. The National Weather Service also notes that with strong winds, particularly with very hot and dry air can become extremely hazardous.

So to stay safe in the heat and humidity that we will likely be seeing in the coming weeks and months here are a few tips. Always remember, NEVER leave pets or kids inside a parked vehicle, always drink plenty of water to stay hydrated. You will want to reduce strenuous outdoor activities, or if you cannot avoid that be sure to take plenty of breaks in the shade or an air conditioned room. Another thing that can help, which I am sure you have heard many people recommend before, is to wear lightweight, light colored clothing when you must be outside.

Do you have a weather question you’ve been dying to ask? Send an email to Matt Mackie ( or Rob Lindenmuth ( to get it answered in the next Weather 101!

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