Heat Index Explained

Heat index explained


We are in that time of the year when you’ll hear meteorologists mention the heat index. You ever wonder what it means and why it always seems to feel hotter than what the thermometer reads?


What You Need To Know

  • The heat index is what it really feels like outside
  • It combines the humidity and the air temperature
  • The higher the humidity, the higher the heat index
  • Know the difference between heat exhaustion and heat stroke

It all has to do with evaporation which is a cooling process.

When it is hot, we sweat. The higher the humidity is, the lower the evaporation rate.

When there is more moisture in the air, your sweat stays on your skin and does not allow you to cool down nearly as fast. We call this factor the heat index or what it actually feels like outside.

On the other hand, the lower the humidity, the more effective the evaporation rate. That is why you feel cooler.

Think about when you get out of the shower. You feel cooler because the air inside your home has relative low humidity. The water droplets on your skin are evaporating rather quickly making you feel much cooler.

Below is a chart of the relative humidity level versus the temperature and what it actually feels like as the humidity climbs. As you would imagine, the higher the humidity, the hotter it will feel outside.

Heat Index Chart

The heat index calculation was derived from a shady area with light wind. In full sunshine, the heat index can easily reach 15 degrees hotter than in shaded areas.

It is also important to note the signs of heat exhaustion and heat stroke. Heat stroke is the most dangerous of the two and requires immediate medical attention.

Heat Exhaustion versus Heat Stroke

Remember to always take frequent breaks if you must work outside in the heat and humidity. Also, drink plenty of fluids, especially water to stay hydrated and know the signs of heat exhaustion and stroke.

It could save your life.



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