In the summer you'll often hear weather reports give you an actual temperature

and then a feels like temperature. This feels like temperature is always much

higher than what the actual temperature is.

This feels like temperature is what's known as a heat index.

The heat index is mainly based on two things:

the actual temperature and humidity.

What is humidity? Well humidity is a general term that's used for the amount of moisture in the air.

This is usually expressed in terms of relative humidity or as a percentage.

Relative humidity is the percentage of moisture in the air

compared to the maximum amount of moisture the air can hold at that temperature.

Warmer air can hold more moisture.

Therefore in the summertime, the air is holding much more moisture than it would

be in the winter.

How does this relate to humans? Well more moisture in the air

slows the amount of the evaporation. Evaporation removes moisture or sweat

from our skin

which also removes heat, which cools our body. With high temperatures and high

moisture, we can sweat but were not able to remove the heat from our bodies.

Our internal body temperature increases, which can lead to such things as heat cramps,

exhaustion or heat stroke.

The heat index was developed from a mathematical formula.

We usually see this as a chart.

For example: with that actual temperature at 90 degrees Fahrenheit,

coupled with a relative humidity of sixty percent this creates a heat index

of 100 degrees.

The National Weather Service issues alerts when the heat index is expected to

exceed 105 degrees for two consecutive days.

The index should be used as a guide when planning an outdoor summer activities.