Hi, this is Duane friend with University of Illinois Extension. 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 the 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
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 we're 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 the actual temperature of 90 degrees Fahrenheit
coupled with a relative humidity of 60%, 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 heat index should be used as a guide when planning outdoor summer activities.