Heat index and heat-related illness

Weather Knowledge

LITTLE ROCK, Ark. – Heat and humidity in the forecast can make it feel even hotter than the temperature reads true. The National Weather Service uses the heat index to help the public understand the effects of the conditions outside, how hot it will really feel and how it may impact

The heat index is a measure of how hot it will really feel outside when relative humidity is combined with the actual air temperature.

In fact, there is a direct realtionship between air temperature and relative humidity and the heat index. When there is a rise in temperature and relative humidity, the heat index will rise. When the temperature and relative humidity falls, the heat index will also decrease.

The following chart from the National Weather Service shows the heat index for temperatures above 80 degrees and relative humidity of at least 40%.

NWS Heat Index Chart

To read the chart, locate the temperature on the top side and the relative humidity on the left side. Follow the column down for the temperature reading and over for the relative humidity percentage – where the column and row meets is the heat index value given those two components.

For example, if the air temp is 96 degrees, and relative humidity is 65%, the heat index is 121.

It’s important to note that heat index values are measured for shaded area where wind is light. Exposure to a clear sky of sunshine can increase the heat index value by up to 15 degrees.

The NWS will initiate alerts when the heat index exceeds 105-110 for at least 2 consecutive days – this varies by location, though.

Here is the criteria for heat alerts in the NWS Little Rock forecasting office area:

Heat Advisory – issued when heat index values are expected to reach 105 or higher or temperatures get up to 103 degrees or higher.

Excessive Heat Watch – issued when heat index values of 110 or higher are possible or temperatures of 105 degrees or higher is expected. Issued 12 to 48 hours in advance.

Excessive Heat Warning – issued when heat index values are expected to reach 110 or hotter or temperatures of 105 degrees or higher.

Heat and Health

High heat indices as a result of heat and humidity can have serious impacts to one’s health.

Our bodies get hot, and we begin to sweat as a result of our bodies trying to cool down. Evaporation is a cooling process, so if the sweat cannot evaporate, the body won’t regulate its temperature. When humid air is present, it’s harder for the moisture on one’s body to evaporate because the air is already saturated. Therefore, the human body will feel warmer in humid conditions because less evaporative cooling is taking place.

As the body temperature rises, you could experience a heat related illness. Knowing the symptoms assosciated with each illness will help you determine which actions to take to alleviate the situation.

Heat index value effects to the body.
Credit: National Weather Service

The Centers for Disease Control (CDC) provides warning signs and symptoms as well as recommended first aid steps for heat related illnesses, below.

Heat Exhaustion

  • Symptoms: feeling faint or dizzy, excessive sweating, skin that may be cool, pale or clammy, nausea or vomiting, a rapid or weak pulse and/or muscle cramps.
  • First aid actions: get to a cooler, air conditioned place. Drink water if conscious. Take a cool shower or use a cool compress.

Heat Stroke

  • Symptoms: throbbing headache, no sweating, a body temperature above 103 degrees, red, hot or dry skin, nausea or vomiting, a rapid or strong pulse, and/or loss of consciousness.
  • First aid actions: call 9-1-1 immediately and cool down as best as you can until help arrives.

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