SAVANNAH, Ga. (WSAV) – Today is the last day of Heat Safety Week. As we end the week, let’s break down something you hear Storm Team 3 say all the time “Heat Index”. This will help you better understand one of the forecasting tools we use to show how dangerous heat can be.

Heat Index

The heat index is a measurement of how hot it actually feels when relative humidity is factored in with the actual air temperature. Relative humidity measures how much moisture is in the air. The higher the number, the more moisture. The more moisture, the harder it is for your body to naturally cool itself down, leading to a higher heat index temperature. If ignored, this can lead to heat-related health problems.

The National Weather Service has a heat index calculator that shows you what the heat index temperature will be and if the heat index exceeds 105°, they will issue alerts and/or warnings. Pay close attention to the heat index and the relative humidity in the summer – both go hand in hand.

National Weather Service Heat Index

Heat Advisory vs Warning

The National Weather Service in Charleston covers most of our communities. They are the ones that issue Heat Advisories or Excessive Heat Watch/Warning. Here are the criteria for each:

HEAT ADVISORY: Through June 30, 105-109° for 2 hours; On or after July 1, 110-114° for 2 hours

EXCESSIVE HEAT WATCH/WARNING: 115° or greater for 2 hours

An excessive heat watch means BE PREPARED for dangerous heat. An excessive heat warning means TAKE ACTION in dangerous heat and stay cool.

Wet-Bulb Globe Temperature

This might be something you probably have never heard us say, but it is a good forecasting tool when it comes to summer heat. The Wet-Bulb Globe Temperature (try saying that 5 times fast) is an indicator of heat stress for those doing outdoor activities in the heat. It combines temperature, humidity, wind, and solar radiation.

When wet-bulb globe temperatures are higher than 78 degrees, the risk of heat illness increases. This temperature forecast is used by the Occupational Safety and Health Administration (OSHA) and the American College of Sports Medicine. Many schools use it as well to use as a guideline for sports in the heat.

Heat Index vs Wet Bulb Globe Temperature

With both of these forecasting tools used to forecast the dangerous heat, what is the difference between the two? The biggest difference is where each is measured. The heat index is measured in the shade. The shade measures the actual temperature. Being in direct sunlight can make the air feel 10-15 degrees warmer than it actually is because of the sun’s rays touching your skin. This would create an inaccurate reading of the heat index.

With that being said, the wet bulb globe temperature is measured in the sun, but it uses different factors to give you an accurate reading like temperature, humidity, wind, and the sun’s angle. This is best used for those who are active in the heat like outdoor workers and athletes. The heat index can be used by everyone as it measures what those summer hot temperatures feel like to the body when humidity is combined with the air temperature.

Heat Tracker

The Centers for Disease Control and Prevention (CDC) have created a Heat and Health Tracker allowing you to look at heat health-related forecasting topics. This will help communities to better prepare for extreme heat events. You can click on different tabs that show current temperatures, weekly heat-related illness, daily heat-related illness, and historical records.