Reaching the time of high heat and high humidity

AUSTIN (KXAN) -- Stop me if you have ever heard this phrase: It's not the heat, it's the humidity. I'm sure many of us have not only heard it but have repeated it to people who visit Central Texas in the late spring and summer.

It's a given that we sweat more in the spring and summer because temperatures warm appreciably in both seasons. But there are times when our bodies are perspiring much more even when we just walk out the door to our cars.

It's called the heat index. This is it's common name. It's the apparent temperature. It's what the temperatures feels like to our bodies when combining the actual air temperature with the relative humidity. If you are new to this area you will recognize this on those days when the humidity is high.

By the way, the heat index has a cousin. It's called the wind chill. Unlike the heat index, the wind chill factors temperatures and wind speeds.

This seems like a good time to address this subject because on Saturday, May 18, a few reporting stations in the KXAN viewing area did have triple-digit heat index readings. More were predicted for Sunday, May 19.

Why is knowing the heat index important? It's all about your health. When your body gets too hot, you begin to sweat in order to cool yourself. That's evaporation.

When the humidity is high, especially in the morning it reduces the rate of evaporation. Your body feels warmer when the humidity is elevated.

The opposite of this is true. When the humidity is low, the rate the body's ability to cool itself is more efficient. The body actually feels cooler.

The heat index is easy to calculate if you know two numbers. You need to know the air temperature and the relative humidity. You cans substitute the relative humidity with the dew point. By the way, the readings require you are using a shady location. All weather service weather stations use shaded areas to house weather instruments.

Is the heat index always higher than the actual air temperature? No. In drier climates, say, our Hill County, you will have hours of the day when the heat index is below the temperature.

Here's an example. On a recent Saturday afternoon, Camp Mabry had a 95° temperature with a 40% relative humidity. That created a dew point of 99°. If the humidity had been much lower, say 25%, the heat index at the same 95° temperature would be 93°.

Listen to your body when spending time outdoors working or otherwise. We're talking about prolonged exposure from physical activity. When the heat index gets to:
90° - fatigue could set in
91° to 103° - heat cramps, heat exhaustion, or heat stroke are possible
104° to 124° - heat cramps or heat exhaustion are likely and heat stroke is possible

These are words you will hear a lot during the summer season. A Heat Advisory is issued when the maximum heat index is forecast to be 100° or higher for at least two afternoons, and when overnight lows will not drop below 75°. An Excessive Heat Watch is issued when it is determined that conditions are favorable for an excessive heat event during the next one to three days.

An Excessive Heat Warning means just one thing: Take Action. These are issued within three days of dangerous heat conditions. It's when the maximum heat index temperature is expected to be 105° or higher for at least two afternoons and overnight lows don't drop below 75°.

It is common in that part of Central Texas to have triple-digit heat index readings to still be above 100° as early as 10 to 11 a.m. and as late as 8 to 9 p.m. This is true of many localities east of I-35.

Long-time Central Texans may be used to the excessive heat and the unusually high heat index readings. But many don't. We remind you, as always, that you are as much a part of the advisory/watch/warning process as our KXAN meteorologists are. When we get information from the National Weather Service about how excessive the heat will be, we pass it on to you both graphically and verbally.

What you choose to do with that information can make a difference in staying safe during what will soon be the long summer season.