Is It Too Hot to Play?

Here in Massachusetts, as in much of the country, we’re heading into a stretch of sweltering days. With that in mind, we’re going to postpone the Worst Dressed poll and talk about the heat.

You already know that during exercise your core body temperature can rise by a couple of degrees. Your body copes with this increase by sweating. As the sweat evaporates, you cool down.

When humidity is high, the air is too saturated for your sweat to evaporate quickly. The higher the humidity, the hotter you’ll feel, and the more dangerous any given temperature will be.

How do you know when it’s too hot to play outdoors? You need to find out the heat index, or the “real feel” temperature, a combination of actual temperature and humidity.

Forecasters will often include the “real feel” temperature on especially hot days. But if you know only the actual temperature and relative humidity, you can find the “real feel” temperature by using the National Weather Service’s heat index chart.

Take a quick glance at the chart, and you’ll see that any heat index of 90 or higher falls in the “extreme caution” zone. The chance of heat-related illness, like heat exhaustion or heat stroke, increases.

What’s not apparent in the chart is that the “real feel” temperature chart was calculated for shade conditions with a light breeze. If you’re running around on a tennis court in full sun and no breeze, the heat index can be as much as 15 degrees higher! To exercise safely in those conditions, you’d need an index of 75.

You won’t find a heat index of 75 on the above chart, but you’re ideally looking for relative humidity below 40% when the temperature is in the low 80s. When the temperature hits 90, you’re already in the “extreme caution” zone, even with no humidity.

To recap, if you’re playing in shade with some air movement, a sub-90 heat index is best. If you’re playing in sun with no breeze, you really want a sub-75 heat index.

I checked my town’s hourly forecast for the weekend on These are the time windows with a sub-90 heat index:

10 p.m. Friday to 8 a.m. Saturday
4 a.m. Sunday to 8 a.m. Sunday

Not ideal hours, but on the plus side, I shouldn’t have much trouble finding shade. Or a free tennis court.

Outside those hours, I’ll have to get my tennis fix by playing indoors or working on our “Wimbledon’s Worst Dressed” post. (Is there anyone you think should be nominated? Let me know in the comments.)

Stay safe and cool, everyone!!!

6 thoughts on “Is It Too Hot to Play?

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  1. Thanks for sharing this important info. You clearly did some research, and it’s so interesting and useful. I had no idea that the heat index was dangerous at these temperatures. I wonder why the parameters are shade with a light breeze. That’s not the most common environment, I would think — although perhaps it’s what people tend to seek when the weather is hot and humid. But you just can’t count on a breeze.

    1. I don’t know why they calculated it for shade. I certainly wasn’t aware of that.

      I find the older I get, the less tolerant my body is of the humidity. I find it more suffocating than I did as a kid. I remember just going outside and playing all day in the summer without being troubled by humidity. Nothing slowed me down.

      Or maybe it’s just more humid than it used to be??? Global humidifying?

  2. Very helpful. I decide based on instinct but this approach is better. You didn’t cover though best way to meet hydration needs. Maybe a follow in?

    1. I left out the hydration because if I packed TOO much useful info into this post, your head would explode.

      Information is power. You can’t be too careful with that stuff.

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