What exactly constitutes a ‘heat wave’ — and does it matter?

We’re definitely in a heat wave, by New Hampshire standards at least, but what that mostly means is: Be careful.

There is no set definition of a “heat wave” the way there are agreed-upon standards for other dangerous weather events such as tornadoes. The term refers to a period in which maximum temperatures exceed the average maximum by a certain amount, but the period of time and amount of excess differ from place to place.

The National Weather Service in Gray, Maine, which covers New Hampshire, defines a heat wave as three days with maximum temperatures topping 90 degrees Fahrenheit at our official weather station at Concord Municipal Airport. We’re there as of Thursday and the streak is expected to continue through at least Saturday.

The nation of India, on the other hand, says it’s not a heat wave until the maximum exceeds 104 degrees on the plains – or 88 degrees in the mountains, meaning the Himalayan foothills – for two days at two adjacent weather stations. Hopefully, we’ll never see an Indian flatland heat wave, although with climate change that’s no longer certain.

The designation of a heat wave doesn’t do anything officially such as free up emergency funding or mandate government or private action. But it is a useful signal for city or state officials to do such things as open cooling centers or, as Concord has done, extend the hours of public swimming pools.

In that way, it is similar to the Weather Service’s designation of various stages of drought, a useful signal for communities to think about imposing water-usage restrictions.

For individuals, being in a heat wave is a reminder to be sensible outdoors. If you start feeling woozy that might be the start of mild heat stroke, so ramp things down and drink a lot of water.

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These articles are being shared by partners in The Granite State News Collaborative. For more information visit collaborativenh.org.