September Heat Wave


After an August that saw high temperatures a few degrees below average, September has seen daily highs near 90. Fall won't officially begin until September 23rd, so it doesn't come as a complete surprise that summer is trying to get in one final word.

As you crank up A.C. and do your best to stay cool, the term "heat wave" may come to mind. A heat wave is a prolonged period of excessive heat, often combined with excessive humidity. A heat wave is going to be different depending on its location and the climate of that region. For example, three straight days of 95 temperatures in Boston would be pretty noteworthy, but the same weather in Phoenixnot so much.

One definition from the Heat Wave Duration Index says that a heat wave occurs when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 C (9 F). For example, if the average high is 85, five straight days at 94 or above would constitute a heat wave. In the Heartland, we should be 81 for early September.

Heat waves are generally the result of trapped air. During the country's last big heat wave in 2012, air was trapped above much of North America for a long period of time. Instead of cycling around the globe, it simply stayed in place and warmed like the air inside an oven. This was due to a growing high pressure system parked over the Central United States. You'll hear meteorologists talk about high pressure all the time. High pressure is associated with stable weather and sinking air. During a heat wave, the sinking air acts as a cap, thus preventing the air at the surface from rising.

During a particularly nasty heat wave, the National Weather Service will issue heat advisories and excessive heat warnings when extreme periods of hot weather are expected. Those weather alerts are crucial because heat waves kill more people in the United States than all other weather-related disasters combined.

The heat index, the "feels like" temperature for how hot it is outside, will coming into play during a heat wave too. If you know the temperature along with the dewpoint or relative humidity, you can even calculate it yourself here. As a general rule, when the dewpoint starts to get above 60 to 65, it can begin to feel uncomfortable. When the relative humidity is above 80%, you'll really start to notice the moisture in the air. Therefore, the worst combination for human comfort would be a high dewpoint and a high relative humidity.

That does it for this installment of the weather blog. Thanks for reading and be sure to email me at if you've got an idea for a topic on a future entry.

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