When Does Summer Start? | Scientific American

What Are Astronomical Summer and Meteorological Summer, and How Do They Differ?

By Andrea Thompson

For many in the U.S., “summer” begins when the three-day Memorial Day weekend ushers in a season of grilling, swimming in pools and drinking lemonade. For others, summer starts whenever school ends or when we reach a specific date on the calendar. For others it might be when the first really hot day happens.

But is there any one solidly official day when we can conclusively say the season has arrived? That depends on who you ask.

An astronomer would likely say it starts on the summer solstice, which happens in the Northern Hemisphere on or around June 21. Its date is based on the Earth’s annual trip around the sun and the angle of our planet’s axis; it occurs when the North Pole is tilted directly toward the sun, giving the hemisphere its longest day of the year. Astronomical summer ends at the fall equinox, when the sun is directly over the equator, and night and day are of roughly equal length. (The exact dates of the solstices and equinoxes vary because the planet takes just slightly longer than 365 days to make its orbital journey—a variation we account for every four years with a leap day. For the Southern Hemisphere, summer begins on or around December 21.)

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But if you ask a meteorologist, they will say the Northern Hemisphere’s summer begins on June 1. This is partly because meteorologists and climatologists need consistent dates in order to look for trends in weather and climate data. For ease, they group the seasons into four three-calendar month periods—for summer, this is June, July and August.

But neither those dates nor the solstice-equinox period necessarily cover the full time span of weather conditions most people associate with the season. “It’s really hard to say what is the start of summer,” says Jared Rennie, a National Oceanic and Atmospheric Administration research meteorologist. “We just like things in three-month increments.”

The time range for what we think of as “summer weather” is changing as the world heats up from the continued burning of fossil fuels. A 2021 study in Geophysical Research Letters found that summers had grown from 78 to 95 days as the other three seasons shrank, based on when the highest 25 percent of temperatures occurred in a given location. In the future, summer could grow longer still unless immediate, aggressive action is taken to curtail greenhouse gas emissions.

Summer also looks like it’s expanding if one considers heat wave data. What constitutes a heat wave of course differs from place to place, with comparatively lower temperatures qualifying in somewhere like Portland, Ore., than in a place like New Orleans. Research shows heat waves are happening more often, lasting longer and becoming more intense all over the country, based on the local criteria. They are also occurring earlier and later in the year than they did in decades past, effectively meaning that the kind of weather associated with summer lasts longer than it used to.

When averaged across 50 large U.S. cities, the heat wave season has grown from about 24 days in the 1960s to 73 days today. This means it is 49 days longer than it used to be.

Longer summers aren’t just a matter of needing to turn on the air conditioning sooner; they pose a major public health concern. Heat is the deadliest extreme weather event in the U.S. When summer temperatures arrive earlier in the season and persist for days as a heat wave, people have less time to become acclimated. And they may be unprepared to take precautions to keep cool—especially if they are thinking in terms of the summers of the past.