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

But is there a real official day when we can say with certainty that the season has arrived? That depends on who you ask.

An astronomer would probably say it begins on the summer solstice, which occurs on or around June 21 in the Northern Hemisphere. The date is based on the Earth’s annual journey 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 the longest day of the year. Astronomical summer ends at the autumnal equinox, when the sun is directly above the equator and day and night are approximately the same length. (The exact dates of the solstices and equinoxes vary because the planet takes just over 365 days to complete its orbital journey – a variation that we account for with a leap day every four years. For the Southern Hemisphere, summer begins on or around December 21.)

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But if you ask any meteorologist, they’ll tell you that summer in the Northern Hemisphere starts on June 1. This is partly because meteorologists and climatologists need consistent data to look for trends in weather and climate data. For convenience, they group the seasons into four periods of three calendar months; for summer these are June, July and August.

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

The time frame for what we call “summer weather” is changing as the world warms from the continued burning of fossil fuels. A 2021 study Geophysical research letters found that summers had grown from 78 to 95 days while the other three seasons shrank, based on when the highest 25 percent of temperatures occurred in a given location. In the future, summer could last even longer unless immediate, aggressive action is taken to limit greenhouse gas emissions.

Summer also appears to be expanding when you look at heat wave data. What constitutes a heat wave obviously varies from place to place, with temperatures in Portland, Oregon, for example, being relatively lower than in a place like New Orleans. Research shows that heat waves are becoming more frequent, longer lasting and more intense across the country, based on local criteria. They also occur earlier and later in the year than in recent decades, which essentially means that the type of weather associated with summer is lasting longer than it used to.

Averaged across fifty major 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 before.

Longer summers aren’t just a matter of turning on the air conditioning earlier; they pose a major public health problem. Heat is the deadliest extreme weather event in the US. When summer temperatures arrive earlier in the season and last as a heat wave for days, people have less time to acclimatize. And they may not be prepared to take precautions to keep cool, especially when they think about summers past.