While the summer solstice is not until June 20, many scientists consider the start of summer to be much sooner. That’s because June 1 marks the start of meteorological summer.
There are two types of seasons: meteorological and astronomical. Meteorological and astronomical seasons have different start and end dates because meteorological seasons are based on the annual temperature cycle, while astronomical seasons rely on the Earth’s position in relation to the sun, according to the National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Information (NCEI).
“Meteorologists and climatologists break the seasons down into groupings of three months based on the annual temperature cycle as well as our calendar,” the NCEI said on its website. “We generally think of winter as the coldest time of the year and summer as the warmest time of the year, with spring and fall being the transition seasons, and that is what the meteorological seasons are based on.”
In the Northern Hemisphere, meteorological spring lasts from March to May, summer from June to August, fall from September to November, and winter from December to February.
Meanwhile, astronomical seasons—what many people consider to be “regular” seasons—are defined by two solstices and two equinoxes, determined by Earth’s tilt and the sun’s alignment over the equator.
In the Northern Hemisphere, the summer solstice usually falls around June 21, the winter solstice around Dec. 22, the spring equinox around March 21, and the autumnal equinox around Sept. 22. In the Southern Hemisphere, the seasons are reversed but have the same dates.
The dates for the solstices and equinoxes can vary depending on the year because the Earth travels around the sun in 365.24 days, leading to a Leap Year—an extra day every fourth year. Because the Earth’s orbit around the sun is elliptical in shape, the lengths of astronomical seasons can also range from 89 to 93 days.
“These variations in season length and start date would make it very difficult to consistently compare climatological statistics for a particular season from one year to the next,” the NCEI said on its website. “Thus, the meteorological seasons were born.”
