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Temperatures hit 90 this week in time for spring in Phoenix. Wait, it’s not spring yet, is it?
It is if you’re a scientist. March 1 marks the beginning of the season, at least according to meteorologists.
But while temperatures are heating up and desert flowers are beginning to bloom, the start of spring is a few weeks off for those who follow the astronomical calendar. Astronomical spring begins on the spring equinox, which occurs on or around March 21. (In this part of the world, it’s March 20 this year.)
Meteorological seasons follow consistent calendar dates, whereas astronomical seasons abide by seasonal equinoxes and solstices.
“Meteorologists often use meteorological seasons because then the same dates can be used every year to gather and compare data,” said Erinanne Saffell, Arizona’s state climatologist. “It helps provide a standard comparison of weather data.”
But what are the differences between meteorological and astronomical seasons, and why do scientists often prefer one over the other for record-keeping?
Here’s everything to know about seasons in the Northern Hemisphere, as Arizona begins a new spring with opportunities to continue breaking weather records:
What are meteorological seasons?
Meteorological seasons are based on the calendar months: in the Northern Hemisphere, spring spans March, April and May; summer lasts June, July and August; fall during September, October and November; and winter in December, January and February.
Each season begins on the first day of the first month of the season.
Meteorological seasons have less variation year-to-year in season duration and start; each season is about 90 days. The consistency makes it easier to determine seasonal statistics.
During meteorological summer and winter, the hottest and coldest times often occur in the middle of the seasons, respectively.
“Even though the sun is the highest in the sky on the June solstice, July is usually the hottest month of the year,” Saffell said. “The same way meteorological winter captures the coldest month.”
What are astronomical seasons?
Astronomical seasons are a bit more complicated. They are based on when the sun reaches specific latitudes.
“The astronomical seasons vary slightly each year because the Earth’s orbit takes just a bit longer than 365 days,” Saffell said. “A leap day/leap year helps to balance out that difference.”
Earth circles the sun in roughly 365.24 days, which creates an extra day every fourth year, or a leap year.
The planet’s tilt and the sun’s alignment over the equator determine the solstices and equinoxes. Equinoxes occur when the sun crosses directly over the equator.
In the Northern Hemisphere, the summer solstice occurs when Earth’s tilt from the sun is at its maximum and the sun is over the Tropic of Cancer. The winter solstice occurs when the tilt is farthest away from the sun, which is located over the Tropic of Capricorn.
These days mark the longest and shortest days of the year, respectively, and the start of the new seasons.
The seasons are reversed in the Southern Hemisphere, correlating with Earth’s tilt throughout the year.
Why do scientists use meteorological seasons?
People have used astronomical seasons since ancient times, but as humans began keeping precise weather records, they needed a more consistent way to track the seasons.
Meteorological seasons date back to the 18th century when scientists wanted to find a better way to align annual temperatures and the calendar. They could deal with entire months rather than fractions with possible overlap.
“Meteorological seasons are set on the same calendar because that makes it easier to track and compare seasonal data,” Saffell said. “Climatologists often write computer codes to analyze data, so using the same dates also helps with the code.”
2025 meteorological seasons
- Spring: Saturday, March 1 – Saturday, May 31
- Summer: Sunday, June 1 – Sunday, Aug. 31
- Fall: Monday, Sept. 1 – Sunday, Nov. 30
- Winter: Monday, Dec. 1 – Saturday, Feb. 28, 2026
2025 astronomical seasons
- Spring: Spring equinox on Thursday, March 20 – Thursday, June 19
- Summer: Summer solstice on Friday, June 20 – Sunday, Sept. 21
- Fall: Fall equinox on Monday, Sept. 22 – Saturday, Dec. 20
- Winter: Winter solstice on Sunday, Dec. 21 – Thursday, March 19
Hayleigh Evans writes about extreme weather and related topics for The Arizona Republic and azcentral.com. Email her with story tips at hayleigh.evans@arizonarepublic.com.
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