When is the first day of spring?

DENVER (KDVR) – Temperatures are slowly starting to warm up and daylight saving time has just started. That must mean that spring is here, right?

Not quite.

Sunday, March 20 – when the spring equinox occurs — marks the first day of the spring season in the Northern Hemisphere. It is known as the astronomical, or "regular" first day of spring. With the equinox comes increasing sunlight hours, meaning earlier dawns and later sunsets.

The equinox actually happens twice each year: once in autumn, and once in spring. At those times, the sun crosses the equator line, resulting in the length of day and night being "nearly equal in all parts of the world," according to the Old Farmer's Almanac.

Then again, the spring equinox merely marks the first day of astronomical spring. There's also a meteorological spring, which began on March 1.

So why are there two different dates?

According to the National Oceanic and Atmospheric Administration, meteorologists and climatologists define seasons differently, which is why there are astronomical and meteorological seasons.

"In short, it’s because the astronomical seasons are based on the position of Earth in relation to the sun, whereas the meteorological seasons are based on the annual temperature cycle," the NOAA says.

The astronomical calendar is based on Earth's rotation around the sun, defining seasons with two solstices and two equinoxes. The meteorological seasons are determined by the thinking that winter is the coldest time of the year and summer is the warmest, with spring and fall considered transition seasons, according to the NOAA.

The solstice also happens twice each year. At those times, the sun reaches its maximum or minimum declination, marking the longest and shortest days. 

The seasons are broken into groupings of three months based on the annual temperature cycle as well as our calendar, the NOAA says.

Here are the meteorological seasons for 2022:

• Spring: March 1
• Summer: June 1
• Fall: Sept. 1
• Winter: Dec. 1

Here are the dates of the astronomical seasons for 2022:

• Spring: March 20 (spring equinox)
• Summer: June 21 (summer solstice)
• Fall: Sept. 22 (autumnal equinox)
• Winter: Dec. 21 (winter solstice)

Speaking of spring, Easter will also be here before we know it, falling on Sunday, April 17, this year.