The Autumn of 2021 arrives  today at 3:20 pm ET. In recent days, portions of the United States have been unusually mild, making conditions feel like more like summer than fall. However, eventually more chilly temperatures are likely as days get shorter and temperatures drop lower in the coming weeks. Autumn runs until December 21; at 10:59 am ET that day, the Winter of 2021 will arrive.

The timing of the four seasons is determined by shifting sunlight, which on its own is determined by how our planet orbits the Sun and the tilt of its axis. Half-way between summer and winter, autumn will see days getting shorter and temperatures getting colder throughout the northern hemisphere (while the days will be getting warmer in the southern hemisphere.)

However, while autumn officially arrived today, meteorologists consider September 1 as the first day of meteorological autumn. For most people in the Northern Hemisphere, fall  begins on Wednesday, September 22 and ends on Tuesday, December 21. But for many scientists, the end of August marks the end of their summer. Meteorologists and climatologists break the seasons down into groupings of three months based on the annual temperature cycle. In the world of meteorology and climatology, autumn includes September, October, and November; winter includes December, January, and February; spring includes March, April, and May; summer includes June, July and August.  Because the seasons are based on months and not traditional astronomical ones, the length of the meteorological seasons is  more consistent, averaging about 91 days per season. By following the civil calendar, meteorologists and especially climatologists can better calculate seasonal statistics from monthly ones.