Season in a year

How it works

The year seasons are a filing system for weather and daylight, laid over twelve months. In much of the United States and Europe the four names are spring, summer, autumn or fall, and winter. The year still contains the same 365 or 366 dates either way. Seasons do not add a thirteenth month.

Two common rulers exist. Meteorological seasons are three-month blocks: in the Northern Hemisphere, spring is March–May, summer June–August, autumn September–November, and winter December–February. Astronomical seasons turn on the March equinox, the June solstice, the September equinox, and the December solstice. Those instants fall on a small range of dates, not on the first of a month.

The Southern Hemisphere swaps the temperature story. June–August is winter there in the meteorological grouping, and the June solstice is a winter mark, not a summer one. The calendar months keep their names. Only the season labels flip.

Near the equator, four temperate seasons are a poor fit. Many tropical places group the year as wet and dry, or as more than two monsoon-related stretches. That is still a season-in-a-year scheme. It is not the four-name temperate set, and forcing “fall” onto a place without a fall is a caption, not a climate.

The year seasons on a grid

Meteorological blocks are tidy because they follow month boundaries. You can say “summer starts 1 June” in that system and be speaking a convention, not a sunrise. Astronomical seasons are tidy in a different way: they follow the Earth’s tilt and the Sun’s apparent path, so the start is an instant in a time zone you must name.

The June solstice is the longest daylight in the Northern Hemisphere and the shortest in the Southern. The December solstice reverses that. Equinoxes are the two instants when day and night are closest to equal worldwide, with local sunrise and sunset still depending on latitude and the atmosphere. None of those four instants is “the season” by itself; each is a hinge.

Northern meteorological spring: March, April, May. Northern meteorological summer: June, July, August. Northern meteorological autumn: September, October, November. Northern meteorological winter: December, January, February.

Astronomical hinges: March equinox, June solstice, September equinox, December solstice. School years, fiscal years, and sports seasons are other overlays. A school “fall term” can start in August, before meteorological autumn. A fiscal year can start in July and contain two summers if you are sloppy with hemisphere words. Those overlays do not rewrite the weather file; they just borrow the names.

What a season is not

A season is not a quarter of exactly 91 days. 365 divided by four is 91 days with a remainder of one; 366 divided by four is 91 days with a remainder of two. Month-based seasons are 90, 91, or 92 days depending on the months inside them. February makes winter (in the northern meteorological set) shorter than summer.

A season is not a promise of a temperature. Early March can freeze after a meteorological spring label has already been applied. Late September can stay hot after an autumn label. The name is a bin. The weather is the contents.

If you needed the first warm Saturday, you needed a forecast. If you needed the first day of spring, you needed to say which ruler you were using. Leap day sits inside northern meteorological winter. It does not create a fifth season. It only makes that winter one date longer in the years that have 29 February.