Unit 12: Seasons

Meteorological Seasons

Meteorological Spring starts on March 1, but you may think of the first day of spring when the equinox happens. Why do we have two ways to measure the seasons?

The Meteorological Seasons have been observed since the early 1900s, according to the National Oceanic and Atmospheric Association (NOAA). This type of season is based on the annual temperature cycle. These were created to have a better way to get seasonal statistics and be able to compare between years.

The meteorological seasons are split up into 3 months. Winter is January, February, and March; Spring is March, April, and May; Summer is June, July, and August, and Fall is September, October, and November. These seasons were created by meteorologists and climatologists to able to compare seasons throughout the years. There is less variation in these seasons because the days from start to end are the same at 90 days. Astronomical seasons are based on the Earth’s tilt and the number of days in each season can vary from year to year.

Spring/Fall Equinox

The Astronomical Seasons have been observed for thousands of years. This is based on the Earth’s tilt and the movement around the sun. Equinoxes happen twice a year marking the start of the Spring and Fall seasons.

An equinox is when the sun is directly over the equator. This creates an equal 12 hours of daylight and 12 hours of nighttime near the equator. In the United States, it’s not exactly 12 hours, because of the Earth’s tilt and where we are located. It takes longer for the sun to rise and set in higher latitudes, so in turn in the United States, we hover from just under to just over 12 hours of daylight. Once the spring equinox passes, our days will continue to get longer until the Summer Solstice.

NOAA SciJinks Lesson on Why Leaves Change Colors

Now, why do we have seasons at all? It all has to do with how the Earth rotates around the Sun during the year. The Earth is tilted on its axis, which causes day and night. The Earth also travels around the sun in an elliptical pattern which is not a perfect circle. This determines when the Earth is closest and furthest away from the Sun. One path around the sun takes 365 and one-quarter days to complete. Therefore, we have a leap year every four years to make up for the quarter of a day every year.

Summer/Winter Solstice

The Winter and Summer Solstices, mark the official start of the winter and summer seasons. The Summer Solstice happens on either June 20 or 21 and the Winter Solstice on either December 21 or 22.

The Winter Solstice marks the day with the least amount of daylight, where the Summer Solstice marks the day with the most amount of daylight. This means we have around 15 hours of daylight on the longest day of the year and around 9 hours of daylight on the shortest day of the year.

The start of this season is the time in which the North Pole reaches its furthest point from the sun.

A common misconception is that the Earth is furthest away from the Sun during the winter because our temperatures are colder. It’s actually the opposite. A few weeks after the Winter Solstice, the Earth approaches its closest point to the sun, which is known as perihelion. The seasons themselves deal more with the tilt of the earth than the actual distance away from the sun.

The Earth is tilted on its axis at around 23.5°. On the Winter Solstice, the Earth is tilted furthest away from the sun, in turn, the Northern Hemisphere won’t see as much sunlight. This would be the opposite during the summer solstice.

Meteorological winter started on December 1st and will go until the end of February.