Two Types of Seasons

In a recent post, we looked at equinoxes, solstices, and why we have seasons. But did you know that we actually have two different types of seasons? There are Astronomical Seasons, and Meteorological Seasons. Today, we're going to dive into why we need the two types of seasons, and how they differ from each other.

Astronomical Seasons

For thousands of years, people have used observable natural phenomena that occurs periodically to mark the passage of time. The natural rotation of the Earth around the Sun forms the basis for the astronomical calendar, which is how we define seasons with two equinoxes and two solstices. The Earth’s tilt and the Sun’s alignment over the equator determines when the equinoxes and solstices occur. The equinoxes mark the times when the sun is passing directly above the equator. The solstices mark the times when the Sun is as far away from the Equator as it can get. The equinoxes and solstices for 2021 are on the following dates:

• March 20th: Spring Equinox
• June 20th: Summer Solstice
• September 22nd: Autumn Equinox
• December 21st: Winter Solstice

These dates vary slightly from year to year. That is because the Earth needs an extra 0.24 days (roughly six hours) to travel around the Sun. Therefore, an extra day is needed every fourth year to account for the extra time the Earth needs to complete its annual journey. This creates what we refer to as Leap Year, which occurs every four years when we add an extra day at the end of February.

Additionally, the elliptical shape of the Earth’s orbit around the Sun causes the lengths of the astronomical seasons to vary in length. The shortest seasons are 89 days long, and the longest seasons are 93 days long. These variations make it difficult to consistently compare climatological statistics for a particular season from one year to the next. To make these comparisons easier, the meteorological seasons were created.

Meteorological Seasons

Meteorologists and climatologists break the seasons down into groupings of three months based on both the calendar and the annual cycle of global temperatures. For the most part, we think of Winter as being the coldest time of the year, and Summer as being the warmest time of the year. Spring and Fall are viewed as the transition seasons. The meteorological seasons are based on this model of thought. Meteorological forecasting and observations led to the creation of these seasons. They are more closely linked to our monthly civil calendar than the astronomical seasons are. The length of the meteorological seasons is also more consistent, only ranging from 90 to 92 days in length. By following the civil calendar and having less variation in when the seasons start and end, it became far easier to calculate seasonal weather statistics from the monthly weather statistics, both of which are very important for a wide range of purposes, including agriculture and commerce.

Here in the Northern Hemisphere, Meteorological Spring includes the entire months of March, April, and May. So when the clock strikes midnight on the morning of March 1st each year, Spring has sprung in the meteorology world, even though it doesn't begin for a few more weeks on the astronomical calendar. So you could argue that meteorology always has an early Spring, regardless of whether Punxsutawney Phil sees his shadow or not on Groundhog Day (which occurs each year on February 2nd).

Meteorological Summer includes the entire months June, July, and August. So while we think of Labor Day, which takes place in September, as being the last holiday of Summer, meteorologists could view it as being the first holiday of Fall. However, on this point, I have to say that pretty much all of the meteorologists I've ever seen while watching television here in the Pacific Northwest seem to agree with the astronomical calendar, and view Labor Day as the last holiday of Summer. That may be so they don't confuse the general public, which largely follows the astronomical calendar. Technically though, meteorological Fall includes the entire months of September, October, and November.

Meteorological Winter includes the entire months December, January, and February. So, thanks to leap year, Winter is one day longer every four years. While some people probably like that very much, others would probably prefer to have an extra day every four years in a different season. While we don't really have the choice to move the extra day to another season, it may be fun to think about where we would move it if we could. If you'd like, you can share your thoughts about this on the Coastal Interpretive Center's Facebook page.

References

1. https://www.ncei.noaa.gov/news/meteorological-versus-astronomical-seasons
2. https://www.britannica.com/story/whats-the-difference-between-a-solstice-and-an-equinox

© Ian D. Caldwell, March 2021