Meteorologists and climatologists have a different start date to when seasons begin that differ from the calendar, or astronomical, seasons. It all has to do with helping to simplify the data. When you look at a calendar year to year, you will notice that the date of spring, summer, fall and winter can vary between the 20th or the 21st of March, June, September, and December respectively.
The reason that happens is because of how we measure the Earth’s revolution around the Sun which occurs in 365.24 days. Similarly to why we have a Leap Year every four years, that extra time causes the exact date of when the Earth’s tilt relative to the sun passes over the equator.
Trying to collect data for different seasons becomes more difficult when accounting for that variability. Meteorologists and climatologists simplify things and look at the broad picture. In the calendar month, we tend to find the coldest temperatures on average to occur in December, January, and February here in the Northern Hemisphere. Similarly, the hottest temperatures typically occur during June, July and August. Transitional times between these two extremes happen in March, April, May, September, October, and November, leading folks to break the calendar up into four sections as the dedicated meteorological seasons.
This helps to make data collection, interpretation, and calculations much easier to work with and understand. As we close out the month of February, this brings an end to what is known as meteorological winter. March will kick off meteorological spring. The vernal equinox will occur on March 20, 2021, at 4:37AM as the sun passes over the equator.
–Meteorologist Erika Paige