For example: the autumnal equinox was yesterday, but meteorological autumn starts September 1st. Why the disconnect?
In: Planetary Science
Imagine you are filling up a bucket that has a hole in it. As long as you are pouring water in it faster than it can leak out of the hole, the water level rises. As soon as you start pouring water in it slower than it leaks out of the hole, the water level goes down.
Heat is the same way. The summer solstice is the day of the year when you have the most sunlight being pumped into your hemisphere. But that doesn’t make it the hottest day, because all of the days after that still are pumping in a lot of heat. More than that part of your hemisphere is losing. So you actually tend to see the hottest days of the year in August in the northern hemisphere. After the summer solstice because the heat is still building up.
Same thing happens in reverse in the winter. The winter solstice is the day when the least heat is being put into the system, but the days following it still have very little heat being put into the system so it still gets colder and colder so you tend to get the coldest days in late January or early february.
Meteorology is concerned with weather patterns. In that regard, spring and fall are both transition periods, where the weather patterns are changing and blending from summer to winter and back. They base this on historical weather patterns, and historically, September is when the weather begins to transition. Sometimes it’s hot, sometimes it’s cool, and often it’s back and forth for the entire month.
For autumn, the days are getting shorter, nights getting colder, there’s usually much less humidity even on hot days, and it’s a period of energy loss.
Astronomical seasons are based solely on the relative angle of the sun. The actual full effects of that angle don’t usually appear for about two months. You’ll notice the summer solstice is in June, but August is the hottest. Winter solstice is in December, but February is often coldest. So there’s a lot of factors that go into the weather, and the meteorological aspect is just trying to fit the weather into a pattern we can determine. Plus, people get cranky when you say winter doesn’t start until mid December when they’ve already had snow and below freezing temperatures for weeks.
Meteorological seasons are based on grouping the year into three-month chunks. The goal is to divide the year up equally and not overthink it. When does fall-like weather start? Sometime in September, so we’ll just set the start date at September 1 and run it from there.
Astronomical seasons are based on actual events: the solstices and equinoxes. This grounds it in something observable — but it also makes it more confusing. The seasons start on different days every year depending on when the event takes place, and the seasons are all different lengths. Also, the first day that feels like Spring doesn’t necessarily happen on the Spring equinox, so it’s not even that helpful.
So meteorological seasons are better for general use, where astronomical seasons are better for astronomers and other nerds.
Lets say you put a pan of water on the stove. The water does not immediately boil when you put on the heat. It takes some time for the water to heat up and eventually boil. And when you turn off the heat the water does not immediately get cold. In fact it will probably still boil for several seconds, maybe a minute and then slowly cool down over the next hour.
It is the same with the Earth. When the Sun gets higher in the sky during the spring the ground and bedrock is still very cold. There may still be snow on the ground even at spring equinox. It takes months for the Sun to be able to heat up the ground so you can enjoy a proper warm summer day. And the ground is still quite warm even as the Sun gets lower in the sky. So you still have warm weather even at autumn equinox.
The first day of the season is based on the calendar date. It’s always the 21st. The equinox/solstice is based on astronomical factors, and might vary by a day or two from the “official” first day of a season.
It’s the same reason why we have leap years: the earth’s orbit isn’t exactly 365 days.
Some places define autumn differently than others. I thought it weird when i heard places start autumn on sept 1. Where im from , the US, fall does start with the equinox. I believe places that do start on the first have weather changes a bit earlier, hence the earlier autumnal starting point.
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