There is often much to do about the official start of seasons in general and Autumn of course today. So what actually happens weather-wise on the day of a season change? Short answer: not much!
It’s not even a day really. The astronomical start to each of the four seasons is actually a point in space and time in the Earth’s orbit around the sun. It happens at a precise time. In the case of today, the Autumnal Equinox starts at 9:49 am CDT.
The most significant thing about today is the length of day and the position of the sun. The length of the day is twelve hours everywhere on the planet. It would be this way everyday if our planet were not tilted on it’s axis at 23.5°. But the tilt provides a more complicated geometry to the path of the sun across the sky which then affects the length of the day.
The enclosed diagram shows the arc length of the sun at the four seasonal points. Summer has a long arc length and the highest sun angle therefore we have the best quality and quantity of sunlight at this time. Winter has the shortest sun arc length across the sky and the lowest sun angle therefore more limited energy.
Notice at the two equinoxes, the sun rises and sets in due east and west. This has consequences to drivers on east-west roads around sunrise and sunset. At these times, the sun would be right in your eyes on such streets making visibility of traffic, traffic lights and pedestrians almost impossible. Be careful this evening and for the next few days!
This day really has no practical affect on the weather. Again it just a point in space along what has been a gradual shift in sun angle and path which started back on the Summer Solstice. Too subtle to have a direct effect on daily weather!
Another good write-up on the seasons can be found here.