The astronomical autumn does not start until September 21st. For meteorologists however, autum starts on the 1st of September. The question is why? And what's the differnce in the first place?
The position of the sun has been examined in the astronomical division of the seasons. The sun is right above the equator at the start of the astronomical autumn and spring. The sun is the furthest away from Europe and we have to deal with the worst day in terms of sunlight on the 21st of December (the start of the astronomical winter). The sun is right above the Tropic of Cancer on the 21st of June (the start of the astronomical summer). That's the day we are dealing with the longest day (in terms of sunlight) in Europe. The three months after June 21st are called the astronomical summer.
The problem is that this classification is pretty limited in meteorology. Let's have a look at the summer. Summer is the warmest time of the year in meteorology. The maximum duration of sunshine is around the summer solstice (June 20th of 21st), but because of the inertia of the atmosphere, it takes another three weeks before the average temperature is generally at its maximum, ie mid-July.
This moment (mid-July) is regarded by meterologists as the middle of the summer. Summer starts in meteorology in early June and ends at the end of August. With the same reasoning, sunshine is minimal at the winter solstice (21 or 22 December). But with the inertia of the atmosphere, the peak of winter is only mid January. The meteorological winter therefore starts early December and ends at the end of February.
Because of this classification, the meteorological autumn has already started while we are still in the astronomical summer. That may seem confusing, but the good news is that meteorological autumn has already started and we have come one step closer to winter. And for those who do not want to say goodbye to summer: the astronomical summer lasts another 3 weeks.