When Does Fall Start?
Some questions just don’t have simple answers and “when does Fall start?” is one of those. A totally unsatifying answer is that it depends on where you live and who you ask! So let’s get the least satisfactory answers out the way first.
From a purely sociological viewpoint, the Fall season starts on the day after Labor Day. For a few months we’ve been vacationing, participating in outdoor activities and sports, and our children have been out of school. Labor day kind of marks the border between that and a life of returning to work and school, a changing of the weather, changing from outdoor activities and sports to some indoors and to different ones outdoors, etc. There’s certainly little justification for this determination of when Fall starts except our frames of mind.
The definition of when Fall begins from a strict weather outlook is September 1st. Meteorologists actually define Fall as the months of September, October, and November. It’s just easier for them to keep seasonal weather pattern records by the month so that definition doesn’t mean much to each of us.
The most common answer to the question, “When does Fall start?” is that it begins at the moment of the Fall Equinox. That’s when the rays of the Sun are exactly perpendicular to the Earth at the equator. This year (2009), that occured at 5:18 PM eastern daylight time in the US on September 22nd.
That, of course, only applies to the Northern Hemisphere. In the Southern Hemisphere, that same moment marked the beginning of Spring! The first day of Fall there was on March 20, 2009 and you probably guessed that was also the first day of Spring in the North. This makes sense if you think about the fact that from the moment the Fall Equinox occurs, the intensity of the Sun’s rays will be increasing in the Southern half of the Earth and decreasing in the Northern half.
So even the most scientifically justified and most commonly accepted answer depends on where you live. But that’s not nearly the strangest fact about when seasons start. June 21st is typically the day when the Sun’s direct rays are the farthest north so shouldn’t that be the hottest day in the North? And yet, that’s simply the first day of Summer! Shouldn’t it be the middle of Summer?
Again, there’s a simple explanation. The Earth’s surface is approximately 75% covered by water. Water temperatures change at a much slower pace than land temperatures. It just so happens that it takes water temperatures about one and a half months to “catch up” with the warmer land temperature in Spring or with the cooler land temperature in Fall. Since the temperature that we experience is both from the land and from the air (which is influenced by water temperatures), the seasons are delayed from what one would expect if the intensity of the Sun was the only factor.
The distance between the Earth and the Sun also has a very slight effect on temperature. The Northern hemisphere actually benefits from this because the Earth is closest to the Sun in Early January and furthest in early July. Again, this is a very small influence on temperatures.
The Earth’s orientation to the Sun is the reason for seasonal changes. The Earth is tilted in relation to the Sun by a bit over 23 degrees. For one half of its year of revolving around the Sun, the Earth’s Northern Hemisphere is tilted towards the Sun and the North experiences Spring and Summer. The other half of the year, the Southern Hemisphere is tilted towards the Sun and the North experiences Fall and Winter.
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