Autumn is a transitional season. A bridge between summer and winter, but the steepness/bumpiness of that bridge can vary greatly from year to year. However, it’s always a safe bet that the end of autumn will be significantly colder than the beginning. The reason being astronomical… the tilt of the earth and the position within its orbit (both relative to the sun) meaning that we lose an average of about 3 and a quarter minutes of daylight per day, such that we start December with almost 5 hours less daylight per day than we finished August with. Not only do the daylight hours decrease significantly, but the same is true for the solar intensity. This is because the sun’s elevation (its height above the horizon) also decreases each day. This means that each ray of sunshine no longer beats its energy intensely onto a small patch of earth but rather spreads it diffusely over a vast swathe, riddled with long shadows, cast by every hint of a hill, tree or building.
Of course, this isn’t happening equally for the whole Earth… whilst we experience autumn in the Northern Hemisphere, the opposite (spring) is happening in the Southern Hemisphere. Furthermore, the transition that these seasons make (between summer and winter) are much more extreme near the poles than they are near the equator. As such, huge temperature contrasts tend to develop between such land masses as Scandinavia and Africa, meaning that the quirks of the weather pattern over the Channel Islands (such as persistent northerlies or persistent southerlies) can really spell the difference between one autumn and the next. Some years, see a very late departure of summer, some see a very early arrival of winter, some blend seamlessly, some oscillate violently.
When do the seasons begin and end? Well, this is a source of mass debate. The corner stones of the year are the two Solstices (the moments when the tilt of the earth is most extreme, relative to the sun, such that one hemisphere is at its most illuminated and the other is at its most shaded) and the two Equinoxes (the moments of equilibrium when the tilt of the earth is changing, such that one hemisphere begins to be more sun-facing as the other hemisphere ceases). However, although those moments are more real than any conventionally agreed human calendar, it is very difficult to decide whether they should mark the beginnings of their respective seasons or the mid-points. That, combined with finding the simplest way to divide climatological data, has led Meteorologists across the world to agree that three whole calendar months should be attributed to each season…
Dec/Jan/Feb = Northern Hemisphere winter/Southern Hemisphere summer
Mar/Apr/May = Northern Hemisphere spring/Southern Hemisphere autumn
Jun/Jul/Aug = Northern Hemisphere summer/Southern Hemisphere winter
Sep/Oct/Nov = Northern Hemisphere autumn/Southern Hemisphere spring
Therefore, by this time next week, we will officially be in winter according to the meteorological calendar. So be sure to ‘watch this space’ and read the next Jersey Met blog, when we intend to pull apart the facts and figures which, for better or worse, have made autumn 2023 a particularly interesting and memorable season.