On 21st June we in the Southern Hemisphere get our shortest day of the year. This corresponds to the longest day in the Northern Hemisphere of course, and my wall calendar, which originates from the Northern half of the planet says that 21st June is the start of summer. I believe that the official start of winter, here in New Zealand, is 1st June.
That started me thinking. One would expect that 21st June, the southern winter solstice, would be the middle on winter, since the earth is tilted furthest away from the sun in southern latitudes at that date, and that the seasons would change mid way between the solstices and the equinoxes in both hemispheres for similar reasons. The equinoxes are the days when the night and days are the same length in both the northern and southern hemispheres. (Pedants will notice that I’m not being precisely correct in my explanations of equinoxes and solstices, but that doesn’t matter for my purposes.)
It is obvious to anyone who has reached a sufficient age that the warmest and coldest parts of the year don’t correspond to the solstices and that the change from higher than average temperatures to lower than average temperatures and vice versa don’t happen at the equinoxes, though these latter events are probably not that noticeable. There is obviously some seasonal lag.
So I browsed to Wikipedia, which is a useful place to start, even if some people question its accuracy and veracity. I’ve not found it too bad, myself. Sure enough, there is an article on seasonal lag, and I’ve no reason to doubt the information there. To summarise, the authors of the article attribute the lag to the oceans which, because of the latent heat of their water absorb heat energy and release energy as the seasons change. I’m not sure that I completely understand the reasons for this, but there are undoubtedly deeper analyses on the Internet. The Wikipedia article contains one reference.
Apparently the seasonal lag is different in each half of the year. I believe that means that the four seasons are not all equal in length. Hmm, summer does seem shorter than the other seasons, but that may be only subjective. However, our shortest day is only four weeks away, so we will at least be seeing more daylight each day from then on. We will be on the upwards slide to Spring and Summer, even though Winter will not have bottomed out, and we can look forward to barbecues and a summertime Christmas!
Me on the net 