Most of us have our own sense of when the seasons change. Even though December 21 (or 22nd for some of us) is a Solstice that is officially the shift of a season, when did (or will) winter/summer actually begin for you?
Ted has thought quite a bit about the causes of the lag in heating and cooling over summer/winter.
There are many major factors involved, but the main ones are – the amount of heat lost to space at night, vs the amount of heat absorbed from the sun during the day.
So the net amount of heat increase usually reaches a maximum on the longest day (all other things being equal – weather effects, ocean current effects, etc), and the earth continues to warm for so long as there is more energy from sunlight coming in during the day than is lost during the night. So the naive view of this would have the heat increasing until the equinox.
So it is not the absolute amount of heat coming in that matters. Rather it is the balance between heat coming in and heat going out.
But the hottest day is usually somewhat before the equinox due to a couple of major factors.
One is that air currents move heat from one place to another (this is actually what causes wind, differential heating and cooling at different scales). This tends to reduce the amount of heating (by increasing cooling effects) and so shifts the hottest say back towards the longest day.
Lots of other things can modify this.
Ocean currents can move a lot of energy.
Local mountain ranges can significantly alter air flows and heat/cooling effects. Areas downwind of the prevailing air-stream over mountains tend to be hotter and drier due to rising air causing condensation (releasing heat) and rain, while descending air gets hotter and drier.
This can also be modified by types of vegetation, local water bodies, local rock types, etc.
Prevailing air streams can be modified by large scale heat transfers in the ocean which can alter probabilities of things like monsoons and tropical cyclones. These are all very complex systems, with many essentially random (mathematically chaotic) elements.
So some major factors, and a vast range of local modifiers causing many local variations on the major theme.
Yep – that is what I said, and it is hard when you are talking to both northern and southern hemisphere folks at the at the same time.
The longest day (Dec southern, June Northern, solstice) is when the most energy comes in, but there is still more energy coming in (during daylight) than goes out (during night-time) until close to the next equinox (March Southern, Sept Northern), so things continue to get warmer, though more slowly.