The 4 Seasons

"In traditional reckoning, the seasons begin at the cross-quarter days. The solstices and equinoxes are the midpoints of these seasons. For example, the days of greatest and least insolation are considered the "midsummer" and "midwinter" respectively.
This reckoning is used by various traditional cultures in the Northern Hemisphere, including East Asian and Irish cultures."

Seasons fixed mid-points; to sun cycle (sun god)
The important point to grasp here is that mid-summer (and so summer) is defined by the mid-summer solstice, that is, summer is defiened by solstice and the sun, the length of days and the light levels. (so too mid-winter (middle of winter) with the winter-solstice).
That is what the seasons mean.

Nature Green reacts to Solstice and equinox light (Green Jack)
(nature's green leaves react to the light levels and so plants react to the solstice defined year ; ie the agricultural natural seasons are defined by the plants and their reaction to the light levels of the sun and solar year; soo too then do people). This was more important in the civilized agricultural based socieities.


Modern temperature-chart fad?

They do not therefore describe the thermal temperature zones of hottest and coldest (as such).
This is a fundermental misunderstanding of the attributes of the seasons. There is therefore no season lag ; the seasons are fixed to solstice and eqinox mid-points! There is a thermal lag (heat/temperature lag) compared too....the seasons. (which are defined by solstice and equinox mid-points). This does not matter, and did not matter! It is thermometer ivory tower academica.
Seasons define what nature is doing; what the green plants and crops are doing. 

Green Jack or the Green Man prefers the Sun God of Light rather than the dubious heat goddess of the therometer...?