Seasons are determined by earth's position relative to the sun. Anyone who bases the seasons on weather just does not know what a season is.
The Summer Solstice (start of summer for the northern hemisphere/winter for the southern hemisphere) is on June 21st, every year.
The Autumn Equinox (end of summer/winter and start of Autumn/Spring in the North/South) is on September 23rd, every year.
The Winter Solstice is on December 21st, every year.
The Spring Equinox is on March 23rd, every year.
These are the starting and ending points of every season, every year. Duolingo needs to correct their spreading of misinformation or put an * with a cultural appendix.
That is certainly astronomically true, though banal, even tedious. The seasons, as experienced in the Mediterranean Basin, were indeed imported into the language of astronomy and used the world over. The word "season," though, refers not only to these astronomical divisions, but also to local divisions of the year, such as the rainy season and the dry season, as experienced in many of the monsoon-affected regions of the Indian Ocean basin.
As you were said, I think the concept of summer is previous to the astronomical definition. Maybe you need to be flexible.
Anyway, Duolingo must teach the popular usage.
I remember in Bogotá, Colombia they call "winters" to each of the serveral cold periods they have along the year.
In previous exercises with months and seasons, I remember one of the sentences saying "June, July, August are the months of summer." I understand that the seasons take place at different times of the year depending on when in the world you are, but for the purposes of trying to learn the language and trying to use clues, let's keep a bit of consistency in these exercises. It didn't help me at all having a sentence that stated that Summer goes from April to June.