The earth's axis at the same time of year the northern hemisphere receives both light. Making summer way hotter then the rest because it is closer to the sun.
How & Why Does Autumn Occur?
Due to the tilt in the earths axis at this time of year the northern hemisphere receives both less light and a slightly weaker light from the sun. This causes the chlorophyll to stop causing most plants to stop being green and go into a hibernation state. The beautiful colors are the trees losing the excess parts it can no longer support