Forgive my ignorance of the subject but I was always wondered about the exact reason of this phenomenon.
Vernal equinox happens around March 20, whereas autumnal equinox happens around September 22, so wherever you are in Northern Hemisphere, the length of the day, and consequently the amount of solar energy that reaches the place should be almost the same.
However the average temperatures differ widely, for example Toronto has average temperatures of 2°C to 6°C on March 20th, and 14°C to 19°C on September 22nd. So around 12°C difference [link].
So obviously there is some sort of temperature inertia, as temperatures seem to experience a delay in responding to changes in day length.
What is the main reason for it? Is it effect of sea ice or snow-covered land albedo? Energy stored in oceans? Energy absorbed by melting snow and ice?