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?

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    A similar thing happens in the southern hemisphere, but the other way around: warmer in March than it is in September. Allied to this is at the start of spring and autumn, autumn nights are generally warmer than spring nights. – Fred Mar 10 '15 at 5:01
up vote 20 down vote accepted

The phenomenon is called seasonal lag.

There's a more extensive answer elsewhere on this site but the basic idea is that temperature lags behind insolation by several weeks, because it takes time to change the mean temperatures of the land, the atmospehere, and especially oceans change their mean temperature. This diagram tries to show the lag, along with various ways of reckoning the seasons:

enter image description here

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    Great, that was what I was looking for, it is much easier to google it, once you know the name :) – Sebastian K Mar 10 '15 at 13:43

It mainly depends on where you are "coming from," rather than where you are "going to."

In September, you are "coming from" a hot summer. The earth has been hot recently, and will take some time to cool down (in your part of the world.

In March, you are "coming from" a cold winter. The earth has been cold recently, and will take some time to heat up.

"Cold and "hot" are where you are "going to," in September and March, respectively, but the weather on the equinox days are dictated by the "rearview mirror."

Put another way, the amount of solar energy that reached the earth "today" is less important a factor in the heat than what happened "recently" or just before.

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