Clearly, sunlght passes through cloud cover, either directly or indirectly by re-radiation from the cloud. How much infrared radiation is filtered out by cloud cover? There is a rider here, in that all radiation, upon absorption by a surface, is transformed into heat.

So, the question is transformed into:

 

What fraction of solar radiation is absorbed or reflected by cloud?

How is this distributed between visible, UV and infrared radiation?

How much is transmitted through cloud and how much is absorbed and re-radiated downwards as light and heat?

 

Some of these questions will be interrelated and overlap.

 

But, it is spring here, so how much of spring warming is due to "direct" solar radiation and how much is due to seasonal changes to weather patterns bringing warmer airstreams across the land, which is transferring solar radiation from a differnet zone?

This post on flickr seems to give you relevant hard data for Toronto.

 

Insolation on a cloudy day in W/m2. It also provides a link for a sunny day.

Edited February 21, 201114 yr by CaptainPanic