Dear Solar Guy,

        I have heard that even if my house's windows are shaded from direct
sunlight, I might pick up substantial unwanted summer heat from something
called diffuse sunlight. What is diffuse sunlight, and is it important?
A. J. Beam, Shiner, TX.

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Dear A.J.,

        Solar radiation falling on surfaces comes from two sources: the
parallel or direct rays from the sun and the diffuse radiation from the sky
and the surroundings. The diffuse from the sky is due to the sunlight that
'bounces' off particles and molecules in the atmosphere and that from the
surroundings is due to reflection from those surfaces.  For a relatively
clear sky the diffuse is small, but as the sky becomes less clear, the
percentage of diffuse radiation becomes greater.  The percent reflected from
surrounding solid surfaces is typically about 10-20%.   Thus 'diffuse'
sunlight from the sky or surrounding surfaces may readily fall on a window,
even though the window may be shaded from direct sunlight by roof overhangs
or other shading devices.  In the summer when we need to air-condition
(cool) a building any sunlight passing through windows is detrimental.  On
the other hand, during cold seasons (when heating is needed) such energy is
helpful in offsetting the heating requirement.  Sunlight is used for natural
lighting of occupied spaces, by means of light pipes or light shelves, and
the source may partially be diffuse radiation.  So, diffuse sunlight
(radiation) is that which comes other than directly from the sun, and it is
important in that it may be either helpful or detrimental.  

The Solar Guy.