IR exposure on overcast days

[RLCaldwell@xxxxxxx]

I recently read Laurie White's book on IR photography and was puzzled by one 
of her exposure recommendations.  She said that on overcast days, one should 
under-expose from the meter reading.  Apparently she uses an SLR and meters 
through a #25 filter on the camera's lens.

At first consideration, this seems a little counter intuitive, but I tested 
it, and it appears to be true.  If am not totally confused, this indicates 
that on overcast days there is a higher proportion of IR in the ambient light 
than there is on sunny days.

Could someone enlighten me on this?

In a similar vein, do IR practitioners adjust their EI for the season?

Thanks,
Bob Caldwell
San Jose, California  (Where we are entering our overcast winter.)
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