How do "Spread" spell effects ... spread?

What does the Spell Effect "Spread" mean? Examples: Sleet Storm, Obscuring Mist. Does the spreading happen instantly, or at a set rate per round, or at the listed rate per round?

Obscuring Mist: Range 30'; Effect: centered on you, spreads 30'; Duration: 1 min/lvl

Sleet Storm: Range: Long; Effect: spreads 40'; Duration 1 round/lvl


What does it all mean?!?!?

Is there a rule somewhere in a book that describes this effect that I am missing, or is it just something we are supposed to make assumptions about?

Thanks,

Brian
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from Pg.175 of the 3.5 PHB, second column, 6th, 7th, 8th, and 9th paragraph.

Basically though, most of those are Instantaneous. Fireball has a 20' radius spread. The difference between a spread vs a burst or emanation is that spread fills up area around corners if it has an opening for the "magic" to pass through.
Burst spells are like spread, but they can't turn corners. Both the spread and burst are instantaneous spells.
Emanation is like burst, it cannot turn corners, but almost all of these spells have more than instaneous duration. For example, Detect magic is an emanation.

Calrin Alshaw

D'oh!

I knew it was around here somewhere...

Thanks,

Brian
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I tend to think of a spread more like water flowing. Its going to get around corners etc. Also, you can assume that spell has somewhere from 6-2 or 3 seconds to spread in, very reasnoble for only 30 feet.