"Can someone explain the rules regarding de-icing?
How much precipitation etc?"
Well not really. And the reason is that every company will have a slightly different version of the rules approved in their Ops Specs.
Likewise each different chemical manufacturer will have a slight different instruction manual. And of course each aircraft will have it's own set of manuals with their own sets of instructions.
Rules of thumb are cold soaked wings, temperature below the freezing point, but it can get so cold that de-ice is not needed. Visible moisture, snow or ice build up on the wings.
I suspect that what you saw sprayed was Type I propylene Glycol. It would have a red or brownish orange color, which again will vary by manufacture or mix. It is the same stuff that is used in those "environmentally friendly" car anti-freezes and is also used as a food preservative. I have a bag of strawberry muffin mix that lists it as an ingredient. Keeps the berries in the mix moist I guess.
I'll give you a couple example.
One company I worked for allowed 1/4 inch of frost on the bottom of Cold Soaked wings. Another allowed 3/8 ths on the bottom surface.
One company had charts for type one mixture ratio's and sprayed different mixes dependent on weather conditions. Another company just sprayed 50/50 because that was all that was available at the remote site.
One company had the rampers performing the De-ice, another it was Maintainence's job.
I suspect that the reason for the spray in Tucson happened was simply because the manual said it had to be done. Alaska used to have (and may still) a policy of always spraying the wings of their MD
-80's down on short turns.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.