Around here, roofs become covered with yellowish pollen. A very large amount of it. For the ponderosa pines, which abound, you can tap a branch with a stick or even toss a rock at one and a cloud of pollen will waft away. Our car windshields, my greenhouse, etc., all covered. Of course, that isn't all year.
Don't know, maybe that's it. It is difficult to filter out, the grains are tiny.
But maybe it isn't that. I collected rain water for some years and the first rain following a dry spell would give us muddy-looking water off the roof for a while til the roof was washed. The water in the buckets beneath the down spouts would be dark brown. A lot of the pollen floats til it becomes waterlogged. It can make a foamy top layer and coat the sides of the containers. In short, it makes a real mess.
When it was time to re-roof the house, I put up a metal roof. [replacing a cedar shingle one] That helped a lot, as the wood [like asphalt] has plenty of places to hole dust, dirt, bird poop, pollen, etc. That stuff still gets on the metal, of course, but it tends to wash off much faster. A lot of roof collection systems are set up to dump the first few gallons from each downspout before any rain is collected into the holding containers. These can be automated or manually operated, though for the latter someone has to be on site to do it.
