Central Valley summer dust — wind, fields, baked-on accumulation.
Short answer: The Central Valley generates a baseline of fine agricultural dust between June and September that lands on panels at 2–4% drop per month. The dust itself is benign — silica + plant matter — but Yolo's daytime temps in July routinely hit 105°F, and that heat bakes the dust into a thin varnish-like film. Mid-July is the ideal clean date because production loss is becoming material (8–12%), the heat is established (single-pass cleanable), and three more months of dust + fire-season ash are coming.
Where the dust comes from
- Plowed agricultural fields — Yolo + Solano have ~1.2M acres of working farmland. Summer wind picks up fine particles from harvested + plowed fields.
- Almond orchard floor — late-summer dust from harvested almond orchards (the trees are shaken; floor debris becomes airborne).
- Construction sites + grading — Roseville and Folsom growth means active construction year-round; nearby areas downwind get extra deposition.
- I-80 and I-5 corridors — vehicle-generated dust adds to baseline in highway-adjacent neighborhoods.
- Cottonwood Creek + Putah Creek dry-season dust corridors — Davis-specific.
The heat-bake effect
Fresh dust on cool glass washes off with light rain. Once panels hit 130°F+ in afternoon sun (which they do most July days), the dust + atmospheric moisture + heat cycle creates a thin polymerized film. By August, this film is the main efficiency loss — not loose dust on top.
You can demonstrate this yourself: hose your panels in May. You'll see clean glass within minutes. Hose them in August after months of unwashed accumulation. You'll see slight haze that won't fully rinse off. That's the baked film.
Why mid-July is the right clean
- Production loss is material — 8–12% across most installs in unwashed Yolo/Sac systems
- The film is still soluble — single-pass clean removes it
- You get clean panels for the August dust month + September fire risk
- If September brings smoke, the post-fire clean is layered on cleaner glass — more effective
Neighborhoods worst-hit by dust
- Dixon — surrounded by agriculture in all directions
- Winters — adjacent to working ag
- Davis Mace Ranch + Wildhorse — east-side new developments adjacent to ag land
- Folsom edge developments — adjacent to active construction grading