Weather Windows & Temperature Inversions

Wind: the range, not the maximum

Most labels allow roughly 3–10 mph. Under 3 isn't "safest" — it's an inversion warning. Over 10, drift potential climbs fast and your swath displaces downwind. Measure at the site (a $30 handheld anemometer at boom height beats a phone forecast) and note direction relative to sensitive areas.

Delta T: the number pros use

Delta T is the gap between dry-bulb and wet-bulb temperature — a proxy for how fast droplets evaporate and shrink into driftable fines. Rough guide widely used in aerial application: Delta T 2–8 is ideal, 8–10 caution, above 10 stop for fine sprays. Hot + dry days blow past 10 by mid-afternoon; that's why quality windows are early and late.

Inversions: the field signs

• Wind under ~2–3 mph, clear sky, cooling evening or just after dawn
• Dust, smoke, or fog hanging flat instead of rising and dispersing
• Dew forming; sounds carrying unusually far
• Two thermometers (canopy vs ~8–10 ft) reading warmer up high = inversion confirmed

In an inversion, fine droplets sit in a stable layer and can move laterally for miles. Do not spray — this is the single most common cause of long-distance drift incidents.

Your daily decision routine

1. Night before: check tomorrow's wind direction vs your sensitive neighbors; plan field order around it.
2. At the field: wind, temp, humidity, inversion check — 60 seconds with the conditions checker.
3. Mid-job: re-check when the sun climbs; conditions that were fine at 8am often aren't at noon.
4. Log every reading in the record.

Delta T guidance follows standard aerial-application practice; wind limits per product label. See also drift management.