Wash or wait.
One honest number from your local forecast. The index reads rain risk, drying conditions, wind, temperature, pollen and dust for the next 48 hours, then says it straight: money well spent, or water down the drain.
The readout
Scored 0–100 · metric-canonicalWash windows
Best 2-hour slots · daylight-awareNext 48 hours
Hour · index · rain % · tempHow it's scored
No black boxRead the forecast
168 hours of local weather from Open-Meteo: rain probability and actual precipitation, temperature, humidity, wind gusts — plus airborne dust and six pollen species where air-quality data exists.
Score every hour
Each hour starts at 100 and takes the penalties on the right. Scoring always runs in metric, whatever units you display — so the verdict never changes with your settings.
Look ahead, not just out the window
A clear sky right now means nothing if rain lands in three hours. Every hour is penalised for rain arriving within the following 6 hours, and the index counts how many dry days a wash would actually hold.
Call it
70+ = WASH. 45–69 = BORDERLINE. Under 45 = WAIT. The confidence figure reflects how steady the next six hours look — a twitchy forecast earns less trust than a settled one.
| Condition | Penalty |
|---|---|
| Rain probability 60% or more | −70 |
| Thunderstorm conditions | −70 |
| Precipitation falling / snow | −60 |
| At or below freezing (0 °C / 32 °F) | −55, capped at WAIT |
| Rain probability 40–59% | −45 |
| Dust event (150+ µg/m³) | −40 |
| Rain likely within 6 hours | −35 |
| Cold (under 5 °C / 41 °F) | −32 |
| Rain probability 25–39% | −28 |
| Strong gusts (45+ km/h / 28+ mph) | −22 |
| Heavy airborne dust | −20 |
| Hot (over 32 °C / 90 °F) | −18 |
| Very high humidity (over 90%) | −18 |
| Cold and damp (under 10 °C at 85%+ humidity) | −15 |
| Peak pollen | −15 |
Smaller penalties apply on lighter versions of each condition · score clamps 0–100
Q1When is the best time to wash a car?+
When rain probability stays under 20% for the next six hours, humidity sits between 40 and 70%, winds are gentle, and the temperature is between roughly 5 and 28 °C (41–82 °F). Early morning or late afternoon beats midday — direct summer sun flash-dries water into mineral spots. That combination is exactly what the index hunts for.
Q2Should I wash my car if rain is coming?+
Usually no — rain re-deposits road film and dust onto clean paint, and in most regions rainwater dries into visible spots. The index penalises any rain arriving within 6 hours and counts how many dry days a wash would actually hold, so you're never paying for a wash the sky undoes by Tuesday.
Q3Can I wash my car in cold weather?+
Above 5 °C (41 °F), yes — dry door seals and locks well. At or near freezing, don't: water wicks into seals, locks and trim and freezes them shut, and washing chemicals stop working properly. The index treats freezing conditions as an automatic WAIT.
Q4What about pollen season?+
Wash more often during pollen season — pollen is mildly acidic and abrasive when wiped dry — but expect the shine to be short-lived on peak days. Where air-quality data is available, the index deducts for elevated pollen and dust so the verdict already reflects it. Always rinse pollen off with water first; never dry-wipe it.