The UV index is not a constant. It rises and falls with the angle of the sun, the thickness of the ozone layer, the altitude and the season. Knowing what to expect each month — for your latitude — lets you plan vitamin D windows in winter, sunscreen routines in summer, and ski trips without a fried face. Here is the year, broken down.
The annual UV calendar (clear-sky midday)
Average maximum UV index at solar noon, clear sky, sea level. Use it as a baseline — clouds, altitude and snow can shift the actual reading by ±2.
| Month | Tropics (0–20°) |
Subtropics (20–35°) |
Mid-lat (35–50°) |
High lat (50–65°) |
|---|---|---|---|---|
| January | 10 | 7 | 2 | 1 |
| February | 11 | 8 | 3 | 1 |
| March | 12 | 9 | 5 | 3 |
| April | 12 | 10 | 6 | 4 |
| May | 12 | 11 | 7 | 6 |
| June | 12 | 11 | 8 | 7 |
| July | 12 | 11 | 8 | 7 |
| August | 12 | 10 | 7 | 6 |
| September | 12 | 9 | 6 | 4 |
| October | 11 | 8 | 4 | 3 |
| November | 10 | 7 | 3 | 1 |
| December | 10 | 7 | 2 | 1 |
Reference cities by band: tropics — Miami, Singapore, Mexico City; subtropics — Madrid, Tokyo, Sydney; mid-latitudes — Paris, New York, Vancouver; high latitudes — Stockholm, Edinburgh, Anchorage. Southern Hemisphere readers: shift the months by 6 (December = your June).
Month by month, what it actually means
The 4 modifiers that break the table
- Altitude: +10 % UV per 1,000 m. A mountaintop in February can match a beach in May.
- Snow: reflects up to 80 % of UV back at you. Doubles the dose.
- Water and sand: +10 to +25 %.
- Clouds: usually reduce UV, but the "broken cloud effect" can amplify it by up to +25 % at the cloud edges.
How to use this calendar
- Plan vitamin D synthesis only in months where your latitude column hits UV 3 or higher around solar noon.
- Switch sunscreen routine as soon as your column crosses UV 6 (default daily SPF on face/neck).
- Schedule outdoor sport before 11 AM or after 4 PM during UV 7+ months.
- Re-check kids' protection every 6 weeks — children's skin tolerates roughly half the dose adults do.
The Sun Day way
This calendar is the average. Your real UV right now depends on today's clouds, today's ozone, today's altitude. Sun Day fetches all three from the Open-Meteo forecast at your exact GPS coordinates and gives you the personalized number, refreshed every hour, with alerts when you cross your skin type's safe threshold. The calendar tells you what season expects from you; the app tells you what today is doing.
Plan your sun, all year long
Sun Day — the UV calendar that adapts to you, every day.