Method & sources
Every figure on UV Hour is 0.667 × MED ÷ effective UV, applied to a public weather forecast. No model, no training data, no proprietary dataset. This page gives the constants and the sources so you can reproduce any number on the site.
The calculation
effective UV = clear-sky UV
× cloud factor(cloud cover %)
× altitude factor(metres)
× surface factor(sand / water / snow)
minutes before first redness = 0.667 × MED / effective UV
with sunscreen = minutes × SPF
tanning window = the hours where effective UV is
at least 2 and no more than the skin type's ceilingMinimal erythemal dose, by Fitzpatrick type
| Skin type | MED | Ceiling | Reaction |
|---|---|---|---|
| Type I | 200 J/m² | UV 3.3 | Always burns, never tans. |
| Type II | 250 J/m² | UV 4.2 | Burns easily, tans minimally. |
| Type III | 300 J/m² | UV 5 | Burns moderately, tans gradually. |
| Type IV | 400 J/m² | UV 6.7 | Burns rarely, tans easily. |
| Type V | 500 J/m² | UV 8.3 | Burns very rarely, tans very easily. |
| Type VI | 600 J/m² | UV 10 | Almost never burns, always tans. |
Correction factors
- Cloud —
1 − 0.75 × (cover/100)^3.4. Half cover removes about 7% of the UV; full overcast removes about 75%. - Altitude —
1 + 0.12 × (metres/1000), i.e. +12% per 1,000 m. - Surface reflection — grass ×1.03, beside water ×1.2, dry sand ×1.15, snow ×1.8.
- SPF — a straight multiplier, as measured at 2 mg/cm². At the amount people actually apply, protection falls roughly with the square root of the shortfall.
Why 40 minutes
The ceiling for each skin type is not a table we chose — it is the UV level at which the formula returns 40 minutes, which gives UV 3.3 for type I through UV 10 for type VI. The threshold itself is a judgement: long enough to be a session rather than a sprint. It is stated rather than hidden so you can move it. Halve it and every ceiling doubles.
Data sources
| Quantity | Source | Licence |
|---|---|---|
| Clear-sky UV index | MET Norway Locationforecast 2.0 | CC BY 4.0 |
| Cloud cover | MET Norway Locationforecast 2.0 | CC BY 4.0 |
| Erythemal doses (MED) | Published Fitzpatrick phototype table | Public dermatological literature |
| UV index bands | World Health Organization global solar UV index | Public |
| Your location | Coarse geolocation from the network edge | Not stored, not logged |
Weather data from MET Norway (CC BY 4.0). Requests identify themselves as uvhour.com bonjour@agencedebord.com and every location is
cached for 15 minutes, as MET Norway's terms require.
What this site deliberately does not do
- It never recommends staying out beyond the erythemal threshold. Every number is a ceiling to stay under, not a target to reach.
- It makes no health claim about sun exposure, in either direction, and it is not medical advice.
- It assumes a healthy adult on no photosensitising medication. Several common prescriptions lower the erythemal threshold, and nothing here accounts for that.
- It stores nothing about you. There is no account, no cookie and no analytics identifier.
Corrections
If a constant here is wrong, or a factor is applied in a way that does not match the published literature, write to bonjour@agencedebord.com and it will be fixed. The arithmetic is the product; an error in it is a bug.