WhatAWeatherHow long can I stay in the sun?
Safe time hour by hour
Safe time without sunscreen (minutes) by hour of day, with forecast UV.
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Recommendations
- Reapply sunscreen every 2 hours and after swimming or sweating.
- Avoid direct exposure between 12pm and 4pm in summer.
- Use a hat, shirt and UV 400 sunglasses.
- Apply sunscreen generously (30 ml for the whole body) 15-30 minutes before going out.
- Snow, water and sand reflect UV: add 25-80% to the index.
- SPF protects mainly against UVB. Look for "UVA" and "broad spectrum" labels for UVA.
Guide: UV index, MED, sunburn and skin cancer
The ultraviolet (UV) index is an international WHO scale that indicates the intensity of solar ultraviolet radiation at ground level. It runs from 0 to 11+: higher means faster skin and eye damage. In Spain, June to August easily reach UV 9-10 around solar noon, and in the Canary Islands UV stays high year-round.
What is the Minimal Erythemal Dose (MED)
The MED is the amount of UV radiation that produces the first visible reddening on non-acclimatised skin. It depends on Fitzpatrick skin type: skin I (very fair) has MED 200 J/m2, skin VI (black) has MED 1,000+ J/m2. The calculator estimates the minutes to that dose under constant UV.
Fitzpatrick skin types
I: very fair, always burns, never tans (15 min unprotected at UV 5). II: fair, almost always burns, tans lightly (20 min). III: medium Mediterranean, sometimes burns, tans gradually (30 min). IV: olive, rarely burns, tans well (45 min). V: brown, very rarely burns (60 min). VI: black, never visibly burns but underlying damage is possible (90 min).
How SPF works
The Sun Protection Factor indicates how much longer you can be exposed before burning. SPF 30 multiplies real safe time by about 14 (not 30: the curve is non-linear). SPF 50 multiplies by 22 and SPF 50+ by 30. Higher is not always better: what matters is enough quantity and reapplication.
UVA, UVB and biological harm
UVB (280-315 nm) drives visible burning and acute skin cancer. UVA (315-400 nm) passes through glass and clouds, does not burn acutely but accelerates ageing and raises melanoma risk. Check that your sunscreen has UVA filters (look for the UVA-in-a-circle stamp on European products).
NASA and Open-Meteo data
UV values in this calculator come from Open-Meteo, which uses NASA GEOS and ESA CAMS models. They update hourly. For historic data see TEMIS or EUMETSAT.