Sunshine, sky clarity, and UV exposure together determine how much outdoor life is realistic in a city. We measure all three independently because they tell different stories.
The three metrics
- Sunshine hours — Hours per day with the sun unobstructed by cloud cover, averaged by month. A strong predictor of mood and energy. The difference between 3 hours of winter sun in London and 7 in Lisbon is life-changing for many people.
- Clear-sky days — Days per year with mostly cloudless skies. Different from sunshine hours: a city can have sunny stretches but be mostly overcast (maritime climates), or have long clear stretches with intense seasonal contrast (continental climates).
- UV index — How strong the sun's UV radiation is at ground level, on a scale roughly 0–11+. Drives sunburn risk, vitamin D production, and long-term skin exposure. Tropical and high-altitude cities can hit 11+ year-round; northern winters can stay under 2 for months.
Why it matters
Light is the single biggest climate factor people underestimate when relocating. People research temperature and cost but overlook that a city with 1,500 sunshine hours/year feels fundamentally different from one with 2,800. Seasonal Affective Disorder, energy levels, and outdoor activity patterns all track with light exposure.
What it doesn't capture
- Daylight hours (photoperiod) aren't the same as sunshine hours. A city at 60°N has very long summer days and very short winter days regardless of cloud cover.
See it in action
The Climate Lab on every city page breaks down sunshine hours and UV by month. The explore map lets you filter cities by annual sunshine hours.