Finland weather by month

In Helsinki, the average share of the sky covered by clouds shows significant seasonal variation over the course of the year.

The clearer part of the year begins around March 9 and lasts for 6.7 months, ending around September 28. The clearest month is May, when the sky is clear, mostly clear, or partly cloudy 48% of the time on average.

The cloudier part of the year begins around September 29 and lasts for 5.3 months, ending around March 8. The cloudiest month is January, when it is overcast or mostly cloudy 84% of the time.

This is the average sustained wind speed at the standard 10 metres above open ground — the steady flow a weather station reports, not the sharper gusts that come and go, which can run half again as fast. It's the number that says whether a place is generally breezy or calm.

Across the year, average wind speed in Helsinki is typically light breeze, near 14 km/h, and varies little with the seasons.

The windier part of the year lasts 6.0 months, from around Sep 14 to Mar 15. The windiest month is December, averaging about 16 km/h (light breeze).

The calmer part of the year runs from around Mar 16 to Sep 13, with July the calmest month at about 12 km/h.

Solar energy is how much sunlight actually reaches the ground each day, measured in kilowatt-hours per square metre (kWh/m²) — the combined effect of how long the sun is up, how high it climbs, and how much cloud gets in the way. It's the raw fuel a solar panel has to work with: for scale, 1 kWh/m² a day is dim and wintry, while 7+ is desert-bright.

Helsinki receives about 2.8 kWh/m² a day of solar energy on average — moderate — with a clear seasonal swing between 0.2 and 5.9 kWh/m².

The brighter stretch of the year lasts 5.7 months, from around Mar 23 to Sep 11. The brightest month is June, averaging about 5.8 kWh/m² (bright).

The dimmest month is December, down near 0.2 kWh/m² — roughly 97% less than the June peak.