Jersey weather by month

In Saint Helier, 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 May 6 and lasts for 4.8 months, ending around September 29. The clearest month is July, when the sky is clear, mostly clear, or partly cloudy 57% of the time on average.

The cloudier part of the year begins around September 30 and lasts for 5.8 months, ending around March 25. The cloudiest month is January, when it is overcast or mostly cloudy 73% 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 Saint Helier is typically breezy, near 25 km/h, and varies a lot with the seasons.

The windier part of the year lasts 5.6 months, from around Oct 2 to Mar 19. The windiest month is January, averaging about 31 km/h (windy).

The calmer part of the year runs from around Mar 20 to Oct 1, with July the calmest month at about 21 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.

Saint Helier receives about 3.5 kWh/m² a day of solar energy on average — moderate — with a clear seasonal swing between 0.8 and 6.3 kWh/m².

The brighter stretch of the year lasts 5.9 months, from around Mar 23 to Sep 19. The brightest month is June, averaging about 6.2 kWh/m² (intense).

The dimmest month is December, down near 0.8 kWh/m² — roughly 86% less than the June peak.