Best Time to Dive Koster archipelago / Kosterfjord
Across 1 Koster archipelago / Kosterfjord, Bohuslän west coast dive site with multi-year conditions history, May historically shows the most favorable days — 92% of sampled days in the good band. January historically shows the fewest favorable days (62% of sampled days). Average water temperature in the sample runs from about 3°C in February to 19°C in July. Typical seas range from 0.7 m to 1.4 m of significant wave height through the year. These figures are historical climatology computed from measured conditions — patterns, not a forecast or a guarantee.
Computed from our own conditions climatology — measured history, not brochure copy.
Month by month
| Month | Historically favorable days | Water temp | Typical seas |
|---|---|---|---|
| Jan | 62% | 4°C | 1.4 m |
| Feb | 69% | 3°C | 1.1 m |
| Mar | 78% | 4°C | 1.0 m |
| Apr | 90% | 7°C | 0.7 m |
| May | 92% | 12°C | 0.7 m |
| Jun | 81% | 17°C | 1.0 m |
| Jul | 81% | 19°C | 1.0 m |
| Aug | 78% | 19°C | 1.1 m |
| Sep | 79% | 17°C | 0.9 m |
| Oct | 69% | 13°C | 1.2 m |
| Nov | 76% | 9°C | 1.1 m |
| Dec | 63% | 5°C | 1.3 m |
% = share of sampled days scoring in the favorable band, median across 1 dive site with at least two years of history. Ringed = historical peak.
How does this compare worldwide? The Most Reliable Diving Months on Earth ranks 61 countries by the same measure.
Marine life seasons
Sighting patterns across the wider Norway, Iceland & North Atlantic region — not site-specific promises.
- Great white sharkAugust
- Bull sharkMay
- Ocean sunfishNov–Dec
- OrcaNovember
- Humpback whaleMay–Nov
- Thresher sharksOct–Jan
Bars are Jan–Dec — months when sightings historically concentrate (GBIF occurrence records). Wildlife is never guaranteed.
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