When to Switch Suits in Izu — The Odds of Getting Cold in June, Site by Site
2026-03-09
"Is a wetsuit fine from June?" Ask that in Izu — the classic dive region two hours southwest of Tokyo — and the honest answer depends entirely on which site you mean. We counted, across 12,662 measurements from five Izu sites (2006–2026), the odds that a given month serves you water below 20°C — the conventional threshold where drysuits are recommended.
The answer, as a table: odds of hitting sub-20°C water in a wetsuit
| Month | Koganezaki | Hirasawa | Mikomoto | Izu Ocean Park | Futo |
|---|---|---|---|---|---|
| May | 89% | 81% | 85% | 98% | 95% |
| June | 5% | 16% | 29% | 48% | 51% |
| July | 0% | 1% | 10% | 17% | 11% |
| October | 1% | 0% | 0% | 0% | 0% |
| November | 28% | 11% | 13% | 1% | 14% |
| December | 79% | 82% | 69% | 65% | 84% |
Each cell is the share of all measurements in that month below 20°C. Three takeaways:
- "Wetsuit from June" is meaningless without a site name. At Koganezaki it's a 5% risk — near certainty of comfort. At Futo it's 51% — a coin toss. Same region, same month, tenfold difference.
- July is the first month that's safe everywhere — though Izu Ocean Park still carries a 17% chance and Futo 11% (cold-water upwelling, see below).
- In autumn, October is unanimous wetsuit weather (0–1% everywhere). The first site to turn cold in November is, surprisingly, Koganezaki (28%).
Why sites differ this much
Koganezaki sits in a shallow bay: it warms first and cools first — first into wetsuit territory in June (5%), first to want a drysuit in November (28%). Open-ocean Mikomoto and the deeper-water sites Futo and Izu Ocean Park lag behind. Mikomoto's June readings span 15.5–24.7°C, so its 20.9°C average hides real gambling odds.
The other trap is summer upwelling: Izu Ocean Park reads below 18°C on 10% of July days. If you pack a 3mm because "it's July," this is the day that finds you. (Why midsummer is the hardest season to predict is a story we've verified separately.)
The year at a glance (monthly means)
- Feb–Mar (15–16°C): the floor — Futo bottoms at 15.1°C in February. Drysuit plus thick undergarments. The reward: the year's best visibility
- May (17.7–19.0°C): warming, but 8–9 of 10 days are still below 20°C. Stay dry
- June (19.8–22.1°C): the switching month — use the table above, per site
- Aug–Sep (23.5–26°C): the peak. At most sites the warmest month is September, not August (Futo 24.9°C, Mikomoto 25.1°C)
- October (23.5–24.3°C): the sleeper month — summer water, sub-20°C odds near zero, and thin crowds because only the air got cold
- November (21.0–21.7°C): the dilemma month; cold-sensitive divers switch back
- December (18.1–19.0°C): 65–84% of readings below 20°C — drysuit season in practice
Recommended switch timing by cold tolerance
| Type | Spring (dry → wet) | Autumn (wet → dry) |
|---|---|---|
| Runs cold | From July (safe everywhere) | Early November (late October for Koganezaki) |
| Average | June at Koganezaki/Hirasawa; July at Futo/IOP | Mid–late November |
| Cold-hardy | June everywhere (check Mikomoto's latest reading first) | Early December |
Check your site's latest measured temperature in the historical archive, and get a thickness recommendation from the wetsuit thickness tool or our quick-reference guide.
Note: The 20°C threshold is an industry rule of thumb; cold tolerance varies. Cold water is a documented incident factor (DAN Japan). Local shop advice comes first.
Data sources
- Water temperatures: this site's database (five Izu sites, 12,662 readings, 2006–2026, browsable in the archive)
- Regional SST trends: Japan Meteorological Agency / hypothermia: DAN Japan
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