Jellyfish Watch
Estimate jellyfish risk at a coastal spot from water temperature, waves and season — plus what the evidence actually says about treating a sting.
When the wave data is missing, the number quietly means something else
The score is a weighted average, and it divides by the weights it actually received rather than by the full set. Any reading that comes back empty is dropped and the rest are scaled up to fill the gap:
| Factor | With wave data | Without |
|---|---|---|
| Water warmth | 55.0% | 67.1% |
| Calm water | 18.0% | — |
| Season | 15.0% | 18.3% |
| Recent storms | 12.0% | 14.6% |
Sea temperature and wave height come from the same marine endpoint, and wave height is the one that goes missing at sheltered or partly inland points. When it does, water warmth silently becomes 67% of the verdict instead of 55% — the remaining 82% of the weight carrying 100% of the answer.
| Conditions | Factors used | Score |
|---|---|---|
| warm day, moderate swell | 4 | 55 |
| the same day, no wave data | 3 | 55 |
| hot and calm | 4 | 80 |
| the same, no wave data | 3 | 76 |
Identical water, four points apart. Renormalising is the right call — the alternative is treating missing data as zero risk, which would be worse — but it is a decision worth knowing about, because nothing on screen distinguishes the two cases except a bar that is not there.
And a single number is many-to-one by design
Sweeping every plausible combination of water temperature, wave height and rainfall gives 23,125 distinct sets of conditions and only 86 distinct scores — about 269 states per number. The commonest single value covers 414 of them.
| Water | Waves | Recent rain | Score |
|---|---|---|---|
| 27.5°C | 0 m | 9.5 mm | 82 |
| 32.5°C | 0.8 m | 0 mm | 82 |
Two beaches showing the same number can be in genuinely different states. A weighted average is a summary, and summarising is precisely the act of discarding which input was responsible — some of that is unavoidable, since the score is rounded to a whole percentage and only 101 values exist.
Which is the argument for the bars underneath. They are not decoration; they are the only part of the display that says what is actually driving the answer, and the only way to tell a hot still sea from a merely warm one after a storm.
How to use
- Check the conditions for your beach and season.
- Read the estimated risk level.
- Follow local warnings, which beat any general estimate.
- Know the sting first aid before you need it.
Frequently asked questions
What should I do for a sting?
Get out of the water, remove any remaining tentacles without bare hands, and rinse with seawater rather than fresh. For most stings, immersion in hot water — as hot as can be tolerated without scalding, around 45 degrees Celsius — has the best evidence for pain relief. Seek medical help for severe pain, a large sting area, or any breathing difficulty.
Does urine help?
No. It is a persistent myth with no supporting evidence, and it may make things worse by triggering undischarged stinging cells. Fresh water does the same, which is why rinsing with seawater matters.
Should I use vinegar?
It depends on the species, which is why blanket advice is unhelpful. Vinegar is recommended for box jellyfish stings in tropical waters and can worsen stings from some other species. Where local guidance specifies vinegar, follow it — where it does not, hot water immersion is the safer default.
Which jellyfish are actually dangerous?
A small number, but seriously so. Box jellyfish in tropical Indo-Pacific waters can be life-threatening and require immediate emergency care. Portuguese man o' war stings are severe though rarely fatal. Most species most people meet cause pain rather than danger.
When are jellyfish most common?
Generally in warmer months and warmer water, and often after onshore winds that push them toward beaches. Blooms are patchy and hard to predict, which is why a lifeguard's flag and a recent local report are worth far more than any seasonal estimate.
Is this a substitute for local warnings?
No, and it should not be treated as one. Beach authorities monitor actual conditions and post warnings that reflect what is present today. This is a general seasonal guide, and nothing here is medical advice — a sting causing severe symptoms is an emergency.
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