When are there jellyfish in United Kingdom?

Portuguese man o' war are recorded most in January, February, October and November — and this is the one that matters, because the sting is severe.

Portuguese man o' war (severe sting)

Recorded well above the expected rate in January, February, October and November, peaking at 5.7× the expected rate. Based on 369 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	3.6	1.7	0.2	0.0	0.0	0.0	0.3	0.5	0.9	4.7	5.7	1.0

Mauve stinger (painful sting)

Recorded well above the expected rate in August, September, October and November, peaking at 4.7× the expected rate. Based on 114 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	0.0	0.2	0.1	0.1	0.1	0.6	0.6	1.5	1.6	3.1	4.7	0.5

Lion's mane (painful sting)

Recorded well above the expected rate in July and August, peaking at 3.0× the expected rate. Based on 2,234 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	0.0	0.0	0.0	0.0	0.2	1.1	2.7	3.0	1.1	0.3	0.2	0.0

True jellyfish (varies by species)

Recorded well above the expected rate in June, July and August, peaking at 2.9× the expected rate. Based on 11,984 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	0.3	0.2	0.1	0.2	0.6	1.9	2.9	1.6	0.6	0.3	0.3	0.2

What these numbers are, and what they are not

1.0 means "as often as expected". Nobody counts jellyfish; people report the ones they run into, and far more people are at the coast in summer. So a raw monthly count of sightings is mostly a picture of the holiday season.

To strip that out, each month's jellyfish records are divided by the records of a control group — molluscs, logged by the same people on the same coast in the same months. What is left is the seasonality that is not explained by there simply being more observers about. A 2.0 means jellyfish were reported twice as often as the level of recording that month would predict.

It is not abundance, and it is not your chance of being stung. It is also not comparable with another country: national totals reflect how much recording each country has digitised, not how many jellyfish are in the water.

Sea temperature in United Kingdom

Records from GBIF, the Global Biodiversity Information Facility. How the adjustment works