When are there jellyfish in Japan?

True jellyfish are recorded most in May, August and September .

True jellyfish (varies by species)

Recorded well above the expected rate in May, August and September, peaking at 1.7× the expected rate. Based on 483 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	0.0	0.4	0.7	1.2	1.4	1.2	1.2	1.7	1.5	0.5	0.2	0.3

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 Japan

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