Weather in Kos in January and February
The sun continues to shine for many hours even in the middle of winter. There will still be an average of 7 hours of sun over these weeks with average temperatures in double figures with highs around 17C. The sea will match those highs though may well feel colder but the beaches are still fun
Weather in Kos in March, April and May
Average temperatures climb from March up to 20 by the middle of May. It is unlikely to rain for more than a handful of days, certainly the later you get in this three month period. There are highs into the mid-20s in May with the average being 20C. That is also about the sea temperature you can expect.
Weather in Kos in June, July and August
These months mean cloudless skies and temperatures over 30C in the middle of the day. Even when the sun sets temperatures fall only slightly. By now the sea is around 24/25C. It is the peak of the season and certainly a time to enjoy water sports.
Rain is an extreme rarity but welcome and warm if it comes.
Weather in Kos in September and October
Those preferring slightly cooler weather select these two months for a visit to Kos. The average still exceeds 20C with highs still towards 30C though it is cooler at night than in the height of summer. The sea is slightly cooler but very pleasant as evidenced by the numbers who swim each day.
Weather in Kos in November and December
Kos becomes quieter and cooler at the same time. The locals enjoy the income tourists provide and the rest they have earned. While there may be around 10 days with rain in these two months there is also plenty of sun. The sea temperatures slowly drops but remains higher than mid-summer ones in the UK as an example.