Map from Google - Kos

In Kos (or Cos), an island of the Eastern Aegean Sea, belonging to the Dodecanese group, located a short distance from the Turkish coast, the climate is Mediterranean, with mild and rainy winters, and hot and sunny summers. The average temperature of February is 9.5 °C (49 °F), that of July is 25.5 °C (78 °F).
Here are the average temperatures.
Average temperatures - Kos
KosJanFebMarAprMayJunJulAugSepOctNovDec
KosJFMAMJJASOND
Min (°C)8710121519212120161210
Max (°C)131215182227302927221713
Min (°F)464550545966707068615450
Max (°F)555459647281868481726355

Precipitation amounts to 730 millimetres (28.5 inches) per year, and is concentrated between October and March; it often occurs in the form of thunderstorm, and sometimes can be abundant. In summer it almost never rains. Due to the length of the dry period, the landscape is semi-arid, except in the inland hills, which reach about 800 metres (2,600 feet) above sea level.
Here is the average precipitation.
Average precipitation - Kos
KosJanFebMarAprMayJunJulAugSepOctNovDecYear
KosJFMAMJJASONDY
Prec. (mm)170115753520825156090135732
Prec. (in)6.74.531.40.80.30.10.20.62.43.55.328.8
Days12109753333681281

Winter, from December to mid-March, is mild enough: daytime temperatures hover about 12/14 °C (54/57 °F). The rains are quite frequent, but there are also sunny days. Sometimes it can be a bit cold, with highs below 10 °C (50 °F). The wind can increase the feeling of cold. Sometimes you can even see a bit of snow, though usually without accumulation, at least in the plains.

Spring, from mid-March to May, is mild, and over the weeks becomes progressively warmer, sunnier and less rainy.

Summer, from June to mid-September, is hot and sunny. The Meltemi, the wind from the north-west, blows from mid-May, cooling the air, but with lower intensity than in the central islands of the Aegean Sea. In addition, sometimes there may be a period of intense heat, with maximum temperatures around 35 °C (95 °F).

Autumn, from mid-September to November, is often sunny and pleasantly warm at the beginning, and gradually becomes milder, while the rains become more frequent.

Beach in Kos

The amount of sunshine in Kos is great in summer, when clear skies prevail, it's very good in late spring, while in autumn and winter sunny periods alternate with others characterized by clouds and rain.
Sunshine - Kos
KosJanFebMarAprMayJunJulAugSepOctNovDec
KosJFMAMJJASOND
Sun (hours)56791113141311865

The sea is warm enough for swimming from July to September, and reaches 24 °C (75 °F) in August and September.
Here are the average sea temperatures.
Sea temperature - Kos
KosJanFebMarAprMayJunJulAugSepOctNovDec
KosJFMAMJJASOND
Sea (°C)171616171922232424221917
Sea (°F)636161636672737575726663

When to go

The best time to go to Kos, for beach life, runs from June to mid-September. In early June it can still be a little cool in the evening: for safety you can bring a sweatshirt.
In May, not only the sea is still a bit cold, but also the air can be cool for sunbathing, at least on certain days.
To visit the ruins of ancient Greece or the Middle Ages, the best time is spring, especially the month of May, since in summer it can be hot. You can also choose autumn, especially October, whereas November is cooler and rainier.

Back to the Dodecanese.