Map from Google - Karpathos

In Karpathos, an island of the southeast Aegean Sea, belonging to the Dodecanese archipelago and having an area of 324 square kilometres (125 square miles), and in the neighboring islet of Saria, the climate is Mediterranean, with mild and rainy winters, and hot and sunny summers. The average temperature of January and February is 11 °C (52 °F), that of August is 25.5 °C (78 °F).
Here are the average temperatures.
Average temperatures - Karpathos
KarpathosJanFebMarAprMayJunJulAugSepOctNovDec
KarpathosJFMAMJJASOND
Min (°C)889121519212220161310
Max (°C)141516192327292927231916
Min (°F)464648545966707268615550
Max (°F)575961667381848481736661

Rainfall amounts to 670 millimetres (26.5 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, basically it never rains; because of the long summer drought, the landscape is arid.
Here is the average precipitation.
Average precipitation - Karpathos
KarpathosJanFebMarAprMayJunJulAugSepOctNovDecYear
KarpathosJFMAMJJASONDY
Prec. (mm)150105753015511106580140670
Prec. (in)5.94.131.20.60.2000.42.63.15.526.4
Days1197421001561157

Winter, from December to mid-March, is mild: daytime temperatures hover on average around 14/16 °C (57/61 °F). The rains are quite frequent, but there are also sunny days. Sometimes it can get a bit cold: lows can drop to around 3/5 °C (37/41 °F), and highs to 7/10 °C (45/50 °F). The wind can increase the sensation of cold. In the plain it hardly ever snow (sometimes you can see the snow falling, though it does not accumulate on the ground), while on the highlands, whose highest point is Mt. Lastos (or Kali Limni), 1,215 metres (3,986 feet), sometimes it can snow.

Spring, from mid-March to May, is mild, and over the weeks becomes gradually more warm and sunny, and less rainy.

Summer from June to mid-September, is hot and sunny. The Meltemi, the northwest wind, which blows from the middle of May, and can reduce the sensation of heat, is felt mostly on the west side of the island. There can be periods in which the wind slows down, and the heat can become intense, with highs around 35 °C (95 °F), though they are very rare: usually the temperature remains around 28/30 °C (82/86 °F).

Autumn, from mid-September to November, is often pleasantly warm and sunny at the beginning, then becomes gradually milder and also more rainy.

Amoopi Beach

The amount of sunshine in Karpathos is excellent in summer, when clear skies prevail, and is very good in spring, while in autumn and winter, sunny periods alternate with others characterized by clouds and rain.
Here are the average sunshine hours per day in nearby Rhodes.
Sunshine - Rhodes
RhodesJanFebMarAprMayJunJulAugSepOctNovDec
RhodesJFMAMJJASOND
Sun (hours)56791113141311865

The sea in Karpathos is warm enough for swimming from July to September, and reaches 25 °C (77 °F) in August.
Here are the average sea temperatures.
Sea temperature - Karpathos
KarpathosJanFebMarAprMayJunJulAugSepOctNovDec
KarpathosJFMAMJJASOND
Sea (°C)161616171922242524222018
Sea (°F)616161636672757775726864

When to go

The best time to go to Karpathos, for beach tourism, goes from June to mid-September. You can bring a sweatshirt for windy evenings. In June, the sea is still a bit cool.
In May, not only the sea is still a bit cold, but also the air can be cool for sunbathing, at least during some days.
For walks on the island, you can choose spring, in April and May, since summer can be very hot (although, as we have seen, the periods of very hot weather are rare). You can also choose autumn, although it is rainier.

Back to the Dodecanese.