Map from Google - Thasos

The climate of Thasos (or Thassos), an island in the Northern Aegean Sea, a short distance from the Thracian coast, is transitional Mediterranean, given the northern location, with quite cold and rainy winters, and hot and sunny summers. The average temperature of January is 4.5 °C (40 °F), that of July and August is 23.5 °C (74.5 °F). The island in winter is subject to outbreaks of cold air from the Balkan Peninsula, while in late spring and summer it can be affected by thunderstorms coming from the mainland.
Here are the average temperatures.
Average temperatures - Thasos
ThasosJanFebMarAprMayJunJulAugSepOctNovDec
ThasosJFMAMJJASOND
Min (°C)114813171919141162
Max (°C)891216212628282419149
Min (°F)343439465563666657524336
Max (°F)464854617079828275665748

The interior of the island is occupied by hills and low mountains, culminating in Mount Ypsarion, 1,170 metres (3,839 feet) high. The north side is green and covered with forests, while the south side is more arid.

The rainfall is not abundant, since it amounts to 620 millimetres (24.5 inches) per year; the rain pattern is almost Mediterranean, because winter is the wettest season and summer is the driest, but the rains increase slightly in late spring, and in summer they remain a bit more frequent than in the southern Greek islands. Here is the average precipitation.
Average precipitation - Thasos
ThasosJanFebMarAprMayJunJulAugSepOctNovDecYear
ThasosJFMAMJJASONDY
Prec. (mm)706050455040302535557090620
Prec. (in)2.82.421.821.61.211.42.22.83.524.4
Days98878654468982

Winter, from December to mid-March, is pretty cold: normally the maximum temperatures hover around 8/9 °C (46/48 °F), but they may remain at certain times below 5 °C (41 °F), in addition, the wind can increase the feeling of cold. Moreover, sometimes, and not too infrequently, it can snow, and there may be night frosts. The rains are quite frequent. However, there are also mild days, with highs about 13/15 °C (55/59 °F).

Spring, from mid-March to May, is quite mild, but with some periods a bit cold, windy and rainy, which gradually become less frequent.

Summer, from June to August, is hot and sunny, with rare showers. Typically, the maximum temperatures hover around 28 °C (82 °F) in July and August, but there are also hotter periods, when they reach 32/34 °C (93/97 °F). Luckily, sea breezes blow.

Autumn, from September to November, is mild or pleasantly warm at the beginning, and then becomes gradually cooler. The rains become gradually more frequent, although in September there are still entire weeks without rain. In November, the first days a bit cold and windy begin to occur, and sometimes also in October.

Beach in Thasos

The amount of sunshine in Thasos is very good in summer, when clear skies prevail, and it is also good in May and September, while in the rest of the year it's not high, since there are several periods in which the sky is cloudy.
Sunshine - Thasos
ThasosJanFebMarAprMayJunJulAugSepOctNovDec
ThasosJFMAMJJASOND
Sun (hours)456891112119754

The temperature of the sea, still cold in May, around 18 °C (64 °F), and cool in June, around 22 °C (72 °F), becomes warm enough for swimming from July to September, and reaches a peak in August, of 25 °C (77 °F). This northern part of the Aegean Sea, close to the mainland and also less windy, in winter is slightly colder, but in summer becomes even warmer than the central and southern part.
Here are the average sea temperatures.
Sea temperature - Thasos
ThasosJanFebMarAprMayJunJulAugSepOctNovDec
ThasosJFMAMJJASOND
Sea (°C)131313141822242523201715
Sea (°F)555555576472757773686359

When to go

The best time to go to Thasos, for swimming and sunbathing, is summer, from June to August. In June, and sometimes in early July, there may be some cool evenings, so it's better to bring a sweatshirt or sweater for the evening, and those who are sensitive to cold can do it also in mid-summer.
In May, not only the sea is cold, but sometimes there may be some quite cool days, with highs below 20 °C (68 °F), especially in the first part of the month.
For excursions throughout the island, you can choose May and September.

Back to the North Aegean Islands.