The northern Aegean Islands are the islands of the northern part of the Aegean Sea. They can be divided into two groups: the islands of the Thracian Sea (ie the northern part of the Aegean Sea), and the Northeast Aegean islands.

Islands in the Sea of Thrace, map

Above the 40th parallel, the islands of the Thracian Sea are the most northern, and have a transitional Mediterranean climate, characterized by fairly cold winters (with possible outbreaks of cold air from the Balkan Peninsula, associated with snowfall and frost), and hot and sunny summers, but shorter than in the southern islands; they are Samothrace, Thasos and Imroz (the latter belonging to Turkey).
Here for example, the average temperatures of Samothrace: in January and February highs hover around 9/10 °C; summer is hot, since highs in July and August are around 31 °C, however, already in September the temperature drops by 5 degrees.
Average temperatures - Samothrace
Samothrace J F M A M J J A S O N D
Min (°C) 3 3 5 9 13 18 20 21 16 13 8 5
Max (°C) 9 10 13 17 23 28 31 31 26 21 15 11

The rains are not very abundant, since they hover around 600 millimetres per year, and follow an almost Mediterranean pattern, since they are more frequent in autumn and winter, however, in late spring and summer sometimes you may experience some thunderstorms, coming from the continent, especially in Thasos, which is closer to the mainland.
This is the average rainfall in Thasos.
Average precipitation - Thasos
Thasos J F M A M J J A S O N D Year
Prec. (mm) 70 60 50 45 50 40 30 25 35 55 70 90 620
Days 9 8 8 7 8 6 5 4 4 6 8 9 82

The temperature of the sea in the Thracian Sea in summer is even slightly higher than in the Aegean Sea, because the latter is affected by the Meltemi, a cool wind from blowing the north and able to blend the waters, thus reducing their heating.
Here, for example, the temperature of the sea in Thasos.
Average sea temperatures - Thasos
Thasos J F M A M J J A S O N D
Sea (°C) 13 13 13 14 18 22 24 25 23 20 17 15

Northeast Aegean islands, map

The Northeast Aegean Islands, lying further south, become progressively warmer in winter, and less subject to cold spells, while summer becomes progressively longer. They are, from north to south: Lemnos (still pretty cold in winter, given the northern location), Agios Efstratios (or Saint Eustratius), Lesbos, Psara, Chios, Oinousses (or Aignousa, small island to the east of Chios), Samos, Ikaria (the westernmost, a short distance from the Cyclades), and the Fournoi Islands (or Fournoi Korseon), namely Fournoi, Thymaina and Agios Minas.
This is for example the average temperatures of Lesbos.
Average temperatures - Lesbos
Lesbos J F M A M J J A S O N D
Min (°C) 7 7 8 11 15 19 22 21 18 15 12 9
Max (°C) 12 13 15 19 24 28 30 30 27 22 17 14

As mentioned, the temperature of the sea in this central part of the Aegean is never very high, in fact it does not exceed 23 degrees in July and August.
Here are the temperature of the sea in Chios.
Average sea temperatures - Chios
Chios J F M A M J J A S O N D
Sea (°C) 15 15 15 16 18 22 23 23 22 20 18 16

The amount of sunshine in the northern Aegean Islands 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. As mentioned, in the northernmost islands, thunderstorms can break out also in the middle of summer.

The best time to travel to the Northern Aegean islands, for swimming and sun bathing, is the summer, from June to August in the islands of the Thracian Sea, and from June to mid-September in the Northeast Aegean Islands.

Back to the Greek Islands.