Map from Google - Lemnos

The climate of Lemnos, an island of the Northern Aegean Sea, is Mediterranean, with relatively mild and rainy winters, and hot and sunny summers. The island is not as warm as the southern Greek islands, it is windy, and in winter it's subject to cold air outbreaks from the Balkan Peninsula.
Here are the average temperatures.
Average temperatures - Lemnos
LemnosJanFebMarAprMayJunJulAugSepOctNovDec
LemnosJFMAMJJASOND
Min (°C)446913172020171396
Max (°C)111113172227292925201612
Min (°F)393943485563686863554843
Max (°F)525255637281848477686154

Precipitation follows the Mediterranean pattern, is more frequent between November and March, and is not abundant, since it does not reach 500 millimetres (20 inches) per year. In summer the rains are very rare, in the form of short downpour or thunderstorm. No wonder the island's landscape is arid.
Here is the average precipitation.
Average precipitation - Lemnos
LemnosJanFebMarAprMayJunJulAugSepOctNovDecYear
LemnosJFMAMJJASONDY
Prec. (mm)6550504025159725457585485
Prec. (in)2.6221.610.60.40.311.833.319.1
Days10999752237101285

Winter, from December to mid-March, is not so mild: there are several days with highs below 10 °C (50 °F), and sometimes even below 5 °C (41 °F), and with a moderate or even strong wind. Every now and then, it can be very cold and it can even snow, and at night there may be slight frosts. Rainfall is quite frequent. There are also mild days, with highs around 15/18 °C (59/64 °F).

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

Summer, from June to August, is hot and sunny, with rare rains. Typically the maximum temperatures hover around 28/32 °C (82/90 °F), and there's a wind from the north, the Meltemi, which blows with a certain intensity in periods of a few days, though less than in the Cyclades. However, there may be periods of intense heat, in which the maximum temperature reaches 32/34 °C (93/97 °F).

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

Myrina

The amount of sunshine in Lemnos is very good in summer, and also in May, when clear skies prevail, while in the rest of the year it is not so remarkable, since there are many periods in which the sky is cloudy.
Here are the average sunshine hours per day in nearby Lesbos.
Sunshine - Lesbos
LesbosJanFebMarAprMayJunJulAugSepOctNovDec
LesbosJFMAMJJASOND
Sun (hours)55781013141311854

The temperature of the sea, still cold in May, and cool in June, around 22 °C (72 °F), becomes acceptable for swimming in July and September, around 23 °C (73 °F), and reaches a peak in August, of 24 °C (75 °F).
Here are the average sea temperatures.
Sea temperature - Lemnos
LemnosJanFebMarAprMayJunJulAugSepOctNovDec
LemnosJFMAMJJASOND
Sea (°C)141414151822232423201715
Sea (°F)575757596472737573686359

When to go

The best time to travel to Lemnos, for swimming and sunbathing, is the 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 for the evening.
In May, not only the sea is cold, but some days can be a bit cool, with highs below 20 °C (68 °F), especially in the first part of the month.
For excursions in the island, you can choose May, June and September.

Back to the North Aegean Islands.