Map from Google - Greece

The climate of Greece is Mediterranean on coasts and islands, with mild and rainy winters, and hot and sunny summers. The northern cities, located in inland areas of Epirus, Macedonia and Thessaly, have a slightly continental climate, with quite cold winters, during which air masses coming from the north can sometimes bring snow and frost, and hot summers, sometimes torrid. Finally, in the northern mountainous areas, there are cold and snowy winters, and mild summers, with possible thunderstorms in the afternoon.

Athens

Winter

On the coast of central and southern Greece and on the islands, where the climate is Mediterranean, winter is mild and rainy. The rains are abundant on the coasts exposed to the west, for instance in Corfu, where even 200 millimetres (8 inches) of rain fall in December, and on the islands close to Turkey (see Rhodes).

In the islands, during winter the wind often blows; precipitation often occurs at night and in the early morning. In the islands, snowfalls are rare, and generally occur only in the northernmost ones, during the most intense cold spells (eg in the Sporades, see Skiathos, and in Euboea).

In winter, temperatures in Greece progressively decrease as you head north: the average daily temperature in January ranges from 12.5 °C (54.5 °F) in Iraklion (Crete), to 9.5 °C (49 °F) in Athens, to 5.5 °C (42 °F) in Thessaloniki. In plains and valleys of the north (see Thessaloniki), where the climate is more continental, winter is a bit cold, though precipitation is not abundant. In Thessaloniki, there is an average of one day out of three with freezing nights.
Here are the average temperatures of Thessaloniki.
Average temperatures - Thessaloniki
ThessalonikiJanFebMarAprMayJunJulAugSepOctNovDec
ThessalonikiJFMAMJJASOND
Min (°C)125812161918151173
Max (°C)91114192529323127211511
Min (°F)343641465461666459524537
Max (°F)485257667784908881705952

In Thessaloniki, rainfall, not abundant, sees a maximum in autumn and winter, and a minimum in summer, when rare thunderstorms occur. Here is the average rainfall.
Average precipitation - Thessaloniki
ThessalonikiJanFebMarAprMayJunJulAugSepOctNovDecYear
ThessalonikiJFMAMJJASONDY
Prec. (mm)453540404040201530555555470
Prec.(in)1.81.41.61.61.61.60.80.61.22.22.22.218.5
Days75666432478765

In inland and mountainous areas, winter is cold, especially in the north, on the border with Macedonia and Bulgaria: here snow and frost prevail. At the same altitude, the temperatures in the mountains of northern Greece are lower than those recorded in the Italian Apennines.
In Florina, located in the extreme north, on the border with Macedonia, at 800 metres (2,600 feet) above sea level, the average temperature in January is 1 °C (34 °F). In a normal year, there are an average of 27 days with snow. Here are the average temperatures.
Average temperatures - Florina
FlorinaJanFebMarAprMayJunJulAugSepOctNovDec
FlorinaJFMAMJJASOND
Min (°C)-3-225101315151283-1
Max (°C)571217222729292519116
Min (°F)272836415055595954463730
Max (°F)414554637281848477665243

On Mount Olympus, the highest mountain in Greece with its 2,917 metres (9,570 feet), it is possible to cross-country ski. In the Peloponnese, the large peninsula connected to the mainland in the isthmus of Corinth, the highest peak is Mount Taygetos, 2,404 meters (7,887 feet) high.

Sometimes Greece is affected by cold air masses coming from Northern Europe or Russia: in these cases, the temperature drops remarkably even in mild areas, and snow can occur in the northern plains (see Thessaloniki); even in Athens, which generally has mild winters, during these brief periods the weather becomes cold and rainy, and during the most intense cold waves, it can even snow.

Spring

The spring is pleasant and sunny in most of Greece, with less and less frequent rainy days.

Starting from the month of May, in northern areas and in the mountains ranges, thunderstorms can erupt in afternoon.
In addition, in the Aegean Sea, a wind that blows from the north or north-east, called Meltemi (or Etesian), begins to blow on sunny days in May, and continues especially during summer (see below).

The following average temperatures, relative to Methoni, a small town situated on the southwestern coast of the Peloponnese, are representative of southern islands and coasts of Greece.
Average temperatures - Methoni
MethoniJanFebMarAprMayJunJulAugSepOctNovDec
MethoniJFMAMJJASOND
Min (°C)87911151821221916129
Max (°C)151516192226282927242016
Min (°F)464548525964707266615448
Max (°F)595961667279828481756861

In the coasts and the islands, the rains have a maximum in winter and a noticeable minimum in summer, when it almost never rains. Here is the average rainfall in Methoni.
Average precipitation - Methoni
MethoniJanFebMarAprMayJunJulAugSepOctNovDecYear
MethoniJFMAMJJASONDY
Prec. (mm)110806535255233575125135695
Prec.(in)4.33.12.61.410.20.10.11.434.95.327.4
Days111085310136101269

Summer

Summer in Greece is hot and sunny. Although Greece is not protected by the Azores High, the weather is consistently sunny.

Only in the north and in mountainous areas, where the sun usually shines anyway, at times there can be showers and thunderstorms in the afternoon, especially in June (in July and August they are a bit more rare): in Thessaloniki, in June there are six days of rain on average. Of course, in mountainous areas, the temperature decreases with increasing altitude, especially at night.
While in the islands and along the coasts the heat is tempered by sea breezes, in the continent, especially in inland areas and in large cities, the heat is intense. In Thessaloniki, in July and August, the average maximum temperature is 31/32 °C (88/90 °F), but on hotter periods it can easily rise to around 35/36 °C (95/97 °F).

Athens, though it overlooks the sea, in summer is the hottest capital in Europe: the average daily temperature in July and August is 26 °C (79 °F) in the coastal area, but it reaches 28 °C (82 °F) in the central districts. Maximum temperatures of 36/38 °C (97/100 °F) are not rare. In addition, it should be remembered that Athens is a polluted city, burdened by a nasty blanket of smog.
Average temperatures - Athens
AthensJanFebMarAprMayJunJulAugSepOctNovDec
AthensJFMAMJJASOND
Min (°C)77812162023231915118
Max (°C)131416212631333329241814
Min (°F)454546546168737366595246
Max (°F)555761707988919184756457

In summer, in the Aegean Sea the Meltemi, a northerly wind typical of the warmest months (July-August), often blows, and even though the weather is nice, it can make the sea dangerous. This wind can blow for periods of two to four days, but sometimes even for weeks. The Meltemi is caused by the difference in atmospheric pressure, between the western Mediterranean, where the Azores High dwells in summer, and the Eastern Mediterranean, in which, along with the Middle East, a low-pressure area forms, due to the exceptional heating of the Iranian plateau and of the deserts of the Middle East. For this reason, these winds can not be considered as mere breezes, and can be pushy and annoying.

Autumn

Autumn in Greece comes quite late, so much so that in September, a summer-like weather is still prevailing, especially in the south and in the islands. The weather remains mild even in November (except in mountainous areas), however, starting from October, rainy periods begin to occur, and at the same time the Meltemi ceases to blow.

Santorini

The temperature of the sea in the central part of the Aegean Sea (see Naxos, Mykonos), never becomes very warm, because the prevailing wind, the Meltemi, prevents it from warming up, mixing the upper layers of its waters. Here is the sea temperature in Naxos.
Sea temperature - Naxos
NaxosJanFebMarAprMayJunJulAugSepOctNovDec
NaxosJFMAMJJASOND
Sea (°C)161515161821232323211817
Sea (°F)615959616470737373706463

In the southern part of the Aegean Sea (see Crete), and in the Ionian Sea (see Corfu), the sea temperature in the summer is a little higher, and it's warm enough for swimming from July to September. Here is the sea temperature in Heraklion (Crete).
Sea temperature - Heraklion
HeraklionJanFebMarAprMayJunJulAugSepOctNovDec
HeraklionJFMAMJJASOND
Sea (°C)161515161922242524222018
Sea (°F)615959616672757775726864

When to go

- For swimming and sunbathing, the best period is the long summer, from early June to mid-September. In this period there isn't a cloud in the sky for long periods, at times the heat is intense, but on the coast and islands it is tempered by the breeze. In the Aegean Sea, as mentioned, the Meltemi often blows.

- For visiting historic and archaeological sites and cities, the best seasons are spring and autumn. Summer is often too hot, especially in the mainland, and in July and August; however, even June and September can be hot, with average highs around 28/30 °C (82/86 °F) in the capital.
Winter is cold in the north and in mountainous regions, while it becomes windy and rainy in the south and in the islands, and therefore is not the best season to visit Greece. On the other hand, in this period, for the low amount of tourists it's possible to make excursions and visits to historic and archaeological sites, with greater tranquility and solitude.
In general, the best times are the month of May, and the period from mid-September to mid-October; the former offers longer days and the nature in bloom, while the latter offers a warmer sea, which can be attractive in the pauses between excursions.
In October, temperatures are pleasantly warm and resemble those of May, but the days are shorter, and sometimes it rains, especially in the second half of the month.
In November, the sky is often cloudy and the rains are frequent, even though it's generally not cold.
In April, temperatures are already mild, and the weather is often sunny, though some disturbances may still pass, so all in all a trip to Greece may be advisable even in this month.

What to pack

In winter: in inland and mountainous areas: warm clothes, down jacket, hat, gloves, scarf. In Athens and in central and northern coastal areas: warm clothes, sweater, jacket, raincoat or umbrella. In Crete and the southern islands: clothes for spring and autumn, sweater, jacket, raincoat or umbrella.
In summer: light clothing, sunglasses, sunscreen, a light scarf and a sweatshirt for the evening, especially in the islands of the Aegean Sea, where the Meltemi blows. For the mountains, sweater, jacket, hiking shoes, raincoat or umbrella.

See also the climate in the Greek Islands.