It's hard to summarize the climate in a land with 2000 islands, high mountain peaks, and exposure to air masses that range from sub-tropical to polar. Nearly all of Greece is ruled by a Mediterranean regime, which means that precipitation is focused in the cool season. Summers are virtually rain-free: the best chance for a shower is toward the north, but even Thessaloniki only gets 1 or 2 a month at best. Since temperatures heat up sharply at lower elevations, the relative humidity isn't the best guide to how sultry the air can feel. A typical Athens night in July and August stays above 22°C/72°F. Greece's hottest pockets include the interior lowlands of Peloponnesia and Thessalia. Along the coast, the mid-day heat is often broken by a light sea breeze, and many spots get the persistent northerly wind in summer known as the etesian. Elevation is another cure for the swelter. The average temperature drop with height is sharper in Greece than in many other locations. On a hot afternoon, you can expect it to be as much as 8°C/14°F cooler for every 1000m/3300ft you climb.
Occasional rains start making their presence felt in October, and late autumn can be a thundery time. Even in mid-winter, however, substantial rains are usually punctuated by sunny spells known as halcyon days. The heaviest amounts fall on the western slopes of the Pindus and Peloponnes mountains, often in the form of snow. Cold air has trouble making it through the maze of Greece's mountains and islands. This means that coastal cities in the north are substantially cooler on average than Athens and other southern destinations. Hard freezes are rare even on the northern coast, but a round or two of light snow each year may sweep as far south as the shores of Peloponnesia, and frost is frequent in some valleys. The winter rains slacken gradually through spring; it takes until June for Thessaloniki to enter its dry summer regime. Spring is an ideal time to catch the Grecian sun and enjoy the still-green countryside before the temperatures hit their summer peaks.