April & May
The island has a short spring when temperatures range from 15 - 25 degrees Celsius. The weather is changeable still with the risk of sudden showers. Sea temperatures start to rise and many people, especially those from colder climates, enjoy a swim but are regarded as a bit soft in the head by the locals! Spring is a particularly pretty time for the island with lots of flowers and greenery.
June - August
The weather on Hydra during high season is predictably hot, ranging from 25 - 38 degrees Celsius during the day. Humidity is low and rain fall during high season is rare but not unheard of. Sea temperatures increase to approximately 26 degrees by the end of the summer. August often brings windy weather that blows up from northern Africa so it's a warm/hot wind. Temperatures have been known to rise to 44 degrees during July and August.
September - October
One of the most popular seasons on Hydra. The weather is still warm and while it's a little changeable, is still dependable enough to eat outside in the evenings. Temperatures start to drop but the sea temperatures are high enough still to make bathing less bracing than it is in the spring.
November - March
The winter months are generally cold and damp with lots of rain. Most of the island is closed during these months. Temperatures rarely drop below freezing. January - March are often punctuated with really sunny, warm days. These days though, aren't to be relied upon but viewed as a bonus. Very rarely there will be a sprinkling of snow on the very tops of the mountains but it doesn't settle.