Climate in Jordan is conditioned partly by altitude, with the lowest areas such as the Jordan Valley and Gulf of Aqaba suffering from the worst summer heat and humidity. The higher central and northern areas, in contrast, can be quite cold in winter (November to February).
When to go
For a small country, Jordan has an extraordinary range of climates. The best time to visit climate-wise is in spring (March to May) and autumn (September to November), when the daytime temperatures aren't too extreme. April is probably the best month, when temperatures are warm and wildflowers are in bloom. March can be cold and rainy in the north but is balmy by the Gulf of Aqaba and the Dead Sea.
Average daytime maximum temperatures in Amman range from 12.6°C in January to 32.5°C in August.
Winter can be surprisingly cold. Snow in Amman is not unheard of (even Petra gets the occasional fall) and the deserts can be freezing, especially at night. Make sure you have plenty of warm clothes and a windproof and waterproof jacket. Aqaba is the one exception, with average daytime maximum temperatures of around 20°C in January, and is quite a hit with deep-frozen northern Europeans during winter.
In high summer (July and August) the weather in the humid Jordan Valley is extremely oppressive - it feels like you're trapped in an airless oven - with suffocating daytime highs well in excess of 36°C. It's also fiercely hot in the desert (including Wadi Rum), though this is a dry heat and thus easier to deal with. The tourist authorities usually plan festivals (such as the Jerash Festival) for the summer period. If you do visit in summer, come well prepared with a hat, sunscreen and protective clothing.
The month of Ramadan is a time when visitors should not eat, drink or smoke in public during the day so it's a tricky time to visit. Eid al-Fitr, the great celebration at the end of Ramadan, is a fun time to visit but it's best to bunker down for a few days because public transport is heavily booked and hotel rooms are sometimes hard to find, especially in Aqaba.
Note also that most of the excellent ecotourism projects operated in Jordan's Dana, Wadi Mujib and Ajlun nature reserves only operate between April and October.