Depending on the altitude you're at the climate in Jordan will change greatly. Lower areas such as the Jordan Valley and the surrounding Aqaba area are warm during winter (January-February) with colder evenings, and extremely hot in summer (June-August). The higher areas in central and northern Jordan such as the Eastern Desert, can get really cold and dry in the winter and intensely hot in the summer. In Jordan rain usually falls between November and March.
The best time to visit Jordan to enjoy the climate is in spring (March to May) and autumn (September to November), when the daytime temperatures aren't excessively high.
At the peak of summer (July and August) the weather in the Jordan Valley is extremely high with average temperatures in excess of 36°C. It's also extremely hot in the desert (including Wadi Rum).