Uzbekistan has an extreme continental climate. It is generally warmer in the south and colder in the north.
The average temperatures in December are -8°C (18°F) in the north and 0°C (32 °F) in the south. However, extreme fluctuations are mesured as low as -35°C (-31°F).
During the summer temperatures can reach 45°C (113°F) and above. Humidity is low.
The Spring (April to June) and the Fall (September through October) are in general the most pleasant times to travel. In the Fall is a harvest time, and the markets are full of fresh fruit.
If you're interested in trekking, then summer (July and August) is the best time, because summers are almost dry.