The weather in Sichuan varies greatly from place to place. It is a big province. But the province is divided into three major geographical regions. There is a region of high mountains in the southwest, a high plateau in the northwest, and a lowland region in the eastern part of the province. The whole province is impacted by the Southeast Pacific Ocean Monsoon and the Southwest Indian Ocean Monsoon. Here is a description of the weather in the three regions.
The eastern basin area is warmer than the higher elevations, but often cloudy or foggy. To give you an example of how cloudy the basin area is, Chengdu lies in the basin area and people compare Chengdu's weather to that of London and say that it is even cloudier. This area has a subtropical monsoon climate. It is fairly warm in the winter, dry in the spring, hot in the summer, and rainy in the autumn. The average temperature in January is 5-8 degrees Celsius (41-46 degree Fahrenheit). The average temperature in July is 25-29 degrees Celsius (77-84 degrees Fahrenheit). There are 250 to 300 cloudy and rainy days a year in the plain.
The average temperature in Chengdu in January is about 6 degrees Celsius, and in July it is about 25 degrees Celsius. In Chengdu, the rainy season extends from May through September, and the dry season extends from October through April. You can get information about Chengdu's weather including daily forecasts here.
The high plateau in the northwest is typical of high plateaus: there is a long winter, a cold summer, but lots of sunshine. The temperature drops during the night. In contrast to the basin, parts of the plateau may bask in 2,500 hours of sunshine a year. Gangzi Township is nicknamed "the small sunshine city".
The climate in the high mountain region in the southwest of course depends on the altitude. The climate is alpine and even arctic in the highest peaks. The Huanglong Scenic and Historic Interest Area has a glacier. The valley of the Jinshajiang River has a tropical climate. In the valleys, the dry season and the rainy season are obvious. May to September is the wet season, and October to April is the dry season.