People don’t move to Beijing for the weather, and most locals consider it something to be tolerated and survived rather than enjoyed. You’ll need clothing for every imaginable weather condition, and even some that may be unimaginable to newcomers from Europe or the United States, such as the sandstorms.

Beijing’s climate is influenced by the Mongolian plateau to the northwest and the East Asian monsoon from the east. Sitting at 39° 54′ 50 N, Beijing has a similar latitude to Philadelphia, Madrid and Ankara. It has distinct seasons characterized by extreme conditions that are some of the harshest in China. Summers, influenced by the monsoon, are long, hot and humid, with an average temperature in July of 30oC (87oF). High temperatures accompanied by heavy humidity frequently top 40oC (104oF) and occasionally reach 50oC (122oF) in August. Rain is most likely in the summer and can cause flooding, although this is normally temporary. More than half of the city’s annual 700 mm of rain falls in July and August.

Winters are long, cold and dry, with cold fronts from Siberia down through the Mongolian plain bringing bitterly cold winds and crisp, clear skies. Temperatures in January average -10oC (14oF). Due to its proximity to the Gobi desert, Beijing is prone to sandstorms that dump tons of dust onto Beijing from March to May. These are as uncomfortable as they sound, smothering the city in an asphyxiating yellow fog that can linger for days. Unfortunately, they’re increasing in frequency and ferocity because of soil erosion in the surrounding mountains caused by deforestation, farming and overgrazing. Recently, the aptly named Beijing Weather Modification Office has been inducing rain by seeding clouds with silver iodide-laden rockets in an effort to quell the dust. Apart from these storms, spring is quite pleasant. The best weather is in September and October, which is referred to as the golden season. During these precious weeks, locals often take holiday to enjoy the sights and attend festivals.

Source: http://expatessentials.net/beijing-preparations-climate/