There are no moderating marine air influences in Siberia, which makes it, along with Central Asia, much drier than areas closer to Europe. Because land masses heat much more quickly than water, Siberia is dominated in winter by a high air pressure system and in summer by a low air pressure system. The summer low is caused by the rapid heating of the ground and the rising of air into the upper atmosphere. The winter high is created by the cooling of the ground, and the subsequent descending of air from the upper atmosphere.
High pressure systems are always caused by air that is descending from the upper atmo- sphere to the ground. Usually this air is very dry, which is why high pressure systems are associated with clear skies. Conversely, low pressure occurs when air near the ground rises up into the upper atmosphere. If a large source of water is available, this will bring moisture into the upper atmosphere, causing clouds to form, which can then bring precipitation (rain or snow). Another feature of low air pressure and high air pressure is that their winds move in opposite directions. The low pressure winds circulate around the center of low pressure in a counterclockwise direction. High pressure winds circulate in a clockwise direction.
Because the Siberian High in winter and the Siberian Low in summer are so intense, they affect the direction of wind patterns for the entire continent of Asia. In the summer the Siberian Low circulates in a counterclockwise direction pushing air off the Asian landmass, across the Indian Ocean toward India and Southeast Asia, across the South China Sea, and up toward Japan.
In the winter months, the Siberian high circulates in a clockwise direction, pushing extremely cold air from Siberia down across northern China and into Japan, and circulating winds from the South China Sea across Southeast Asia and across the Indian Ocean toward India and Southwest Asia. This change in wind direction is known as a monsoon wind pattern: with a summer monsoon coming from one direction, and a winter monsoon coming from an opposite direction. Because the summer monsoon is so pronounced when it arrives in India and Southeast Asia, most people refer to the summer monsoon simply as the monsoon.