An anticyclone sometimes moves south (e.g. to 45°S) and becomes a blocking high. This means that its movement is stalled, and it travels less than 20 degrees of longitude in a week. Its place at normal latitudes is taken by a subtropical low. Blocking is connected with a parting of polar and subtropical jet streams (Section 12.5; Note 12.L), which appears to be associated with either instability in the flow of upper winds or patches of unusually warm sea.

Blocking in the southern hemisphere is only half as common as in the northern, where there are larger land masses and more mountain ranges to disturb patterns of windflow. A blocking event may last a few days or a few weeks. During that time, oncoming cold fronts are deflected to the south.

Features of northern hemisphere weather, including blocking, are discussed in Notes 13.I and 13.J.

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