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Why does the weather sometimes get 'stuck in a rut'?

At mid to high latitudes in the upper part of the troposphere (above roughly 5 km ), the mean wind flow exhibits a broadly west-to-east motion - this applies in both hemispheres. On many occasions, particularly in mid-latitude/temperate zone regions, the flow is directed more or less directly from west to east, crossing few latitude zones within the same longitude range: this is a 'highly zonal' type - any short-wave disturbances embedded in the flow will be carried quickly along and the weather is ever-changing as a succession of frontal systems, interspersed with transient ridge conditions cross any one point.

However, on both average (e.g. monthly) pressure maps and on individual days, long-wave trough/ridge patterns can be found - some having large amplitude, i.e. the airflow meanders a long way north and south around the loops of the pattern, crossing many parallels of latitude in a relatively limited longitudinal range: a 'meridional' type; Usually, some west-to-east progression of the looped pattern can be seen over a 24 hr period, and the associated surface weather type changes, albeit more slowly than the zonal type described earlier.

However, if the 'loops' in the pattern become locked in one geographical area, then depending where you are in relation to the upper flow, the associated surface patterns are often little changed from one day to another, and in extreme cases, from one week to another - the pattern is said to be 'blocked'.

In, and just to the east of a slow-moving trough in the upper flow, the surface weather will tend to be of a low pressure/convective/showery type, and perhaps cool for the time of year (but not necessarily).

In, and just to the east of a static ridge in the flow, the surface pressure will tend to be high, with settled conditions lasting until the block is destroyed. This latter case is responsible for prolonged dry/hot weather in summer, but cold/sometimes grey conditions in winter, and considerable pollution build-up can occur at all seasons due to the stagnation of the lower level air and high air-mass stability encountered.

For a personal view of some aspects of upper air meteorology, and some further explanation of the terminology used, see "Upper air meteorology".