Sometimes, upper air patterns change only slowly – whats going on?
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, and is a consequence of the global-scale heating differential between the cold/polar regions and the warm/tropical latitudes, coupled to the deflection of air on a spinning sphere: the earth. 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. diagram of a zonal jet arrangement 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,
