The long waves – also known as Rossby waves: What produces these?
A pure west to east flow, perhaps implied by the simple model of warm/low latitudes and cold/high latitudes outlined above, is difficult to sustain in the real atmosphere, with orography, and differential thermal capacities of land versus sea playing a major part in causing perturbations in the broad-scale westerly flow. On a mean (e.g. monthly) analysis, and often on individual days, long wave trough/ridge systems can be found. On average, there are between 3 and 7 such waves in each hemisphere, and considering just for the moment the Northern Hemisphere, there are two that are regarded as semi-permanent features: one roughly downstream of the North American Rockies, and another downwind of the Himalya range. The position of these features, plus their semi-permanence, suggest that the massive mountain chains involved are the primary reason for their existence. The mechanism is almost certainly a need to conserve absolute vorticity in a column of the atmosphere that is forced to climb
