At mid-latitudes (that is, roughly between 30° and 60° latitude), air in the upper troposphere tends to move in relatively narrow, fast-moving "streams" that wobble back and forth as they flow generally eastward. Often, two such jet streams might be visible on these maps: (1) a polar jet stream (generally within the mid-latitudes); and (2) a subtropical jet stream (generally within the subtropics, between roughly 23.5° and around 33° latitude). However, the atmosphere is a complicated place that often defies our attempts to make simple generalizations about it, and sometimes the polar jet stream splits into two branches or one or both jet streams fail to be continuous or well-defined.
The jet streams powerfully influence synoptic-scale weather patterns in the mid-latitudes. ("Synoptic-scale" patterns are typically around 1-2 thousand kilometers across and typically last from around a day up to a week; midlatitude cyclones are synoptic-scale patterns, for example.) As air races through the sinuous jet-stream pattern, there are some parts of the pattern where air tends to converge, thereby increasing the total weight of air above the earth's surface in those areas, which implies higher pressure at the earth's surface below. Moreover, convergence of air within the jet stream tends to force air beneath the jet stream downward. As air descends in the atmosphere, the pressure on it increases and compresses it, thereby warming the sinking air, which evaporates any existing clouds or prevents clouds from forming in the first place. Such areas generally experience clear weather.
However, air in other parts of the jet stream pattern tends to diverge, which reduces the total weight of air above the surface which implies lower surface pressure beneath those parts of the jet stream. Moreover, divergence of air within the jet stream tends to induce air below to ascend to replace the diverging air. Rising air encounters lower atmospheric pressure, so the rising air expands and cools by virtue of the expansion. If the rising air cools enough, clouds form in it and may eventually produce precipitation. Regions of the atmosphere where air within the jet stream aloft diverges, causing surface pressure to fall and forcing air within the troposphere below to rise and cool, leading to large-scale cloud formation and possibly precipitation, are associated with mid-latitude cyclones and their accompanying warm and cold fronts. These storms are often easy to spot on satellite images by their characteristic "comma"-shaped cloud pattern, often extending thousands of miles along the front(s).
As air races eastward through the jet stream at speeds often exceeding 150 m.p.h. (about 130 knots, or 65 meters/sec), the sinuous wobbles of the jet stream pattern tend to shift slowly eastward. The smaller of these wobbles generally propagate faster then the larger ones, which can sometimes stall completely. The regions of convergence and divergence within the jet stream pattern--and the patterns of clear and stormy weather associated with the regions of convergence and divergence--are strongly correlated with certain parts of the wobbles. Hence, since the wobbles tend to propagate eastward, so do both the mid-latitude cyclones and the regions of generally clear weather between the storms. The speed of these storms is typically around 20-30 m.p.h. (10-15 meters/second).
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