Well Developed High Pressure Ridge and Low Pressure Trough

This view shows a very well developed high pressure ridge (isobars are shown by white lines) over the western United States, and a well developed low pressure trough over the eastern United States. Although closed contours, such as the one over the southwestern states, are relatively rare in upper level charts, they do occur. Notice how close together the isobars are at the apex of the ridge and at the vertex of the trough; more closely spaced isobars indicate higher wind velocity. It is very important to observe that the wind arrows are parallel to the isobars at the 500 Mb level, that is, the flow is geostrophic. Notice that the isobar spacing is beginning to narrow in the northwestern United States and southwestern Canada; this is the location to look at for the position of the surface high pressure. Notice also that the isobar spacing increases in the area just above the Great Lakes and again over the southeastern United States. These areas are the locations of the surface low pressure cells. The position of the Sub-polar Jet Stream is marked by the pathway highest velocity high level winds: over the top of the high pressure ridge in southwestern Canada, southwest where it splits into two branches, one north of the Great Lakes, and one south over northern Florida. Convergence aloft (southwestern Canada) creates a surface high pressure, divergence aloft (north of the Great Lakes and again over the southeastern United States) creates surface low pressure.