How are the upper features related to surface weather patterns?
Broadly speaking, msl low pressure is found in and forward of a marked upper trough, and msl high pressure is found in and forward of a marked upper ridge. These troughs and ridges need not be large amplitude affairs that sweep over hundreds of kilometres of the globe: ~~ A minor RIDGE in the upper flow over a synoptically ‘small’ area such as southern Britain, or northwest Germany, can act to dampen any tendency to convection: the ridge will be an area of descent, which will produce subsidence warming/drying of the air, and act against vigorous shower activity. ~~ A minor TROUGH, (or short-wave length trough, ‘shortwave’ for short!) perhaps only just detectable on a chart, can be the cause of much heartbreak for forecasters! The trough may be a slight hesitation in the upper flow, but combined with adequate moisture, and perhaps enhanced by land heating, can lead to notably more intensive shower activity, and operational forecasters are always on the look-out for such irritations at s
