One of a forecaster's first thoughts when confronted with the 300 / 200 mb chart is the jet stream. The jet stream is a high velocity river of air that flows completely around the Earth at the mid-latitudes. During winter, the jet core is located generally closer to 300 millibars since the air is more cold and dense in the vicinity of the jet stream during the cool season. The 200 millibar chart is used for the jet stream in the warm season but either chart in most instances will suffice.
Many hot air balloonists have tried to ride this river of air around the world with not much success for most. The river of air is not continuous. Embedded within the jet stream are higher velocity jet streaks. Jet streaks are segments of faster wind speed within the jet stream. At 300 mb, the air density is much smaller than near the surface. A 100-knot wind at the 200 / 300 millibar level does not feel as strong as a 100 knot wind at the surface. Even though the density is smaller, these air currents have the power to drive the movement of storm systems and build troughs and ridges.
The jet stream is useful for the prediction of temperature. The jet stream divides colder air to the north from warmer air to the south. The transition between temperatures on each side of the jet is very abrupt. Heights are higher to the south of the jet and lower to the north. In the upper levels, this creates relatively high heights to the south of the jet and relatively low heights to the north. The Pressure Gradient Force flows from a southerly to northerly direction. However, the Coriolis force shifts the wind flow to the right of the path of motion. Therefore, the jet stream flows from the west to east. When a trough builds over a region it often indicates cooler temperatures due to CAA which is sometimes combined with cloudier weather (especially on the right side of the trough). A ridge builds by low level (between the surface and 700-mb) warm air advection and upper level forcing (negative vorticity). Air in a ridge is sinking and is thus expanding and creating higher heights. Therefore, temperatures are warmer than normal in a ridge due to warmer temperatures and sunnier weather. This is especially true when a ridge occurs in high latitudes.