A ridge is an elongated area of high atmospheric pressure. Ridges can occur both at the surface of the earth and at higher altitudes. Upper level ridges can have a major impact on the weather at the surface.
Air tends to sink to the east of the ridge axis, which inhibits clouds and precipitation.
On the other hand, air tends to rise to the west of the ridge axis, which can lead to the formation of clouds and precipitation. Extremely hot weather during the summer and unusually mild weather during the winter are often associated with a strong, slow moving, upper-level ridge. On Northern Hemisphere weather maps, upper-air ridges are shown by the path of upper altitude winds, or the jet stream, turning to flow northward and then back to the south.