WX MODEL 18: The Ridge


A ridge is the opposite of a trough. While a trough or shortwave tends to bring in cooler and cloudier weather with an increased precipitation threat especially near the lifting mechanisms, a ridge brings in warmer weather and often a reduced precipitation threat, especially when produced by sinking air. A mechanism that can produce ridging aloft is a warmer air mass replacing a colder air mass. Warm air is less dense than cold air and thus warm air takes up a larger volume. Taking up more space means the pressure surfaces will have to expand upwards. Another process of ridging, that is more pronounced and that generally covers a larger area, is produced from a warm core high (sub-tropical high) and typically results in dry weather with temperatures that are above normal. These ridges tend to have a long wavelength and can influence a forecast region for several days. They are produced from air sinking throughout the troposphere. Sinking air warms adiabatically and this increases the volume of air and thus the pressure surfaces are relatively high. The sinking air also decreases the relative humidity of the air which can lead to reduced cloud cover.

The image below shows ridging occurring over the west and central United States. A warmer air mass and sinking air are helping to contribute to the ridging. Typically ridging aloft is a sign of warmer weather, and also less clouds and reduced precipitation chances when sinking air is involved with the ridge formation.