The science behind “troughs” and “ridges”

ST. JOSEPH, Mo. (News-Press NOW) -- As meteorologists, we sometimes use complex terminology to describe weather patterns. "Ridges" and "troughs" are among these terms, but what do they mean?

When analyzing data and model trends, meteorologists look for ridges and troughs in the upper levels of the atmosphere to help determine what weather can be expected at the surface. Simply put, on a 500-millibar (mb) map, one of the key ways of observing the upper atmosphere, when there is a ridge, it appears as a hill-like shape and indicates high pressure, which generally brings fair and calm weather. 

Conversely, a trough looks like a valley or a U-shape on these maps, signaling a low-pressure system, which brings unsettled weather at the surface. Depending on the season, this can promote thunderstorms in warmer months and snow in colder months.

Aside from influencing precipitation potential, broad upper-level features can influence surface temperatures. Upper-level ridging is generally more conducive to above-average temperatures. Similar to a pressure cooker, an upper-level ridge is essentially a large area of sinking air in the atmosphere, which compresses and warms. This could mean a fairly mild winter day or a searing heat wave in the warmer months. The same sinking motion is also what serves to suppress cloud cover and precipitation, minimizing upward motion, also known as buoyancy. 

Unlike upper-level ridges, upper-level troughs promote buoyancy by bringing colder air aloft in the atmosphere, which exists at a lower pressure. As warmer air at the surface rises to fill the void, it cools and condenses, promoting more cloud cover and, in some cases, precipitation. 

While broad-scale upper-level patterns can help predict weather at the surface, there are also regions of high and low pressure lower in the atmosphere that develop at varying scales. For this reason, the all-important 500-millibar maps are only a starting point to the complex forecasting puzzle.