Weather prediction relies heavily on the interpretation of complex atmospheric data, which meteorologists translate into visual tools. These graphical representations allow forecasters to communicate expected atmospheric patterns and their movements over time. Understanding the symbols and features on these maps is necessary for interpreting a forecast beyond a simple temperature reading.
Defining the Prognostic Chart
A prognostic chart, often shortened to a “prog chart,” is a forecast map that displays the expected state of the atmosphere at a specific time in the future. Unlike a surface analysis chart, which shows current, observed conditions, the prog chart is a product of numerical weather prediction models. These models use current observations and physical laws to simulate atmospheric changes, providing a visual prediction of where weather features will be located. Forecasters use these charts to predict the movement and development of pressure systems, wind patterns, and precipitation up to several days in advance.
What Exactly Is a Trough?
A trough is a meteorological term for an elongated region of relatively low atmospheric pressure. It is similar to a low-pressure system but does not have the complete, closed circular isobars that define a full low. A trough is a V-shaped or U-shaped valley of lower pressure extending outward from a larger low-pressure center. This pressure configuration is associated with convergence, where air flows inward toward the line of the trough. This convergence forces the air to rise. The upward motion of air cools it, triggering cloud formation and atmospheric instability.
Reading the Trough on a Chart
On a prognostic chart, a trough is typically represented by a line that distinguishes it from other features like fronts. While the exact symbol can vary by country or region, it is often shown as a dashed line or a bold solid line, sometimes colored blue or black. This visual representation is distinct from a weather front, which uses solid lines adorned with triangles or semicircles to denote the boundary between different air masses. The trough line often runs perpendicular to the isobars, which are the lines connecting points of equal pressure. These isobars will show a noticeable kink or V-shape as they cross the trough line.
Trough Impact on Weather
The upward air motion associated with a trough has significant consequences for the weather. As the air rises and cools, the moisture within it condenses, leading to increased cloud cover and precipitation. This lifting mechanism commonly results in unsettled conditions such as scattered showers or thunderstorms along and ahead of the trough axis. The passage of a trough also brings a noticeable shift in wind direction. For example, in the Northern Hemisphere, winds ahead of the trough may be from the southwest, but as the trough line moves past, the wind will typically shift to a more northwesterly direction. This wind shift signifies a change in the air flow pattern, which often ushers in cooler air behind the feature.