Weatherz School: Surface fronts
When discussing the forecast, we show maps that track approaching weather system and their fronts. Let’s dive into what these symbols represent.
Fronts are lines on weather maps that represent the boundaries between air masses. Triangles or half-moons point in the direction of motion.
Fronts extend from the center of low-pressure areas, never from the center of high-pressure.
They are detected by wind shifts, temperature differences, and pressure differences.
A cold front is a blue line with triangles. This shows the leading edge of a cold air mass displacing a warmer air mass.
Warm fronts are red lines with half-moons. These how the leading edge of a warm air mass displacing a cooler air mass. Ahead of a warm front is where we often talk about overrunning.
This is where advancing warm air hits cold air already in place. The warm air is forced upwards and overruns the denser cold air. The warm air cools as it rises causing moisture in the air to condense into clouds and precipitation. Eventually, the warm air cools down enough to where clouds and precipitation can’t form. This weather pattern typically brings steady, light rain, thick low clouds, and fog or mist. In the winter, overrunning can cause freezing rain, sleet, and snow.
Warm fronts and cold fronts are the two most common fronts, but there are others. A stationary front is an alternating red and blue line. This indicates a front that stops moving. There’s still a boundary between two air masses, but the incoming air mass isn’t advancing across the region for the time being.
An occluded front is a purple line with alternating triangles and half-moons. This means that the cold front has “caught up” to the warm front and is forcing the warm air mass upwards. Occluded fronts mean the weather system is no longer intensifying.
A trough is a dashed line. A trough isn’t a front because it isn’t a boundary between air masses. It’s an elongated area of low pressure. Shows changes in conditions higher up in the atmosphere, so precipitation can still occur near them.