You’ll hear it fairly often in forecasts, any may have heard it right before it rained: an “upper-level trough” is on the way. The news may be accompanied by colorful charts. It may not be apparent what a trough actually is, or what it means, aside from some weather being on the way.
We’re here to help translate, along with the good folks the National Weather Service, those running the TravisCountySevereWx Twitter feed, and Troy Kimmel, a forecaster who teaches meteorology at the University of Texas.
The short, short version is that a trough means colder and sometimes wetter weather — and tends to be associated with storms.
First, let’s learn how atmospheric pressure works
The longer version starts with a brief explanation of atmospheric pressure (bear with us):
In low pressure, air way up in the sky is colder than it usually is at that height. Being unusually cold makes it unstable, which then makes air in that part of the atmosphere want to rise. These low-pressure situations are called troughs. The word trough — as in, horse trough — is just a metaphor used to describe a line of low pressure stretching from one place to another. This phenomenon is important partly because sometimes a trough passes over a low-hanging front (the front end of a mass of air moving into an area).
A front pushes air upward, and if there is a trough overhead, the air goes up into the trough. If that air in front of that trough has significant amounts of moisture, that moisture collects as clouds, with those clouds sometimes dropping rain as they pass.
Here is a visual explanation, via the weather service:
A trough usually shows up on weather maps as dashed red or brown lines, Kimmel said.
The counterpart to a trough is a ridge. They tend to be less exciting. A ridge is a line of high pressure, where the air way up in the sky is heavier than usual. It pushes down on the air underneath it. The weight makes the air below it stable. That produces the less-exciting kind of weather. A ridge is usually depicted with a zigging line, Kimmel said.
Think of them as water ripples
Paul Yura, the second-in-command of the National Weather Service’s New Braunfels office, compared troughs and ridges to the ripples in water. The atmosphere, like water, has ripples that roll through it. The cold ripples are the troughs. The warm ones, the ridges.
Things get a little more complicated when talking about “lower level” versus “upper level,” which refers to how high in the sky the trough or ridge is happening. For our purposes, an upper-level trough is two to five miles above the Earth’s surface.
Got it? Let’s now use a real-world example
The nice, clear, stable weather we had been enjoying recently here in Central Texas — suck it, Buffalo! — had been coming from a ridge hanging overhead, Kimmel said. We’d had a few cold fronts come through in recent weeks, but they didn’t bring rain because there hasn’t been much moisture in the air. The majority of the moisture around here generally comes from the Gulf of Mexico, but the gulf had not sent moisture this way in a while.
“When (troughs have) gone through, there’s been nothing to lift,” Kimmel said.
Early Tuesday morning, there was moisture in the air. That moisture arrived, from the gulf, just ahead of the trough and the front. The trough therefore had something to lift. Voila: thunderstorms, and more than an inch-and-a-half of rain.