WeatherWeather Blog


WEATHER TERMS EXPLAINED: Fronts, what are they and what does each one mean?

The history and use of the "front"
Posted at 12:04 PM, Jun 21, 2024

If you have ever watched (or read, or looked at) a weather forecast, or taken a middle school Earth Science class, you have been introduced to the concept of a front. Those blue spiky lines racing across the screen, or the red humped line slowly lifting upward. We see them almost every day, so much so that they have become an afterthought. So, let's stop and talk about fronts. Who thought of them? Why are they named "fronts"? What does each one mean?

To begin, let's talk about some history. Fronts, like much of modern weather forecasting, is a 20th-century concept. We can thank the contributions of one man for this, Norweigan scientist Vilhelm Bjerknes (1862-1951). Originally from Norway, he was living in Germany when World War I broke out in 1914, and by 1917 he had to return to his home. During the war, the transmission of weather information ceased, lest the enemy get ahold of them. As Norway was a neutral nation during World War I, it was cut off from all weather information. So, Bjerknes decided to create his weather information system. In the process, he and his students discovered a revolutionary concept that came to be called the "Norwegian Cyclone Model", but we know them today as "high and low-pressure systems".

Vilhelm Bjerknes.jpg
Often called the father of modern meteorology, Wilhelm Bjerknes and his Bergen school of talented meteorologists revolutionized how we understand the weather today. Among those discoveries was the concept of the front.

During his new theories, Bjerknes noticed that connected to these low-pressure systems are boundaries that separate colder and warmer air masses. He compared the clashing air masses with the battle lines of the war, which were known as fronts (think the Western front etc.). Thus, the term for the boundaries separating cold and warm air became known as "fronts".

Low Pressure.PNG
One of the contributions of Bjerknes to meteorology is our modern conception of how low-pressure systems form and operate. A wedge of warm air to its southeast provides fuel for a low pressure to strengthen. The borders of this warm air wedge is the cold front and warm front.

Okay, this is not This Week in Weather History (yet...), history over, so let's talk about what they mean.

COLD FRONT - One of the most common fronts, the cold front simply separates warmer air ahead from colder air behind it. A cold front almost always extends south and west of the area of low pressure. The front is denoted as blue on the map with triangles (known as pips). A rule of thumb for fronts is the pips are always pointing in the direction that the front is moving toward. Cold fronts are often known for bringing in colder weather but are also major triggers for thunderstorm development.

Cold Front.PNG
An example of a cold front

WARM FRONT - The other of the most common fronts, the warm front separates colder air ahead from warmer air behind it. A warm front almost always extends east from the area of low pressure. The warm front is blue on a weather map with semicircles as pips. Like the cold front, the semi-circles always point in the direction the front is moving toward. Before a warm frontal passage, it is typically cool and rainy, then the front passes and it gets warm and sunny. While often not a focus for thunderstorms, warm fronts could add enhanced rotation along them, making them greater tornado threats if a storm goes up on them.

Warm Front.PNG
An example of a warm front

STATIONARY FRONT - In the summer, cold fronts can often lose steam as they become parallel to the jet stream or move far enough away from the low pressure. As they do so, they stall out and do not move for several days. This is when we draw the stationary front on the map. A stationary front is denoted by a combination of the blue spiky cold front and the red semi-circled warm front. The blue spikes will always point toward the warmer air, while the semicircles will point toward the cooler air. Stationary fronts can often bring rounds of heavy rain along them, which increases the risk of flash flooding.

Stationary Front.PNG
An example of a stationary front

OCCLUDED FRONT - Rarely on the map you might see a purple front, this is known as an occluded front. Cold air moves faster than warmer air. As the cold front swings around the area of low pressure, there will be a time when the cold front catches up with the warm front. Since warm air is less dense, the cold front forces the warm air to rise, known as occlusion. This is what the occluded front denotes, the cold air catching up with the warm air. Occlusions are a sign that the area of low pressure is decaying as it is cut off from the supply of warm air.

Occluded Front.PNG
An example of an occluded front.

DRY LINE - If you live in the plains, chances are you have heard of the dry line. A dry line is not a "front" per se, but it is an important boundary to weather in the Great Plains. Unlike fronts, which separate air masses of different temperatures, the dry line separates air masses of different moisture content. Ahead of the dry line lays moisture-laden air from the Gulf of Mexico, while behind it is desert air from the southwest US. This boundary often sets up in the Great Plains and wobbles through the warm season. Dry lines are important focal mechanisms for supercell thunderstorms and tornadoes if other ingredients are present. On weather maps, dry lines are yellow with semi-circles that point toward the moist airmass.

Those are the main examples of fronts. Hopefully, when you see a weather forecast, you have a new appreciation for fronts and what they are.