Most weather apps and weather forecasts give viewers, listeners, and users a brief rundown of the weather in their area. These forecasts may talk of cold and warm fronts, precipitation, and storms, but very few mention the term ‘occluded front’.
The weather conditions we experience are often associated with a specific type of front. These fronts can either be stationary, cold, warm, or occluded.
But what exactly is an occluded front, how is it formed, and what conditions does it bring with it? Keep reading to find out more.
What Is An Occluded Front?
An occluded front develops when a cold front catches up with, and eventually overtakes, a warm front. The end result is the formation of an occluded front, which happens through a process called cyclogenesis.
You’ll usually see an occluded front marked on a weather map as a line that has both semi-circles and triangles.
The word occluded means ‘hidden’. This is because when the cold front catches up, and eventually overtakes the warm front, the warm air in the atmosphere is lifted to the surface, and becomes hidden.
Because of the way occluded fronts occur, they often have characteristics of both warm and cold fronts.
Knowing the difference between warm and cold fronts can help you understand the nature of an occluded front. So, let’s take a closer look at weather fronts and what they mean.
What Is A Warm Front?
If a warm front is present in your weather forecast, this means that the warm air in the atmosphere is taking precedence and rising over the cold air.
This happens because the warm air is often lighter than the cold air, so it replaces the cold air at the surface.
Warm fronts tend to move from the southwest to the northeast, and the air behind them is often more moist and warm than the air in front. If a warm front passes you, you’ll feel the atmosphere become noticeably more humid than before.
Just because this front is ‘warm,’ it doesn’t mean its effects are all good. Unfortunately, warm fronts usually bring stormy weather with them.
This is because the warm air mass at the surface moves above the cool air mass, which makes storms and clouds.
These fronts also move far slower than cold fronts, and on a weather map, they’ll be marked out by a solid line with red semicircles that point towards the colder air in the atmosphere and point in the direction of the fronts movement.
What Is A Cold Front?
A cold front is usually symbolized on a weather map as a line with triangles that often represent icicles. While warm fronts are labeled in red, cold fronts are often labeled in blue.
If you have a cold front in your area, the cold air in the atmosphere is moving and pushing underneath the warmer air.
This happens because the cold air is usually denser and much heavier than warm air, meaning the cold air replaces the warm air at the surface.
Cold fronts tend to move from northeast to southeast, and the air behind them is much colder and drier than the air in front. If a cold front passes through your region, temperatures can plummet by more than 15 degrees within an hour.
The temperature changes on each side of a cold front are often drastic. For example, some temperatures to the east of the front could be 55 degrees, while those just a short distance behind can go down 38 degrees (also see ‘What Are Climate Feedback Loops? Examples Of Positives And Negatives‘).
Aside from plummeting temperatures, cold fronts will also often bring a band of precipitation that follows the edge of the cold front.
Although this band of precipitation is narrow, it’s often intense, which can cause weather such as snow squalls, severe thunderstorms, hailstorms, and even tornadoes (also see ‘Where Do Blizzards Occur?‘).
How Do Occluded Fronts Form?
The occluded front creation process usually occurs during the formation of a cyclonic system or cyclogenesis.
Cyclogenesis is a term that describes the creation and intensification of an extratropical cyclone, usually around a low-pressure system. As you can tell from the name, this event tends to follow the formation of a cyclone.
Before we talk more about the formation of occluded fronts, it’s important to know that there’s more than one type.
There are usually two types of occluded fronts, and these are:
- A Cold Occluded Front A cold occluded front will develop when the air behind the front is cooler and the air in front is warmer.
- A Warm Occluded Front A warm occluded front occurs in opposite conditions when the air behind is warmer and the air in front is cooler.
To make this process easier to describe, we’ll discuss how occluded fronts form in the Northern Hemisphere, often around a low-pressure system, when a cold weather front catches up with and overtakes a warm front.
As we know, occluded fronts form when the cyclonic formation system is taking place. When circulation around the center of the low-pressure system occurs, the cold weather front will be pushed to move faster than the warm front, which is leading.
When this counterclockwise movement happens, the cold front will at some point start to catch up with and overtake the warm front. When this happens, an occluded front will form at the exact point where both the cold and warm fronts intersect.
At this formation point, the cold air will move directly underneath the warm air, which sits behind the warm front, and catch up with the cold air moving ahead of the warmer system.
The result? A mass of cold and dry air trails in the wake of the occluded front.
What Weather Does An Occluded Front Cause?
As we mentioned earlier, most occluded fronts will have characteristics of both cold and warm fronts. However, not all weather fronts and conditions will manifest in the same way, which can leave these fronts open to variations.
However, specific weather patterns will often be characterized by certain weather systems (also see ‘What Causes Weather To Change And How Do Forecasters Predict Weather Patterns?‘). This is also the same with occluded fronts.
We can understand the weather conditions and patterns associated with occluded fronts by examining the same example we discussed previously.
When the cold weather front moves in front of the warm front, it will push underneath the warm front and lift the warm air up behind it.
When the cold front does it, it can cause significant precipitation to occur (also see ‘What Is Precipitation: All You Need To Know‘). This is because when the warm air is forced upwards, the moisture inside is allowed to cool and later condensates into water droplets.
Mixing air between the cold and warm front will often cause temperatures to drop significantly, too. You may also see strong winds and, in some cases, severe storms.
Are Occluded Fronts Dangerous?
When occluded fronts pass over, they have the potential to be dangerous. But, as we know, there are two types of occluded fronts, and their nature and hazards can vary.
Cold occluded fronts, for example, are usually responsible for causing severe storms that can generate harsh and damaging winds, hail, and even tornadoes.
Cold occluded fronts are often more destructive and move at between 20 and 30mph. They also have a steeper frontal slope, which means they’re more likely to produce the most violent weather activity out of both fronts.
Warm occluded fronts tend to produce conditions such as poor visibility and rain. These fronts move a lot slower than cold occluded fronts, at around 10 to 25mph.
One of the key differences between the two is that a warm occluded front will provide a warning before its advancement, and because it moves slower, it can sometimes take days to move through a region.
However, cold occluded fronts are fast-moving and can occur with little to no warning.
The weather can change in a matter of hours, and considering the turbulent conditions that a cold occluded front can bring, its nature makes it the most dangerous of the two.
Where In The World Do Occluded Fronts Form?
Occluded fronts tend to form around mature low-pressure areas. Geographically, the occluded fronts favored low-pressure systems tend to be located in the lee of the Rocky Mountains and the Tibetan Plateau. However, they can occur in Europe, too.
Occluded fronts will occur in whatever regions are experiencing low atmospheric pressure. Although occluded fronts have a few favored locations, such as those mentioned above, there are no specific geographical preferences.
However, as long as the atmospheric conditions are right, occluded fronts will form, and their effects and hazards will vary depending on the type of occluded front that’s occurring.
Are Occluded Fronts Common?
If you’ve looked at your weather forecast recently, you’ll probably have heard the terms ‘cold’ and ‘warm’ used a lot.
These are the most common weather fronts we hear of, and that’s precisely why so many of us are unfamiliar with occluded fronts, their hazards, and their characteristics.
But just how common are occluded fronts, and are they the rare phenomena, they appear to be?
Cold air occluded fronts tend to be more common than warm occluded fronts. However, occluded fronts, in general, are not a rare phenomenon.
Although these fronts can be more complex than warm or cold fronts, they can occur almost anywhere (with the right conditions), and they’re the weather fronts that are usually responsible for causing periods of stormy weather.
Although occluded fronts form during cyclogenesis, not every occluded front will cause a tornado. However, you should still expect strong winds and thunderstorms.
Remember, though, that the characteristics of an occluded front can vary depending on whether it’s warm or cold.
The Occluded Front Symbol
If you’re unsure whether or not your region is experiencing an occluded front, take a look at your local weather map. Meteorologists will mark most specific weather fronts with particular symbols to help us identify them.
An occluded front will usually be depicted by a purple line interspersed with both triangles and semi-circles. This is because a cold front is depicted by triangles, and semi-circles depict a warm front, and an occluded front has characteristics of both.
The semi-circles and triangles of an occluded front will always point in the same direction that the front is moving.
The Bottom Line
Occluded fronts are intense, complex, and fascinating, but in general, they’re not rare.
Unfortunately, these lesser-known systems are a bit of a mystery to some, mainly because they have a more extensive collection of mechanisms at work that can drive intense and unusual weather patterns across the world.
We hope this article has helped deepen your understanding of occluded weather fronts and what type of weather you can expect to occur from these events.
While these fronts may not occur as frequently as warm or cold fronts, it’s still just as important that we understand them and the influence they can have on the global weather system.