Hello, everyone! Welcome to our first edition of Meteorology 101 on athensgaweather.com. This is the section of the site where we’ll go more in depth on some of the things you see us mention in our daily weather posts. If you have a topic you’d like us to breakdown, let us know! We’re going to start off our first edition with a relatively simple topic: Fronts. You’ve all seen them on surface weather map, like the one above, but you may not have known what they meant or were depicting. Let’s dive right in!
Have you ever wondered why weather fronts came to be known as “fronts”? Well, it all stretches back to the late 1910s when the Norwegian Cyclone Model was developed by Vilhelm Bjerknes and his associates. This model was developed to illustrate the leading theory on the life cycle of mid-latitude cyclones. At the time of the Norwegian Cyclone Model’s development, World War I was ongoing, and since the leading edge of air masses resembled military fronts, the term “front” was chosen to describe the lines in the model. Ever since, fronts have become common terminology in meteorology.
Some Front basics:
- transition zone between two different air masses
- frontal zone is the sloping surface where two air masses meet
- classified by temperature changes
There are many types of fronts in meteorology, some you’ve probably heard of, and some that may be new to you. In this post we’re going to discuss cold fronts, warm fronts, stationary fronts, occluded fronts, and a special feature called a dry line.
Cold fronts are simply when a cold air mass replaces a warmer air mass. As a cold front approaches, temperatures remain steady or rise due to southwesterly winds. Temperatures then quickly drop as the frontal zone passes and the cold air mass moves in. As the cold front nears, pressure decreases, and pressure then rises as high pressure moves in behind the cold front. Northwest winds are common behind the front, and these winds help move cold air into the region. Precipitation in cold fronts is typically confined to the frontal zone, though there are exceptions. Showers and thunderstorms are generally the common precipitation types.
On a surface weather map, a cold front will be depicted by a thin blue line with blue triangles pointing in the direction the cold front is moving.
Check out the video below to see a strong cold frontal passage in Marissa, Illinois, on October 25, 2012. You can see the wind shift in the video too. Notice the steam change directions as the frontal zone passes.
Warm fronts occur when a warmer air mass replaces a colder air mass. As a warm front approaches, temperatures generally reman stable or climb slowly. Once the warm front passes, temperatures rapidly increase. Pressures drop as a warm front draws near and increases behind it, though the difference between pressure falls and rises is not as dramatic as in a cold front passage. Ahead of a warm front, winds are usually from the east and northeast which allows for cooler air to occupy the surface. As the warm front passes, warm air is forced up and over the cold air at the surface. We call this process overrunning. During the winter season, if the cold air at the surface is deep enough, snow can fall. Many different types of precipitation can fall in advance of a warm front depending on the season.
Warm fronts on a surface weather map will be depicted by a thin red line with red semicircles extending in the direction the warm front is moving.
The video below summarizes some of the clouds types common in a warm front.
A stationary front, as the name implies, is a front where two air masses meet but move little or not at all at the surface. Conditions right along a stationary front are typical of a warm front. Overrunning can also occur, and when this happens precipitation can fall and keep falling. Many flooding situations occur when a stationary front is present. Over time, a stationary front may cease to be stationary and can begin moving as ether a cold or warm front. It can also dissipate.
On a surface weather map, stationary fronts are depicted by an alternating red/blue line with blue triangles pointing in one direction and red semicircles pointing in the opposite direction.
Occluded fronts are a bit tricky, in fact, there are two kinds of occluded fronts: cold and warm. Cold-type occluded fronts are the most common, and occur when a low pressure center is maturing and about to weaken. In this case, the quick moving cold front merges with the slower moving warm front. When this happens, warm air is pushed vertically above the cold air behind the cold front and the cooler air out ahead of the warm front. Weather ahead of an occluded front is similar to a warm front, and weather behind an occluded front is similar to a cold front.
Occluded fronts on surface weather map will be depicted by a thin purple line with alternating triangles and semicircles pointing in its direction of motion.
Now, onto our last topic of discussion!
A dry line is determined by moisture rather than temperature. Behind a dry line, dry, continental air exists. Out and ahead of the dry line exists warm, moist air. You can tell where a dry line is by looking at dew point temperatures. Where these two drastically different air masses meet often marks the location of intense thunderstorms. Dry lines are most commonly found in the Great Plains during the springtime, and storm chasers love to see them because it gives them a great indication of where thunderstorms will begin firing throughout the day.
You may not always see a dry line on a surface weather map, but when you do it will be a a think orange line with hollow orange semicircles pointing in the direction of dry line movement. It will look something like this:
Here’s a cool video where you can see thunderstorms begin to fire along a dry line, especially at the end of the video.
Cold Front Experiment:
Finally, here’s a neat video form the MetOffice in the United Kingdom where they carry out a cold front experiment in a tank. It looks fairly easy to replicate if you’re interested in testing out your own cold front!