METEOROLOGIST JEFF HABY
A cold front is an air mass boundary separating warmer air from colder air. While seeing a cold front on a surface chart is most common,
there is an interesting perspective in looking at a vertical cross section of a cold front. Some interesting features include the depth
of the cold air, the density discontinuity and the immediate edge of the cold front boundary.
The depth of the cold air generally increases behind the surface cold front boundary. When a cold front passes, there is typically only
a shallow layer of colder air at the surface with the air aloft containing the same air mass that the cold front will eventually
displace. There is a variation in how quickly the cold front increases in depth at a location depending on factors such as cold
front speed, upper level winds, how far toward the equator the front has penetrated and the wind speeds behind the cold front. It
can be interesting to witness a shallow cold front move through a downtown area. The cold air will be moving through on the lower
floors first while the upper floors have not been influenced by the colder air.
The depth of the cold air can be analyzed on a sounding. Often there will be an obvious demarcation in the vertical boundary between
the colder air below and the warmer air above it. Cold air is denser than warm air, thus the cold air will first occupy the low
elevations before occupying the higher elevations. This process can be witnessed firsthand when a shallow cold front moves into
higher terrain and mountain areas. The valleys will get the cold air first while the mountain tops will stay warmer until the
depth of the cold front is enough to occupy the mountain tops.
The boundary between the cold air and warm air represents a density discontinuity. This has the result of lifting the warm air
out ahead and along the cold front boundary. Similar to a helium balloon rising since it is surrounded by denser air, the less
dense warm air will lift due to the more dense colder air displacing the warmer air.
Interesting weather occurs along the immediate vicinity of a cold front boundary. There is a mixing between the warm and cold air as
the air masses interact with each other. This causes the air behind the cold front to warm. Since the air is most shallow closer
to the cold front boundary, mixing of the warmer and colder air will significantly warm the cold air behind the cold front. It
can be noticed that the temperature generally decreases hour by hour after a cold front passes. This is because the cold air is
deeper and less influenced by mixing with warm air as the cold air becomes deeper. Also, it takes time for surface objects to
cool off after frontal passage. The warmer objects help warm the cold front air especially when the cold front air is shallow. Once
the cold air is deep and the surface has had time to cool off, then temperature will generally not continue to fall as much. The
greatest temperature falls tend to occur just after cold front passage. However, the much colder air may be 100 miles or more behind
the surface cold front.
The image below is a diagram of a vertical profile of a cold front. Notice the pressure levels are closer to the surface and closer
together in the cold air. This is because cold air is denser and thus more compacted as compared to warm air. Also, notice how
the depth of the cold air increases behind the cold front boundary. In some cases, such as shown in the image below, surface
friction can drag on the cold air resulting in the cold air passing aloft just before it passes at the surface. Depending
on the characteristics of the front, the cold air can do the opposite and first pass by on the surface.