Air Masses & Fronts Frontal Classification
We now progress into a more qualitative interpretation of meteorology. We will spend more time developing issues more traditionally associated with weather reports and less on the more quantitative, mathematical developments we have focused on.
Air Masses
As the name implies, an air mass is a mass of air that has relatively uniform characteristics with respect to temperature, and moisture. Air masses are often characterized by both their temperatures and their humidities. Air masses can basically be classified as warm or cold with respect to temperature and moist or dry with respect to humidity. The characteristics of an air mass are determined by the region over which it formed. For instance, during the winter, when the nights are long and frigid over the Polar regions, the air tends to become relatively uniform. A very cold and dry air mass develops. Occasionally, pieces of these air masses will break free and be transported by the jet stream.
There are a tremendous number of different names given to air masses. We will stick with the basic ones. Below is a table that describes the air masses I would like you to know:
| Air Mass | Symbol | Characteristics/Comments |
|---|---|---|
| Continental Arctic | cA | Form exclusively in the Arctic and Antarctic regions and descend toward the equator. Bitterly cold and extremely dry in the winter, cool and dry during the summer. |
| Continental Polar | cP | Form over dry lands. Cold and dry during the winter, mild and dry during the winter. |
| Continental Tropical | cT | Form over deserts and plains. In the United States, a flow into the US out of Mexico often sends a cT air mass northward. Typically hot and dry during the summer and mild and dry during the winter. |
| Maritime Polar | mP | Marine type humidities with cool or cold weather. Typically provide for miserable, damp, gray days. Mild to cold and humid with low stratus clouds and precipitation is often the rule with Maritime Polar air masses. |
| Maritime Tropical | mT | Hot, humid, sticky weather. A good example of when mT air masses affect the United States is during the summer with the Bermuda High phenomena. A southerly flow of hot, humid, sticky weather is circulated northward into the US. Rarely will mT air masses affect the US during the witner. |
It should be apparent that moist or humid air masses are given the designation m (Maritime) and dry air masses are given the designation c (Continental).
When air masses move into different environments, they can change their designation as they are modified and adjust to their new surroundings. For instance, a cP air mass that migrates into the southwestern US could over time turn into a cT air mass as it will warm over the course of time.
Fronts
Consider an air mass that is beginning to migrate. At the leading edge of this air mass, there will be a sharp contrast in temperature. For instance, suppose there was a cP air mass migrating southward into an mP air mass. The interface of these air masses is called a front.
There are four types of frontal systems we will be studying. We will study their formation, movement, and the type of weather each is often associated with.
Different cloud formations and precipitation patterns occur in different frontal scenarios.
Cold fronts
As a cold, dry air mass migrataes toward a warm, moist air mass, a cold front develops at the air mass interface. Since the cold dry air is heavier and denser than the warm moist air, it is strong enough to push it out of the way and displace it. As a result, the warm air is pushed horizontally and also vertically upward by the under-cutting cold dry air. Since the air is forced upwards relatively rapidly, there are often cumulo-type cloud developments in these situations. The more moisture that is present in the atmosphere, and the faster the warm air is forced upward, the higher in the atmosphere those cumulus clouds can be expected to propagate. Cumulus cloud development is often associated with thunderstorm activity. Cold fronts are often monitored, particularly during the spring and summer months, as prime areas for the development of thunderstorm activity.
Warm fronts
Now consider a situation where a warm, moist air mass is migrating into a cold, dry air mass. At the interface of the two air masses, a warm front will exist. Since the warm, moist air is less dense than the cold, dry air, the warm air will rise up and over the cold, dry air. This is an example of a situation called overrunning.
The air rises much more slowly and gradually than in the cold front situation. As a result, the cloud development will be less vertically developed and will be more stratiform than cumuloform. Far ahead of a warm front, cirro type clouds are often observed. These thicken into alto type clouds as one moves toward the surface front. These eventually thicken into low level stratus and nimbostratus in the vicinity of the warm front.
Occluded front
Suppose we have a cold front and a warm front from west to east respectively. We would have alternating cold air - warm air - cold air. Cold fronts generally move along more quickly than warm fronts. What happens if the cold front catches up to the warm front? They will merge into one front called an occluded front.
The warm air in between the two fronts is called the warm sector. What will happen in an occlusion, is the warm air will be forced aloft. Occlusions are often the sites of precipitation and convective type thunderstorm activity. Occluded fronts typically do not last too long. The warm air that is forced aloft cools. Then the east-west temperature gradient that "powered" the front, diminishes and the front weakens and potentially dies.
Stationary front
What if a cold air mass is advancing southward into a warm air mass, while at the same time a warm air mass is advancing northward into a cold air mass. What happens if there is a draw and neither front makes any progress either northward or southward? The result is a stationary front.
Stationary fronts are battlegrounds between warm and cold air masses. If there is an active jet stream in the vicinity of a stationary front or if the front is near a continuous supply of moisture, stationary fronts are often regions of prolonged precipitation and cloud development.
In our next lecture, we will learn about the different cloud and precipitation structures that are often associated with our four major frontal forms.