A front is the boundary between two different airmasses. Air masses are categorized by their temperature and moisture properties. Let's say an airmass is located in the Canadian Prairie provinces. At any time of year this airmass will be relatively cold or cool in comparison to airmasses farther south, in the Great Plains for instance. It will also be rather dry, since there are no major bodies of water for this airmass to obtain water. This specific airmass is typical behind a cold front that we may encounter in the eastern two-thirds of the United States.
A front is defined by the properties of the airmasses associated with it. However, another aspect of this definition involves which airmass is doing the pushing. There are five major types of fronts defined by this method. They are: cold front, warm front, occluded front, stationary front, and dry line.
- Cold Front: the boundary between a cold airmass and a warm air mass, where the cold airmass is pushing the warm airmass out of the way. Denoted in Figure 1.1 by CF.
- Warm Front: the boundary between a warm airmass and a cold airmass, where the warm airmass is pushing the cold airmass out of the way. Denoted in Figure 1.1 by WF.
- Occluded Front: the boundary between a cold airmass and a warm airmass, where both airmasses are pushing in the same direction. This only happens when a cold front begins to overtake a warm front near the center of a low-pressure system. Denoted in Figure 1.1 by OF.
- Stationary Front: the boundary between a cold airmass and a warm airmass, where neither airmass is moving, but the temperature difference across the boundary remains (the airmasses don't mix to reach equilibrium). Denoted in Figure 1.1 by SF.
- Dry Line: the boundary between a moist airmass and a dry airmass, where the dry airmass is pushing the moist airmass out of the way. This generally occur in the southern Great Plains (TX, OK, and KS), but have occured seldomly east of the Mississippi River. Denoted in Figure 1.1 by DL.
On a surface anaylsis, the fronts will be colored accoding to the image below (Figure 1.1).
Figure 1.1: Frontal symbols with correct colors.
Abbreviations noted in text.
To define a front, think about what temperature and moisture parameters the airmasses around the front will have. It may be good to look at the surface observation plot to better define what kind of front you are dealing with (look at winds, temperatures, and dewpoints). Also, try to figure out which direction the front may be moving based on the surface winds.