If you were to compare a weather chart that had been prepared a century ago with the one appearing on this page today, you would notice several differences between the two. On the earlier map, the isobars would sweep across the chart in graceful curves, delineating oval, regular and smoothly-drawn depressions.
On modern weather maps, however, depressions have an eccentric, pointed shape, and on their southern flanks the isobars often form acute angles by bending backwards on themselves. And there are also the thick black lines with "barbs", the lines we call the "fronts".
A "front" may be thought of in either of two ways. From a purely practical point of view, it is an elongated zone of rain which moves steadily across the surface of the globe - and usually from west to east. But meteorologists have a much more complex view of these phenomena; they see a front as the sharp dividing line between two masses of air of differing characteristics, and which - almost as an afterthought - produces rain.
In general, the air on one side of a front is warm and humid, and that on the other side is cooler and relatively dry. The boundary between the two is surprisingly well defined, and as the two air masses sweep along, side by side so to speak, and one following the other, a sudden change in temperature and humidity is noticeable as the boundary passes each particular spot.
If the passage of a front results in the existing air mass being replaced by a warmer one, temperature and humidity rise rapidly, and the front in question is called a warm front; if the temperature after the event is lower than it was before, a cold front has passed. In either case, the interaction between the two air masses is likely to bring a period of rain.
The theory of fronts and air masses was developed shortly after the first World War. It was a time, obviously, when military terminology was very much in vogue, and meteorologists found it easy to grasp the analogy between two masses of air side by side, and two great armies facing each other across a long line, or "front".
On the weather map the barbs on a front are merely labels of a kind. A warm front is marked by semi-circular barbs, and a cold front by tri-angular ones, and the barbs are placed on the side of the line towards when the front is moving. And when the weather map is shown in colour, the difference between the two is further highlighted by showing warm fronts red and cold fronts blue.