Transcription
Understanding weather. Weather fronts.
You might have heard "weather fronts" mentioned in a weather forecast, but what are they and how do they affect our weather? Describing weather features as 'fronts' dates back to 1919. Just after the First World War, Norwegian meteorologist Jacob Bjerknes used this war analogy to describe the transition zone between two contrasting bodies of air. He likened the air masses either side of the front to two warring armies with the front as the 'battlefield' where the two sides meet.
Put simply, a front is the boundary between two air masses which have different temperature and humidity, and at this boundary, the reaction as the air masses meet often brings with it a dramatic change in temperature or weather conditions. In fact, the larger the difference in temperature between the two air masses, the more intense the resultant weather that the front brings. The three most common weather fronts are warm fronts, cold fronts, and occluded fronts.
On a weather chart, warm fronts are drawn as solid lines with semicircles and are often coloured red. The edges of the semicircles indicate the direction of movement of the warm air. The presence of a warm front means that warm air is replacing colder air at the surface. As warm air is 'lighter' or less dense, than cold air, it rises up over the colder air it is replacing. As this warm, moist air is forced to rise, it cools, forming clouds. These can first be seen high in the atmosphere hundreds of kilometres ahead of the surface warm front. This cloud gradually thickens and lowers as the warm front gets closer. Precipitation can start falling long before the front arrives, getting heavier as the warm front approaches, followed by milder, but often cloudy, conditions once the front has passed.
Cold fronts appear on weather charts as solid lines with triangles and are often coloured blue. The tips of the triangles indicate the direction of movement of the cold air. The presence of a cold front means that cold air is replacing warm air at the surface. Cold air is heavier, or denser, than the warm air it is replacing. The incoming cold air pushes underneath the warm air like a wedge, forming a steep, sloping boundary between the warm air and advancing cold air. The warm air is forced to rise up over the advancing cold air and forms thick layers of cloud. This process will often bring a narrow band of heavy rain and thunderstorms. There is often a dramatic clearance, with blue skies and bright sunshine. The sunnier conditions can offset the drop in temperature that follows a cold front, but cloud will gradually build up, producing showers.
Occluded fronts are slightly more complex than cold or warm fronts. 'Occluded' means 'hidden', and these fronts occur when the cold front 'catches up' with the warm front and lifts the warm air, hiding it from the surface. Because of this process, they can be thought of as having the characteristics of both warm and cold fronts. They are shown on weather charts as a solid line with both a triangle and a semicircle and are often coloured purple. Occluded fronts bring a band of thick cloud and rain, but the change in weather before and after the front is not as pronounced as that with a warm or cold front.
Since the advent of satellite and radar imagery, the theory for how these weather fronts form has changed somewhat. However, the theories developed nearly 100 years ago still have merit, and fronts are still drawn on weather charts in the same way.