The Highs and Lows of Weather: Part 2 -- The Low

Large bodies of water, such as the Great Lakes, can be breeding grounds for low pressure systems, or assist in the further development of weak lows moving over them. Cyclogenesis over water occurs when the water mass is significantly warmer than the air passing over. By imparting heat into the system from the warm waters below, updrafts are formed that start the development process. Over the Great Lakes, particularly Superior and Huron, the autumn is the peak season for low formation.

There are also several cyclone spawning regions around the globe. One, the Aleutian Low off Alaska, spawns the storms that worry North America's northwestern coast each winter. Its counterpart off Iceland sends storms into northern Europe. Tropical regions too have preferred locations that produce tropical storms, hurricanes and typhoons during specific periods of the year. In the Atlantic Ocean, areas west of the Azores and in the Gulf of Mexico are preferential locations for tropical storm and hurricane development from June through November.

Frontal Low Formation

In the middle latitudes, many low pressure cells and cyclonic weather systems develop in the zone between two contrasting air masses (Highs). The zone between air masses of polar origin and those of tropical birth is termed the Polar Front, and therefore, the explanation for this storm development process is known as the Polar Front Theory of Cyclonic Development. The theory, developed by Norwegian meteorologists about a century ago, was a significant step in the understanding of weather and weather forecasting. It also gave us the now-common term front for the boundary between contrasting air masses.

In the polar frontal zone between the air masses, the complex process begins. Simplistically, it usually starts when some combination of surface and upper-level factors cause the frontal boundary between the air masses to kink. This kink, known as a frontal wave, pushes cold air under warm air, forming a cold front, and warm air over cold, forming a warm front. This causes air to converge and then ascend around the "kink." If upper air conditions are right, the ascending air produces a region of lowering pressure, and a spinning cyclonic system with characteristic frontal boundaries begins to develop. These Lows are called frontal lows, and the inverted V-shaped frontal region around the Low is termed the frontal wedge.

Eventually, the contrasting air masses around the low pressure cell mix well enough to weaken their contrast, thus ending a cyclone's driving force. Its storminess weakens and its pressure rises until the cyclone eventually disappears from the weather map. One cyclone's

Go To Page: 1 2 3