Review the major characteristics and stages of development of a midlatitude cyclone.

In Section 4.2, we learned that a cyclone is any meteorological system that rotates counterclockwise (in the Northern Hemisphere) around a low-pressure center. When large cyclonic systems occur at midlatitudes, they are called midlatitude cyclones (or extratropical cyclones). They are also called depressions, lows, or low-pressure systems, names reflecting the fact that they are geographically extensive regions of low barometric pressure. In contrast to hurricanes, which are fueled by the release of latent heat, midlatitude cyclones form as a result of temperature contrasts between air masses.

Midlatitude cyclones bring storms from fall through spring between approximately 30° and 70° latitude in both hemispheres. In the Northern Hemisphere, the United States, most of Canada, Europe, and Asia are influenced by these large storm systems, which move from west to east with the westerlies. Some 10 to 20 midlatitude cyclones are in progress at any given time worldwide. They are the largest storm systems on the planet, having a diameter of 1,600 km (1,000 mi) or more. In some cases, they become as strong as hurricanes at sea level. In mountainous regions, they commonly produce hurricane-force winds.

There are many different permutations of midlatitude cyclones, depending on the types of air masses that are interacting and the characteristics of geostrophic winds aloft. Most midlatitude cyclones are composed of a warm front and a cold front. A warm front is produced when warm air advances on and flows over cooler, denser air. Warm fronts may bring precipitation, but they are rarely associated with severe weather. A cold front is a region where cold, dense air advances on relatively warm and less dense air. Cold fronts are sometimes associated with severe weather. The different densities of warm and cold air prevent air masses from mixing together as they converge. Without this characteristic, frontal systems would not form. Figure 5.22 illustrates how fronts may combine to form a midlatitude cyclone.

As a midlatitude cyclone moves over a region, the weather experienced in that region will reflect the type of front moving through. In most cases, a warm front moves through the region first and is followed by a cold front. Warm fronts are usually associated with nimbostratus clouds that bring steady precipitation. Cold fronts are usually associated with cumulonimbus clouds that bring short bursts of rainfall and potentially severe weather. Figure 5.23 diagrams the typical characteristics and weather patterns of frontal systems.

Like a single-cell thunderstorm or a hurricane, a midlatitude cyclone experiences stages of growth, maturation, and dissipation over a period of about one to two weeks (Figure 5.24A). Although midlatitude cyclones do not all look and behave alike, temperature gradients and, therefore, pressure gradients give rise to all of them. In addition, they must have upper-level support to persist. In other words, they do not form unless geostrophic winds are lifting air.

Most midlatitude cyclones begin as waves in the subpolar low. If an upper-level Rossby wave trough is present (see Section 4.3), the low pressure at Earth’s surface will deepen (decrease) and the cyclonic system will strengthen (Figure 5.24B). Upper-level troughs maintain surface-level low pressure by pulling air from the surface to higher altitudes.

One particularly important type of midlatitude cyclone is called a nor’easter. These powerful storms bring blizzard-like conditions from the Mid-Atlantic states north to New England (see Figure 5.28). Nor’easters form where mT air from the Gulf of Mexico meets cold air from the Great Plains. Their name derives from the direction of the wind (from the northeast) that they bring to the regions where their precipitation falls.

After midlatitude cyclones move over the Great Lakes, the cold air behind them often creates lake-effect snow. Lake-effect snow is heavy snowfall that results as cold air moves over large, relatively warm bodies of water, such as the Great Lakes. Heavy snows downwind of the lakes sometimes bury towns in snow. The warm water readily evaporates and increases the atmospheric humidity. Picture This explores this phenomenon further.

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