Question

Of all the air masses, which are of greatest significance to the United States and Canada? What happens to them as they migrate to locations different from their source regions? Give an example of air mass modification.

Short answer

The most significant air masses to the United States and Canada are the Continental Polar and Maritime Tropical. As these air masses migrate from their source regions, they modify the temperature and humidity characteristics of the regions they pass over. An example of air mass modification is the 'lake-effect' snow, that happens when the cold cP air mass moves over the warmer Great Lakes during the autumn and winter months causing heavy snowfall.

Step-by-step solution

Identify Important Air Masses

Firstly, consider which air masses most significantly impact the United States and Canada. Continental Polar (cP) and Maritime Tropical (mT) air masses are the most significant. The cP originates from the cold, dry regions of North America and the mT originates from the warm, humid areas over the Gulf of Mexico and the Atlantic Ocean.

Understand Air Mass Migration

Secondly, understand what happens to these air masses as they migrate to locations different from their source regions. As these air masses move, they modify the temperature and humidity characteristics of the regions they pass over. For example, as mT air mass moves northward, it may cause a rise in temperature and an increase in humidity in the region over which it passes.

Provide an Example of Air Mass Modification

Lastly, provide an example of air mass modification. An example of this would be when a cold cP air mass moves over the warmer Great Lakes area during the autumn and winter months. This phenomenon is known as 'lake-effect' snow. The warmer water of the Great Lakes heats the base of the cold cP air mass, causing the air at the bottom to become warmer and less dense, and thereby rising, cooling, and causing precipitation in the form of snow on the lee shores of the Great Lakes.

Key concepts

Continental Polar (cP) Air Masses

Continental Polar (cP) air masses are vast bodies of air characterized by low temperatures and minimal moisture content. These air masses are formed over the northern plains of Canada and interior Alaska, where the terrain is primarily flat and surface temperatures are cold during the winter months. As these air masses move southward into the United States, they influence the weather by bringing cold, dry air.

Because of their continental and polar origins, cP air masses are responsible for crisp, clear winter conditions, and when they move out of their source regions, they can cause sharp drops in temperature. Over time, as they travel over land, they may become less cold, but will usually remain drier than other air masses. One remarkable aspect of cP air masses is their ability to influence a wide geographic area, altering temperature patterns far from their source regions.

Maritime Tropical (mT) Air Masses

Maritime Tropical (mT) air masses are quite the opposite of their Continental Polar counterparts. They originate from warm, tropical waters, such as the Gulf of Mexico and the Atlantic Ocean near the equator. These air masses are loaded with moisture and bring warm, humid conditions to the regions they affect.

For the United States, mT air masses are particularly influential during the summer months, where they contribute to hot, sticky weather conditions. When an mT air mass moves northward and encounters a colder region, it can cause a significant rise in humidity and create a muggy atmosphere. The interaction between mT air masses and other weather systems can lead to the development of severe weather events, such as thunderstorms or hurricanes, due to the high energy and moisture content of these air masses.

Air Mass Migration

The concept of air mass migration is central to understanding weather patterns and forecasting. Air mass migration describes the movement of air masses from their source regions to different geographical areas. This migration is driven by the prevailing winds and is influenced by the Earth's rotation which deflects the path of air masses in a phenomenon known as the Coriolis effect.

As air masses migrate, they interact with the local environment, leading to weather changes. For instance, a cP air mass moving from the frigid Canadian plains into the United States can trigger a cold spell. Similarly, a mT air mass drifting northward may raise temperatures and humidity levels. These migrating air masses adapt to the topography and surface conditions they encounter, often leading to significant weather events. An understanding of air mass migration is vital for predicting how these large bodies of air will modify the weather in their new locations.

Lake-Effect Snow

Lake-effect snow is a fascinating example of air mass modification that occurs when a cold Continental Polar (cP) air mass travels across the relatively warm waters of the Great Lakes. As the cold, dry air moves over the warm water, the lower layer of the air absorbs moisture and warmth, causing it to rise due to its lower density.

As the moist air rises and moves over land, it cools and condenses, forming clouds and eventually precipitating as snow on the leeward (downwind) shores of the lakes. This localized snowfall can often be heavy and concentrated in narrow bands, creating significant snow accumulations in areas like Upstate New York and the regions east of the Great Lakes. This effect is most pronounced during the late autumn and winter when the temperature difference between the cold air mass and the warm lake water is at its greatest.