Air masses are vast volumes of air that acquire relatively uniform temperature and moisture characteristics from their source regions. Meteorologists classify these bodies based on whether they originate over land (“continental”) or water (“maritime”), which determines moisture content. They are further categorized by the latitude of their formation, such as “polar” for cold regions or “tropical” for warm regions, which governs temperature.
The Origin and Temperature Characteristics of Continental Polar Air
Continental Polar (cP) air masses develop over immense, high-latitude land areas of the Northern Hemisphere, such as interior Alaska, northern Canada, and Siberia. The “Continental” designation means the air mass forms over land, while “Polar” indicates a source region in the higher latitudes. These regions are often covered in snow and ice during the winter, which helps define the air mass properties.
The lack of solar radiation and rapid heat loss from the land surface, particularly in winter, allows the air to become extremely cold. Prolonged contact with this frigid surface ensures the air mass develops homogeneous, cold temperature characteristics throughout.
The Humidity Profile of Continental Polar Air
Continental Polar air has inherently very low humidity, making it one of the driest air masses encountered in the middle latitudes. This dryness results directly from its continental origin and extremely cold temperature. Since the air mass forms over land, it has little opportunity to evaporate moisture, contributing to a low water vapor content.
The primary reason for this low moisture content is the relationship between temperature and the air’s capacity to hold water vapor. Cold air holds significantly less water vapor than warm air, known as a low saturation point. Therefore, even if a cP air mass has high relative humidity (nearly saturated), its absolute humidity remains minimal. This minimal water vapor content is reflected in a very low dew point temperature.
Weather Impacts Associated with Continental Polar Air
When a Continental Polar air mass moves away from its source region, it brings a distinctive set of weather conditions. The stability and low moisture typically result in clear skies with few clouds. This absence of cloud cover allows for maximum incoming solar radiation during the day but also maximum heat loss at night, leading to large daily temperature swings.
The cold, dense air is often associated with high atmospheric pressure, contributing to calm, settled weather and light winds. The low absolute humidity means the chance of precipitation is generally minimal, as there is not enough water vapor available to form significant rainfall or snowfall. However, a notable exception occurs when the cP air travels over large, unfrozen bodies of water, such as the Great Lakes.
In this scenario, known as the lake effect, the cold, dry air rapidly picks up heat and moisture from the relatively warmer water surface. This addition of warmth and moisture causes the air to become unstable, leading to the formation of localized, heavy snowfall on the downwind shores. Once the air moves inland again, it loses its moisture source, and the characteristic cold, dry conditions quickly return.