Air Masses

An air mass is a huge, identifiable body of air possessing a relative homogeneity of temperature and moisture characteristics within compared to the air surrounding it. Most air masses have dimensions of hundreds of thousands of square kilometers and are sometimes pushed thousands of kilometers from their source regions. Boundaries between air masses are known as fronts and the various types of fronts are described elsewhere.

Although air is a continuous fluid, its properties are such that it frequently organizes into air masses having differences substantial enough to cause significant weather changes as they pass. When air of the lower troposphere passes over Earth’s surface, it exchanges energy and moisture characteristics with the surface. Air slowly moving large horizontal distances starts to take on the characteristics of the surface, be it tropical ocean or polar tundra. Some areas are well known as air mass source regions. Source regions occur in all latitudes and are characterized by non-mountainous terrain and frequent dominance of high pressure. High pressure is associated with light winds and this allows air to take on the surface temperature and moisture characteristics over the course of several days. As air masses move out of their source regions, they are modified. It is not rare for Arctic air to leave Siberia and travel over central North America to the Gulf Coast. While its original temperature might be −40°C, it might moderate to temperatures slightly below freezing. This is a cold shock to the Gulf Coast but a testimony to air mass modification.

One well-appreciated instance of air mass modification is lake effect snow. In early winter the Great Lakes of the United States and Canada are not yet frozen over. Polar and Arctic air masses stream over the lakes. These cold air masses do not contain much moisture but have high relative humidities. As the air passes over a few tens of kilometers of lake surface, the air mass gains water vapor and saturates. Downwind of the lakes, the arrival of the air over land initiates a small bit of lifting that cools the air lower than its dewpoint temperature. Condensation and the precipitation processes begin and make copious amounts of snow without the presence of a winter storm.

A simple air mass classification scheme considers the surface and latitude over which air passes. For instance, consider the differences in air types that can be generated over the great Antarctic ice sheets versus the tropical reaches of the Indian Ocean. Air masses having their origins in these places will provide vastly different weather as they progress over a location.