A source region in meteorology is an extensive geographical area where a large body of air, known as an air mass, remains stationary long enough to acquire distinct characteristics of the underlying surface. This allows the air mass to take on a relatively uniform temperature and moisture content across its vast horizontal expanse. Source regions are fundamental to understanding global weather patterns, as they are the birthplaces of atmospheric systems. The properties an air mass gains here are used by meteorologists to forecast how that air will influence local weather.
Defining Characteristics of a Source Region
For any area to qualify as an air mass source region, it must possess specific geographical and atmospheric requirements. The surface must be extensive and physically uniform, such as a large, flat plain or a vast ocean body. This uniformity allows the overlying air to develop consistent temperature and humidity properties across its entire breadth.
Atmospheric stability is a second requirement, typically found in areas dominated by high-pressure systems. These high-pressure zones feature gentle, divergent air circulation and light surface winds, which prevents the turbulent mixing of air. This stable environment allows the air to stagnate over the region for an extended period. Stagnation is necessary for the air mass to achieve thermal and moisture equilibrium with the surface below.
Categorizing Global Source Regions
The properties of an air mass lead to a standardized classification system based on two main criteria: latitude (temperature) and surface type (moisture content). Latitude divides air masses into Polar (P) or Arctic (A) for cold masses originating poleward of 60 degrees, and Tropical (T) for warm masses originating in lower latitudes. Surface type further categorizes the air as Continental (c), meaning it originated over land and is dry, or Maritime (m), meaning it formed over water and is moist.
Combining these distinctions yields the main air mass types that influence global weather. For instance, a Continental Polar (cP) air mass forms over high-latitude landmasses like Siberia, making it cold and dry. Conversely, a Maritime Tropical (mT) air mass originates over subtropical oceans, such as the Gulf of Mexico, resulting in a warm and moist air body. These categorized air masses interact to create weather fronts and storm systems worldwide.
How Air Masses Acquire Identity
An air mass acquires its distinct identity through continuous physical exchange processes between the Earth’s surface and the lowest layers of the atmosphere. This transfer involves both energy (heat) and moisture, occurring slowly and often requiring the air mass to remain stagnant for a week or more. Heat energy is transferred directly from the surface to the air through conduction, particularly in the layer closest to the ground.
Radiation also plays a significant role, where the surface emits longwave energy that is absorbed by the lower atmosphere, warming the air mass. For moisture content, air masses over water bodies gain humidity through evaporation, while those over dry land remain dry due to the lack of moisture transfer. The longer the air mass resides in this stable environment, the more completely the vertical column of air reaches equilibrium with the surface.
Modification Outside the Source Region
Once an air mass is fully developed, global atmospheric circulation steers it away from its source region and over different surfaces. As it travels, the air mass undergoes modification, meaning its original temperature, moisture, and stability characteristics begin to change. Thermal modification occurs when a cold air mass moves over a warmer surface or a warm air mass moves over a cooler surface. For example, a cold, dry Continental Polar air mass moving over a large, warmer lake will be heated from below, increasing instability and often leading to snow.
Moisture modification happens when a continental air mass passes over a large body of water, causing it to pick up substantial water vapor through evaporation. Conversely, a maritime air mass moving inland over a dry continent will lose moisture. Mechanical modification, such as being forced upward over a mountain range (orographic lifting), causes the air to cool, leading to condensation, cloud formation, and precipitation.