Definition
The geographic area over which a large body of air rests long enough to take on the temperature and moisture characteristics of the surface beneath it, forming an air mass with relatively uniform properties.
Plain English
It is the part of the Earth where a big patch of air sits still long enough to soak up the temperature and humidity of the ground or water below, becoming an air mass.
Context Anchor
Seen in weather and air mass discussions when explaining why an air mass has certain temperature and moisture characteristics.
Derivation
From 'source' (the place something comes from) and 'region' (an area). Together: the area an air mass comes from. Knowing the source region tells you what kind of weather that air will bring with it.
Why Pilots Care
Knowing the source region lets pilots anticipate whether an air mass will bring stable air, low ceilings, turbulence, or good visibility along their route.
Grounding Statement
Air sitting over the Gulf of Mexico for days becomes warm and moist; air sitting over northern Canada in winter becomes cold and dry. Each surface stamps its character onto the air above it.
Intuition Check
A source region is not the place where a weather report comes from. It is the area where the air itself picked up its temperature and moisture characteristics.
Example Sentence 1
The cold, dry air affecting the Midwest came from a source region over northern Canada.
Example Sentence 2
Before departure the pilot reviewed the source region of the maritime tropical air mass to expect reduced visibility and possible fog near the destination.