An air mass is a large volume of air — typically covering thousands of square kilometres — that has relatively uniform temperature and moisture characteristics acquired from prolonged contact with the surface below. Air masses form over source regions: polar oceans (maritime polar — mP), tropical oceans (maritime tropical — mT), Arctic ice caps (continental Arctic — cA), and continental interiors (continental polar — cP, continental tropical — cT).
As an air mass moves away from its source region, it modifies gradually but retains its core characteristics for days. The boundaries between air masses with contrasting properties are called fronts: a cold front marks the leading edge of cold air, a warm front marks advancing warm air, and an occluded front results when a cold front catches up with a warm front. The interaction of air masses along these boundaries is responsible for most mid-latitude weather.
In the Iberian Peninsula, weather is shaped by five principal air masses: maritime polar (mP, from the North Atlantic — cool and moist, bringing rain), continental polar (cP, from Scandinavia/Russia — cold and dry winter waves), Arctic (A, rare but can bring snowfall to low elevations), maritime tropical (mT, from the subtropical Atlantic — warm and humid), and continental tropical (cT, from the Sahara — hot and dry, carrying calima dust). Identifying the dominant air mass on a given day is the starting point for any weather analysis.