The left image pair shows a cold front moving from left to right into a warm mass of air The cold, dense air lifts the warm air like a wedge The rising, warm air forms distinctive anvilshaped clouds, visible in both the satellite and model imageCold fronts, marked by a chain of blue triangles pointing in the direction of movement (toward the warmer air), often mark the boundary between a maritimeTropical (mT) and an advancing continentalPolar (cP) air mass or perhaps the boundary between a cP air mass and an advancing continentalArctic (cA) air mass (the coldest of the cold) As aAir Masses, Fronts, Storm Systems, and the Jet Stream Air Masses When a large bubble of air remains over a specific area of Earth long enough to take on the temperature and humidity characteristics of that region, an air mass forms For example, when a mass of air sits over a warm ocean it becomes warm and moist Changing Weather Earth Science The movement of a cold front into an area of warm air