Air Masses at Sea
A front is the boundary between the meeting of two air masses of different temperatures. One is a warm and moist tropical air mass while the other is a cold and dry polar air mass.
Due to the tilting of the earth on its axis the polar region (north or south of 67.5 degree latitude) gets less sunlight. Therefore the surface air in its contact is quite cold. Cold air cannot hold much water vapor, so it is dry and dense and has high pressure. The condition of the air mass is roughly uniform throughout the polar zone.
During winter months, the area falling in between latitude 40 degree to 67.5 degree gets cold. This means the air in contact becomes drier, colder, and denser. However this air is not as cold as in the polar zones and its spread is not uniform due to the presence of ocean and continental land masses. Air above cold continent becomes almost as cold as polar region, while the air over oceans is humid and warmer then polar air.
Tropical areas are roughly 90% ocean and 10% land mass. The air in contact is mostly warm, humid, and lighter, and normally has low pressure. Its spread is not uniform due to presence of huge continental mass of Africa, Central America, and the East Indies. In the case of the Atlantic and Pacific Ocean, its spread can be called almost uniform.
Due to various reasons, a surface air mass from a tropic area starts moving towards a pole. At the same time, a surface air mass from a temperate latitude starts moving or spreading southwards. Owing to the presence of large land masses in temperate zones, the movement is not uniform. The hot tropical air mass comes in contact with the cold air. Since the size of tropic air mass or maritime air mass has made inroads in the larger area of cold air mass in the temperate zone, it will appear as if the warm air is making a dent in the cold air.