A "front" is essentially a boundary. In meteorological terms, a warm front is the boundary line between a mass of warm air and the air surrounding it. By contrast, a cold front is the boundary line between a mass of cooler air and the air surrounding it.
Warm air masses are generally found in tropical regions where the temperature is higher. Warm fronts usually form on the eastern side of a low-pressure center, where winds coming from the south push the air northward. In the northern hemisphere, warm fronts generally travel from southwest to northeast. Because the warm air is generally from the tropics, it also carries more water vapor. Despite this extra payload, warm air is lighter and less dense than cool air. This disparity means that the cooler air must retreat before warm air if warm air is to replace it.
Cold air masses generally form near the Earth's poles, far from the equator. As such, they are referred to as polar or arctic air masses. In the northern hemisphere, they generally form to the west of a high-pressure center, where winds from the north push the air southward. The "front," or leading edge of this air mass, usually travels from northeast to southwest. While the air is cooler behind a cold front, the air behind it is at a higher pressure. This generally results in clearer days once the front has passed.
When Fronts Collide
When a cold front meets a warm front, the two cannot mix because of the difference in temperatures and densities. The result is that the warm air, being lighter, will usually be pushed atop the colder air. The air cools as it rises. As this happens, the water vapor in it condenses, forming clouds. Clouds are common along the boundaries of cold fronts. Whether a warm front is meeting a cold front or a cold front meeting a warm front, precipitation is likely. But while both types of collision can result in weather events, they are not equal in terms of severity.
Effects on Weather
Cold fronts generally move more quickly than warm fronts. Cold fronts also have steeper edges. When these edges displace the surrounding warmer air, that warm air is forced to rise rapidly. The rapid advance of a cold front can drop the temperature on the ground by as much a 15 degrees Fahrenheit. It can also produce extreme weather events, including snow squalls in the north, thunderstorms and even tornadoes.
By contrast, warm fronts move more slowly, and the air mass has more of a gentle slope. As a result, the resulting weather events are less likely to be severe. However, the precipitation caused can last for days.