How Cold Fronts Really Work

How Cold Fronts Work

It's warm and comfortable. It's pouring rain. Then all of the sudden it's freezing cold!
Yes, when a cold front is passing over an area, all of this can happen in less than a day!
So, what is a cold front? Why does it bring such sudden, dramatic weather changes?
In this lesson, we are going to find out how cold fronts really work.

1. A cold front is a boundary or border separating a cold and warm air mass.
2. The cold air mass is advancing or moving forward to replace the warm air mass.

The main point to keep in mind is that a cold front is a border or boundary. Cold air is behind the boundary and warm air is ahead of it.
In a cold front, the cold air is advancing forward to replace the warm air. This happens because cold air is denser or heavier than warm air.
A cold front often causes rain, severe weather and drastic changes of temperature within a day or less.

Figure 1: How cold air and warm air interact in a cold front.

Knowing that warm air rises and cold air sinks is the key to understanding how cold fronts cause rain, hail, tornadoes and other forms of severe weather.
1. In a cold front, cold air moves towards the warm air. Because cold air is more dense than warm air, it slides beneath the warm air when it comes into contact with it. This forces the warm air to rise much more than it would have if there wasn't cold a front.
2. As the warm air rises, it cools when it reaches the higher levels of the atmosphere. One reason for this is because it is colder at the top of the atmosphere.
3. Now cold air cannot hold as much moisture as warm air. This cooling causes the water molecules within the rising air to slow down and come together (condensation). As the water molecules come together, water vapor, clouds, and eventually rain begin to occur.
4. As the process continues, more than just clouds and rain will occur. Thunderstorms and even severe weather can occur as well. The severity of the weather depends on several factors including how strong the cold front is.

If you guessed in "cold" places, you would not be wrong! The temperature in the tropical areas of the world tend to be uniform. Thus, there aren't significant temperature gradients or major temperature changes in a short distance.
However, this is not the case for the mid-latitude regions of the world. These regions have more pronounced temperature gradients. As a result, cold fronts primarily occur in these places. A different lesson will consider why mid-latitudes have such sharp temperature gradients.

Figure 2: Cold fronts are illustrated as triangle along a line on weather maps.

On weather maps, cold fronts are usually illustrated as triangles along a blue line. The triangles point toward the direction the cold front is moving.
Nonetheless, try not to focus too much on the blue color. On some classic surface analysis maps, you may notice they are only black and white. In such cases, the cold front will be depicted as a triangles on a black line.