If you’re a little fuzzy about what an air mass is, then you should probably read Introduction to Air Masses before continuing.

A front is the region where two air masses of different densities meet. The change in density is usually caused by a difference in the temperatures of the air masses, but could be caused by a change in the water vapor content. In this post, we’ll look at two different types of fronts: the warm front and the cold front.

The cartoon below is of a warm front.

In this example, we have a cold air mass moving from right to left on the left-hand side of the image. On the right, we have a warm air mass that is also moving from right to left; the difference is that it has a greater speed than the cold air mass. When the warm air mass hits the cold one, it is forced to rise because the warm air has a lower density than the cold air. This rising motion causes the air to be cooled until it reaches 100% relative humidity, where clouds form. Precipitation is found on the cold air mass side of the front - which is not shown because it’s hard to draw blue rain on a blue cold air mass. The clouds are generally stratiform - or layered - clouds. Warm front are associated with horizontally distributed clouds with light rain. The weather will obviously be warmer after the passage of a warm front.

The cartoon below is of a cold front.

In this example, we have a cold air mass moving from left to right on the left-hand side of the image. On the right, we have a warm air mass that is relatively stationary. When the cold air mass hits the cold one, it digs underneath the warm air mass because the cold air is more dense than the warm air. The digging motion of the cold air forced the warm air to rise. This rising motion causes the air to be cooled until it reaches 100% relative humidity, where clouds form. Precipitation is found on the warm air mass side of the front. The clouds are generally cumuloform - or vertical - clouds. Cold front are associated with heavy precipitation caused by the vertical developing clouds. However, the rain is generally confined to a small horizontal range. The vertical development of clouds may form hail and severe thunderstorms.

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