Meteorologists focus a great deal on the study of weather patterns known as fronts. A front is a boundary between large masses of air with different densities. Air masses are large bodies of air with roughly uniform characteristics. They form over large areas of Earth’s surface and take on the general temperature and moisture characteristics of the area over which they form. Sometimes these air masses are referred to in terms of where they originate. There are continental air masses that originate over land, maritime masses that start over oceans, and tropical, polar, and arctic masses that originate in different parts of the world. Air masses are generally defined by their overall temperature and water vapor content, or moisture. Examples include very cold and dry, cold and moist, hot and dry, or warm and moist.

When distinct air masses converge, they form boundaries called “fronts.” These fronts are named based on temperature differentials.

If a cooler air mass is moving to replace a warmer one, it is called a “cold front.”

If a warmer air mass is replacing a cooler one, it is called a “warm front.”

If two air masses are next to each other but neither is moving to replace the other, it is called a “stationary front.”

Sometimes a cold front can overtake a warm front, lifting the warm air up from the ground. This is called a “cold occluded front.”

A “warm occluded front” is less likely and occurs when the air behind the advancing cold front is warmer than the cold air it is overtaking.

Keep in mind that “cold front” is a relative term. Cold fronts don’t always bring snow and ice, just cooler air than what was there before. For example, an air mass with an average temperature of 20 degrees Celsius replacing a mass with an average temperature of 30 degrees Celsius would be called a cold front, even though 20 degrees Celsius is about room temperature. As an air mass moves across a region, the fronts also move. Much of the movement of air masses is due to regular, predictable wind patterns like those of the jet stream. The jet stream moves through the United States in a general west-to-east direction, curving and changing directions every now and then. Overall, this means that air masses (and fronts) move west to east in the United States, as well.