FRONTS: When Air Masses Meet
Named after the area where to opposing armies meet, a FRONT is where two air masses meet. The weather along the front (or frontal boundary) may be very different than the weather that occurs within the main body of the air mass, once the boundary has passed.
NOTE: terms like "warm" and "cold" will be used as comparison terms between two air masses, like saying someone who is 5ft 9in is "tall" if everyone else in their family is 5ft 4in. The same person would be "short" if they were surrounded by a basketball team! |
Remember: Warmer air is less dense than cooler air.
When air masses meet, the FRONT that forms is a result of air rising, usually as the warmer air rises over the cooler air. As the warm air rises, it cools, condensation occurs and is followed by precipitation. The amount of precipitation is often determined by how much moisture is in the air mass and how quickly it is forced upwards.
Types of Fronts
There are four types of fronts you will learn about this year. Remember, the front is the BOUNDARY between air masses.
Lines are used to show the edge where two air masses meet. Shapes and colors are added to the line to tell you which air mass is doing the pushing, and which way the boundary is moving. The shape "points" in the direction the front is going. Cold = blue triangle (think "biting" cold) Warm = red half circle (think a nice, warm, rising sun) |
Cold Front
What Happens: Cold Air Mass pushes a Warm Air Mass. The warm air is pushed upward FAST and creates cumulonimbus clouds. Weather along the Front: Creates the strongest storms - thunderstorms and blizzards! |
Warm Front
What Happens: Warm Air Masses pushes a Cold Air Mass. The warm air mass "crawls" up on top of the cold air mass SLOWLY - creating cirrus clouds ahead of the front and stratus clouds along the front. Weather along the Front: steady precipitation, NOT a strong storm! |
Stationary Front
What Happens: A cold and warm air mass meet, but neither is strong enough to push the other, so they stay in one place. Weather along the Front: often, steady precipitation for several days....until the front is able to move. |
Occluded Front
What Happens: Two cold air masses "trap" a warm air mass, pushing the warm air up between the two of them. Weather along the Front: Rain as the warm is pushed up and cools. Strong storms can occur, but are less common than with a cold front. |