Rocket has pushed two chairs together in the middle of the lab. One has a blue cushion, one has a red one.
"Cold air mass, warm air mass," he announces. "They bump. Then what?"
Raven pulls a printed surface map from the log. Blue lines with triangles and red lines with half circles curve across it.
"What do you notice about where the lines sit?" she asks.
"They all come out of the L," Rocket says. "The low pressure center."
Nova traces one blue line with light. "That line is a front," she says. "Which way do the triangles point?"
Rocket squints. "The way the cold air is moving," he decides. "So the shapes are arrows."
A front is the boundary between two air masses, for example warm air to the south and cold air to the north.
A front is often stationary at first. When the air masses begin to move, it becomes a cold front and a warm front.
A cold front marks cold air advancing. A warm front marks warm air advancing.
The cold front moves faster than the warm front. In time it catches up and overtakes it, forming an occluded front.
The occlusion removes the warm, moist air from the center of the low. Without it, the low pressure system fades away.
| Front | Drawn as | Shapes point |
|---|---|---|
| Cold front | Blue line with triangles | The way the front is moving |
| Warm front | Red line with half circles | The way the front is moving |
| Stationary front | Alternating red and blue line | Both ways, since it is not moving |
| Occluded front | Purple line with triangles and half circles | The way the front is moving |
Cold fronts nearly always extend south and west of the center of a low pressure area, and never from a high.
Warm fronts also extend from a low, nearly always on its east side.
A front separates two air masses of different density. The cooler air mass is the denser, heavier one.
Precipitation develops along a front, and it is heaviest right along the line. Clouds form where air rises, expands, cools and its water vapor condenses.
As a weather system intensifies, its fronts become sharper. The temperature change grows, and the wind direction changes distinctly across the line.
| Statement | True or false? |
|---|---|
| A front is the boundary between two air masses. | ? |
| A warm front moves faster than a cold front. | ? |
| Cold fronts extend from high pressure centers. | ? |
| An occluded front is drawn in purple. | ? |
Well done. Tomorrow is Station Lab: you make two air masses meet in a clear box.