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What causes stationary fronts?

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Posted by KWWL Storm Track 7 on Sunday, July 12, 2020

Today's question comes from Angela Beauchamp: "What causes a front to become stationary and stall out? Seems like it happens a lot in Iowa."

Fronts are the division between air masses. A cold front is the leading edge of an advancing cold air mass and a warm front is the leading edge of an advancing warm air mass.

The movement of these fronts depends only upon the cold air, not the warm air. Why? Well, cold air is heavier than warm air because its molecules are closer together and are more dense than the same volume of warm air. Therefore the cold air can "bully " the warm air.

Think of it as a battle between two armies trying to advance into each other's territory. If the cold air is marching forward, with winds blowing into the boundary (or front line), it pushes the warm air up and over the cold air (which in a general case develops clouds and storms).

If the warm air is marching forward, the only way it can advance is if the cold air is retreating and the warm air takes its place.

If they are marching toward each other, the cold air will win because it is heavier. The last one is known as an "active front" where convergence is maximized and there is a greater chance for storms.

A stationary boundary is the result of two air masses that aren't moving. Again, this is all dependent on the movement of the cold air. Essentially, you need a calm wind on both sides or if the wind on the warm and cold side are moving parallel to the boundary, there will be no movement.

Stationary boundaries can be the result of localized heavy rain and training thunderstorms.

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Brandon Libby


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