When neither air mass displaces the adjacent one, their boundary is called a(an) front.

Stationary front

Explanation:

A stationary front is usually defined as a front when both the warm as well as cold front does not move and remain stationary. It is because the force exerted by both the air masses against each other is not so high to displace one another. In this case, the wind blows parallel instead of blowing perpendicular to the front.

Generally, in weather maps, the stationary front is shown by the alternate use of blue triangles and red semi-circles, where the triangle faces the warm front and the semi-circle faces the cold front.

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