The front of an air mass that moves either very slowly or not at all is called a stationary front.

Here's why:

* Stationary fronts occur when two air masses of different temperatures meet but neither has enough force to push the other aside.

* This results in a prolonged period of cloudy skies, precipitation, and relatively stable weather conditions.

* The front itself is often marked by a line of thunderstorms or other severe weather.

* They can persist for days or even weeks, as the air masses struggle for dominance.

Let me know if you'd like to know more about air masses or weather fronts!