The boundary between two air masses have unique densities. When it does now not move, it is known as "stationary"; "warm" when hotter air replaces cooler air; "cold" when cooler air replaces warmer air. Hope this helps!
Make it interesting, and make sure it's on topic the whole time, don't get off topic and be descriptive! Stuff like that so it'll make the reader want to read it more. :) hope this helpes!
