Answer: I believe the answer is pressure
Explanation:
Cold air IS less dense, sinks, holds less vapor
A fan-shaped deposit <em>(of sediment</em><em>)</em> at the mouth of some rivers is called b) an alluvial fan.
A <em>dune</em> is a mound of sand shaped by wind.
A <em>delta</em> is a sort of "tree" of splitting rivers at the mouth of a river.