A new and denser air mass has moved in to Boulder from the east at 19:00 to 24:00, blocking the westerly winds coming off the mountains. Which two variables suggest that this new air mass is indeed denser than the air coming down off the mountains

The two other variable are font and pressure.

Explanation:

1. It is the fronts between air masses that create a strong wind. This happens when one mass pushes into another along their front and one is forced upwards. This forced uplift produces adiabatic cooling and so precipitation and storms.

Because cold air is denser than the warm air it is invading, it can push the more buoyant warm air out of the way very fast. Cold fronts are, therefore, fast moving. A warm front, in which the invading air mass is warmer and, therefore, lighter than the one it's invading. Warm fronts are found east of cold fronts.

Air always moves as a result of temperature differences. It moves from high pressure areas to low pressure areas in an attempt to balance out the differences in temperature

i would think form in reponse to tensional forces