ASK THE METEOROLOGIST
“Why do lenticular clouds form on Mt Shasta?”
Lenticular clouds, or Altocumulus Standing Lenticular (ACSL) form from waves in the atmosphere that develop when fast moving stable air is forced upwards over a mountain. Mountains cause what’s called Orographic Lifting, which means air is forced to rise due to the mountain, this defection upward is like throwing a pebble into a calm pool, creating a ripple effect called gravity waves downwind of the mountain. When there is enough moisture above the mountain when the air rises it cools at the adiabatic lapse rate and if it’s able to tap into enough moisture the temperature of the air parcel is able to cool to the dewpoint and condensation can occur. On the opposite side of the mountain you get sinking air, which warms at the dry adiabatic lapse rate so that the temperature and dewpoint grow further apart. The wave continue to rise and sink downwind of the mountain and lenticular clouds can form with each lift in air but will be smaller than the original lenticular cloud. On a stable day no other lifting occurs in the atmosphere and so you can get a single cloud to form over a mountain while no other clouds are in the sky. This can occur in the unique way of a lenticular cloud, which looks like a spaceship floating over a mountain. Lenticular clouds are also a good indicator of turbulence.