Momentum transfer

Wind has momentum: After all, it is a moving mass of air. The magnitude of this momentum usually varies with height in the atmosphere, because aloft the density is lower but the wind is typically stronger. Convection in the atmosphere can mix the winds from one level with those of another resulting in momentum being transferred between levels.

Such a transfer of momentum is easily seen almost any day around the Lake. In the early morning, the surface wind is light and the water unruffled. As the day progresses, the Sun warms the ground causing convection. This mixes the stronger winds aloft with the gentler surface winds with the result that by mid morning, the Lake has become rough. This behaviour is repeated almost daily. 

Here, I show a different consequence of the transfer of momentum between different levels in the atmosphere: a pileus forming over a cumulus. 

The word, pileus, originally described a felt cap worn by freed Roman slaves to signify their new status. Biologists now apply the word to the cap of a mushroom. Less familiar is the use by meteorologists to represent the cap cloud that forms over a growing cumulus. The meteorologist’s pileus is a result of momentum transfer.

Just as a boat moving through water has to push water aside to move forward, a growing cumulus must push the air above it. The lifting of that air can produce another cloud, a pileus, atop the cumulus. A pileus formed in this way is fairly symmetric.

There is another more important mechanism that produces a pileus. The air in the cumulus has a horizontal velocity characteristic of the lower level in the atmosphere from which the cumulus started growing. Generally this will mean that it is travelling slower than the surrounding air. Just as water in a creek will often flow up and over a bolder in its path and winds that encounter a mountain often flow up and over it, so will the faster moving surrounding air flow up and over the cumulus that lies in its path. As the air rises over the blocking cumulus, condensation can produce a pileus. A characteristic of a pileus formed as a result of these momentum differences is usually asymmetry.

An asymmetric and laminated pileus sits atop a vigorous cumulus. The fact that the pileus is steeper on one side than the other reveals its cause as the difference in momentum between the cumulus and its surroundings. The laminations in the pileus are a consequence of moisture layering in the air.