The University Presbyterian Church is on College Road and I get a clear view of the University of Alaska Fairbanks on my way there. Today, I noticed an unusual structure in the cloud of exhaust coming from the power plant.
It is common to see the hot, moisture-laden exhaust rise until it reaches an equilibrium altitude, then spread horizontally. The exhaust gases rise because their temperature reduces their density and the difference in density between the exhaust and the atmosphere results in a buoyant force which lifts them upwards. When the exhaust reaches an altitude at which the air density matches the exhaust density, the buoyant force disappears and the exhaust cannot rise further. It simply spreads sideways and dissipates in to the surrounding air.
The uncommon feature in today’s image is the overshooting top. This structure forms when the exhaust gases rise quickly enough that their kinetic energy can do work against the force of gravity to lift them above the equilibrium altitude. When the kinetic energy has been expended, the gases drop back down to the equilibrium altitude before spreading out.
An overshooting top is a common feature in the cumulonimbus clouds we get here in the summer. They are a good indication we will have afternoon showers. Fortunately, the moisture content of the power plant exhaust is to small to pose such a risk. At this time of year, it would result in a localized snow fall!
