Just sent another article to Canadian Aviator magazine. Here it is in its “raw” version.
Temperature Warm Ups (Inversions)
A Weather Warm Up (Inversions)
Temperature Warm Ups Aloft
Any pilot knows temperature decreases about 2°C per 1000 feet, but meteorologically inquisitive pilots want to know more about lapse rates and what exactly is going on aloft. I recently gave two talks to local COPA (AOPA equivalent) chapters on lapse rates. They discovered weather balloons launched twice a day from over 900 sites globally gather information while ascending to about 100,000 feet where the balloon bursts and tumbles back to earth slowed by a parachute. From these soundings air is found to sometimes warm with height. These inversions occur in four different scenarios.
Nocturnal inversion. During the night under light winds cooling is more rapid over land than over water. This nocturnal cooling leads to stability in the lower layers as an inversion develops and may lead to the formation of low cloud or fog. Smoke rising in these inversions spreads out horizontally or even sinks as the warm air seeks the cooler air below. Any place with a smoke stack will depict such an inversion, however, unpleasant smells may ensue. I frequently see those low based plumes from pulp mills while flying into Vancouver from the east. Smoke from wood stoves will also form a plume during such an inversion, but the smell is much more pleasant. Nocturnal inversions generally mean smooth flight conditions, but sometimes non-convective low-level wind shear (LLWS) can be present when a surfaced-based inversion results in the development of a low-level jet maximum at the top of the inversion. This inversion decouples the wind just above the surface and allows the winds to accelerate unencumbered by surface friction. Nocturnal inversions can trap many pollutants and moisture, possibly resulting in IFR conditions. If you fly up north during the Arctic winter (think long nocturnal night) you’ll witness dramatic inversions.
Years ago, at a weather conference in Winnipeg, Manitoba I met a grape grower from the Niagara, Ontario region. His grape-growing operation included hiring a bi-wing aircraft with lots of parasitic drag to churn up the nocturnal inversion, pushing the warmer air to the ground so the grapes would not freeze. Many growers also employ expensive helicopters to do the job.
These inversions can also bend ground-based weather radar beams during early morning. The beams are deflected toward the ground giving false returns called anomalous propagation.
Frontal inversion As warm air overrides a cold air mass, a frontal inversion sets up. At the surface during winter below-freezing temperatures exist, but as one ascends, an above-freezing layer develops, on the order of a few hundred feet to several thousand feet thick. Temperatures then decrease to below zero on top of this inversion. Because of this scenario, snow falling through the above-freezing layer turns to rain. The rain then falls into the below-freezing layer near the surface. Depending on how deep or warm this above-freezing layer is, either freezing rain or ice pellets will form which is conducive to serious airframe icing.
Years ago, during a flight from Halifax, Nova Scotia to Moncton, New Brunswick in a Navajo, we encountered light-to-moderate icing in the climb, but an advancing warm front pushed above-freezing temperatures in a thick layer from 4,000 to 8,000 feet with balmy +5° C temperatures. We stayed in this layer until our descent into Moncton and literally watched the ice melt and wash away.
Subsidence inversion Air sinks within a high pressure system, causing air to heat up due to adiabatic compression. This heating eventually causes clouds to dissipate and is why clear skies are associated with a high pressure system. The sinking (and warming) of the air slows down closer to the surface of the earth, resulting in an inversion in the lowest layer of the atmosphere (typically several thousand feet in height). Clouds may be flattened by this inversion or break up. Stratocumulus is a very common cloud associated with subsidence inversions. Sometimes, this inversion may be so strong that it traps the low-level moisture busting forecasts calling for sunshine!
Often, on descent, I mention the temperature to my flying partner when I suspect a subsidence inversion. I point out that at the cloud top, the temperature will be warmer than the temperature in the cloud. Sure enough, one can watch the temperature sway from, say, plus 6° C at cloud top, to well below freezing a couple of thousand feet inside the cloud. This goes against the standard logic, which says that temperature should increase on descent. The potential for airframe icing exists when the subsidence inversion traps lots of moisture. If you fly near open areas of water such as the Great Lakes during late fall, winter and early spring you may encounter heavy icing conditions in this moisture laden cloud.
The last inversion comes to a surprise for most. Many learn the top of the troposphere (tropopause) has an isothermal layer, but a significant inversion may be present due to warming from ozone in the stratosphere. I am constantly pointing this out to my flying partner (yes, sometimes I get funny looks) and I try to drive it home when teaching new hire pilots who will fly at tropopause heights. The temperature may be -60° C and within minutes it rises to -54°C meaning you flew above the tropopause. On a recent flight from Frankfurt to Calgary at flight level 380 the temperature went from -70°C to -54° C. I haven’t seen such frigid temperatures in a while so I took a picture of the readout. This inversion is why anvils form from thunderstorms.
There are four types of inversions: nocturnal, frontal, subsidence and one at the tropopause.
Inversions imply stability, but LLWS may occur during a nocturnal inversion.
Warm air aloft and cold air below indicates stable conditions.
Canada and Great Britain plot upper air data on charts called tephigrams, but in the USA they are labelled Skew-T log P diagrams.
Many sites and universities supply data from upper air soundings.
Doug Morris is a B787 captain/certified meteorologist.