Thin air and its problems

High flight levels offer speed but require caution as well.

By Karsten Shein
Comm-Inst, Climate Scientist

Thin clouds pass by at FL330 over the western US. Low temperatures and a lack of moisture ensure that most clouds in the upper half of the troposphere are thin and often composed entirely of ice crystals.

One of the great joys of aviation for me has always been breaking out on top of a thick cloud deck, and seeing hundreds of square miles of clear, deep blue skies above an unbroken sea of billowy, white clouds.

While we must often fly through the soup when we take off or land—and at low levels the weather can often be quite dicey—more often than not, when we reach the flight levels for cruise we leave most of that bad weather beneath us. However, just because we're flying above the majority of the "weather," we're certainly not beyond its reach.

Altitudes above the 18,000-ft pressure level (FL180 and above) contain many of their own weather challenges, and it is important that pilots understand these challenges to get the best rides.

To understand high-altitude weather, we must first know what is happening in the atmosphere to generate that weather. Most of what we consider as adverse flying weather tends to occur in fairly close proximity to the surface.

There is a simple reason for this. First, most of the molecules in the atmosphere are within a few thousand feet of the surface. In fact, half of all the molecules in the atmosphere are below about 18,000 ft.

In among these molecules of nitrogen and oxygen are water vapor molecules. Water is the constituent in a large number of our adverse weather conditions, from clouds, fog and storms to icing and blowing snow. And, since the source of water is Earth's surface, most of the water vapor in the atmosphere can also be found in very close proximity to the surface.

The final reason that most adverse weather is near the surface is that the surface is the lower atmosphere's heating source. The atmosphere itself is fairly transparent to solar radiation, allowing it to pass through and be absorbed or reflected by the surface.

Over water, the absorbed radiation is easily stored but can also be used to evaporate water from its surface. Land, on the other hand, is horrible at holding on to the energy, and releases it quickly to the atmosphere as heat. It is this heating from below that drives the winds and weather in the lower atmosphere.

But, since this heating is dependent on either molecule-to-molecule transfer (conduction) or the physical movement of a heated molecule (convection), the surface's ability to heat higher altitudes is severely limited, especially when most of the molecules that would move the heat around are compacted below about 18,000 ft.

The further one goes from the heat source, the colder it gets. Tropospheric temperatures generally decrease with increasing altitude. This also means that the atmosphere's ability to hold water vapor also decreases with increasing altitude.

In the case of temperature the lapse rate is linear, but for water vapor it is exponential—meaning that the flight levels generally are capable of holding very little water vapor, although convective currents may carry ample liquid water (in the form of clouds or supercooled rain drops) well into the flight levels.

All of this works together to keep what we think of as convective weather limited to the lowest layer of the atmosphere—the troposphere. The top of the troposphere is highly variable and is dependent on the decrease in temperature from the surface. The troposphere tops out where the temperature stops decreasing and remains relatively steady with increasing altitude.

That isothermal region—the tropopause—is the boundary between the troposphere and the stratosphere. In general, the tropical troposphere tops out at around 60,000 ft, while the polar troposphere only extends upward to 20,000–30,000 ft depending on the season (higher in summer).

Over the middle latitudes where most of us fly, the top of the troposphere is around 45,000 ft in summer and 35,000 ft in winter. Because these altitudes are dependent on temperature and therefore on pressure, the top of the troposphere can also be defined by a single pressure level regardless of latitude or season.

With some local variation, the top of the troposphere is usually found around 300 mb (hPa). The thickness of the tropopause is also variable, ranging from a few hundred feet to several thousand. Above that, at roughly 33,000 ft (10 km) or 280 mb, begins the stratosphere.

Heated from above

High cirrostratus deck blankets a decaying storm cell as seen from FL310 over the Polish coast. While most moisture is contained below the flight levels, active convection can push the tops of cumulus into the lower flight levels, while some cumulonimbi will push up against the stratosphere at about FL330. Regardless of altitude, all thunderstorms should be given a wide berth.

The stratosphere is a different animal from the troposphere. We tend to think of it as a region without "weather." This is because, unlike the troposphere, with its surface heating and concentration of atmospheric molecules, the stratosphere contains only about a quarter of the atmosphere's mass, spread over about 130,000 ft of altitude, and is heated from above.

Although the atmosphere is mostly transparent to solar radiation, the ozone layer, which is concentrated in the upper stratosphere, is very good at absorbing short-wave solar radiation and releasing it as long-wave, or heat, energy.

This heating from above means that temperatures in the stratosphere increase with increasing altitude and that, even if there were sufficient water vapor at those altitudes, there would be no convection to generate clouds or precipitation.

This stratospheric temperature inversion is also why the largest thunderstorms flatten out when they reach the base of the stratosphere, forming an anvil top that instead propagates downwind.

What does the structure of the lower atmosphere have to do with flight level weather? Everything. Flight levels are pressure altitudes. They begin at FL180, which corresponds not to 18,000 ft MSL, but rather the variable altitude corresponding to about 506 mb (hPa) or approximately 500 mb—above half the molecules in the atmosphere. Since the top of the troposphere is at roughly 300 mb, in the ISA, that equates to FL320. Above that, flight levels can be considered stratospheric.

At FL180, the fewer molecules and greater distance from the heat source provide an ISA temperature of about –5°F (–20°C), or about 64°F (36°C) colder than at the surface. At FL320, the ISA temperature drops to about –50°F (–46°C).


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