WX BRIEF

Keeping weather flying rules in mind

Basic atmospheric knowledge that's essential for pro pilot flying.




Surface weather chart for central North America. At a minimum, every pilot should have the ability to read a weather map, as well as be able to visualize what each symbol means for the weather they might encounter on their flight.

Despite the vast amount of detailed scientific understanding of the weather that gives meteorologists the ability to forecast (with reasonable accuracy) conditions associated with a storm system a week in advance, a pilot needs only a small and largely nonscientific subset of this knowledge. No calculus or physics is required. What is needed, however, is the ability to take this knowledge and apply it to what you observe outside the cockpit window.

The troposphere

Nearly all aviation operations are conducted within about 30,000 ft of the surface. This means nearly all flying takes place in the lowest level of the atmosphere, known as the troposphere.

Unlike the rest of the atmosphere, the troposphere is characterized by convection as Earth's surface heats and cools and causes vertical movement of the air above it. Even those pilots fortunate enough to steer their birds through the next higher layer of atmosphere—the stratosphere—where a largely smooth and cloud-free ride awaits, must still climb and descend through the troposphere.

All the wind, clouds, fogs, thunderstorms, icing and other adverse weather phenomena are ultimately the product of this heating and cooling process. As the air heats or cools, its density changes—and, as the density changes, the air starts to move in an attempt to equalize these differences. The movement is the wind. The greater the density difference, the faster the wind.

Similarly, the density of the air also allows it to contain a certain amount of water vapor. The more dense the air, the less room there is for the water vapor, and the excess vapor is condensed into fogs or clouds, and occasionally into rain, snow, or icing on your wings.

The movement of air to equalize density differences also brings about turbulence, which is simply the result of air moving along a boundary. This boundary is frequently air that is either stationary or moving in a different direction and/or speed.

Examples of this include rising thermals on a warm afternoon, updrafts in a thunderstorm, and the edges of the jet stream. The boundary can also be a physical obstacle such as a mountain range or a TV tower. Any boundary—rock, air or steel—will generate friction with the air moving next to it, causing the moving air to sheer off in eddies.

Regardless of the boundary, the intensity of the turbulence is entirely dependent on the strength of the eddy, which is a function of the velocity of the air.

In the atmosphere, gravity ensures that the air is densest at the surface, becoming less dense with increasing altitude. In fact, even though the atmosphere is roughly 328,000 ft (100 km) thick, about half the molecules in it are below 18,000 ft, and about 75% of the atmosphere's mass is in the troposphere.

The reason all the molecules don't wind up pancaked within a few feet of the surface is that the low density aloft means gravity's pull is countered by the force drawing molecules upward from high density to low.

This equilibrium between gravity drawing downward and density pulling upward means that the average density at any level is more or less predictable. Also, because density, pressure and temperature are all related through the Ideal Gas Law, the average pressure and temperature at a given altitude can also be estimated.

It is this estimation that gives us the Intl Standard Atmosphere (ISA)—or values for temperature and pressure at any altitude. In fact, because density is a complex concept relative to temperature and air pressure, and the latter are easier to measure directly, we tend to refer to atmospheric dynamics in terms of temperature and pressure rather than density.

Also, because we can use the predictable nature of the average atmosphere to tell us temperature or pressure at any altitude, we can also calculate lapse rates, such as ambient temperature decreasing about 3.5°F (2°C) per 1000 ft (300 m) of altitude gained, or roughly 1°F every 300 ft (1°C/150 m).

This lapse rate is an important bit of meteorological knowledge for pilots. Lapse rate and surface temperatures can be used to get a quick estimate of the freezing level of the atmosphere or how much of a descent might be necessary to get into above-freezing air—both critical to avoiding and extricating oneself from potential icing conditions.

Of course, the ISA doesn't hold very well around fronts, upper air troughs and ridges, near convective activity, or close to the surface. These are dynamic places where air is being modified actively. These are also the areas where we tend to run into adverse weather conditions.

Ground effect

Within a few thousand feet of the surface, the heating and cooling of the surface either puts heat into the overlying air or draws heat out of it. During the day, solar heating of the surface creates thermals that mix the heat through the surface layer and give us bumpy approaches or departures.

When there is sufficient moisture, either evaporated from the ground or blown in by larger-scale wind patterns, the rising currents contain water vapor that will condense into cumulus clouds when the lifted surface air has cooled to its dew point.

That lapse rate is about 5.5°F per 1000 ft (10°C/1000 m) or, crudely, about 1°F decrease for each 200 ft (181 ft to be exact). This lapse rate can give you a very quick approximation of both the cloud bases and the level above which you can expect a smoother ride.

At night, the opposite occurs, and the cooling surface draws heat out of the adjacent air. This decreases the mixing of the surface layer and often creates a temperature inversion near the surface.

Night-time temperatures close to freezing, which frost your wings at the surface, are also frequently coupled with warmer temperatures just above the surface. The lack of mixing in the surface layer also means that fog which develops in the chilled air is unlikely to dissipate before morning, and the surface layer often also separates from the overlying atmosphere, allowing a low-level nocturnal jet stream to develop a few thousand feet above the surface. As before, turbulence is generally found around the edges of jets at any level.

Nocturnal cooling can be enhanced with any snow cover. This can be important if you fly in and out of any airport situated in a valley or canyon. The cooling of the air once the Sun goes down means that air from the top of the valley is quickly going to become quite dense and will seek equilibrium by flowing downhill.

The downslope wind over a snow-covered canyon airport can become quite strong, with turbulence shedding off the canyon walls providing dangerous gusts.

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