Weather occurring at low levels in the atmosphere (considered to be altitudes of 24,000 ft. or below) is of great significance to the single engine aircraft that fly there, and to the multi-engine aircraft that pass through when making an ascent or descent. It affects things like visibility, aircraft lift, and even the technique used by a pilot when operating a plane.
Precipitation & Pollution
Most of us have heard the expression "flying blind" before, but in aviation this phrase takes on literal meaning. Events like heavy precipitation, haze, smoke, fog, or the presence of low clouds can impede visibility, making it impossible to view the horizon--a pilot's only point of reference in the sky.
Visibility over horizontal distances isn't a pilot's only concern. There's such a thing as vertical visibility too--the ability to see above and below the plane's position. Vertical visibility is expressed by the meteorological term cloud ceiling, that is, the observable distance between the surface and the bottom of the lowest cloud layer of a broken or overcast sky.
Some atmospheric conditions threaten flight safety, not by affecting a pilot's ability to operate the aircraft, but by endangering the aircraft itself. Two such examples of this are icing and turbulence.
Winter travelers may be most familiar with the concept of airframe icing; it's the reason why planes are routinely de-iced before take-off. But this danger isn't just limited to the winter season. Temperatures in the upper atmosphere are always below freezing (32°F) at varying locations and at all times of the year.
Icing occurs when moisture from cloud droplets freezes onto the surface of a plane. The ice particles reduce lift--the very mechanism by which planes fly--by disrupting airflow over the wings and tail, and can ultimately lead to loss of aircraft control. For this reason it's essential that pilots know at what altitude and at what locations freezing temperatures exist.
Turbulence is another atmospheric danger that can cause structural damage to aircraft. The sheer force of air moving in many different directions and at varying speeds can lead to a "bumpy" flight, and in extreme cases, can even cause an aircraft to break apart.
Two kinds of turbulence are common at low levels: thermal turbulence whose rising air currents are brought on by heating of the surface, and turbulence caused by surface winds blowing over irregular landscapes including mountains, cliffs, and densely populated forests. Turbulence is impossible to detect with the naked eye. Because of this, one of the best practices for avoiding it is to know where thunderstorms and other areas of atmospheric instability are, so that these can be circumvented.
Weather Conditions and Flight Rules
The Federal Aviation Administration’s (FAA) National Airspace System (NAS) has identified a "weather threshold" to outline the minimum acceptable conditions in which a flight can be safely operated by hand versus by automation. Two sets of conditions, one above and one below this minimum, exist.
Visual Meteorological Conditions (VMC):
When fair to good weather prevails, conditions exceed the threshold and visual meteorological conditions (VMC) are present. Under VMC, aircraft are authorized to operate using visual flight rules (VFR); this means that pilots rely on eyesight to see/avoid obstacles and maintain appropriate aircraft separation (the physical distance between it and neighboring planes).
VMC requirements vary depending on country of operation, airspace type, and flight time of day (some countries allow VFR flights at night). VMC requirements for the United States are as follows:For flights in class C and D airspace, and E airspace when at or below 10,000 ft:
- Visibility must be 3 statute miles or greater
- A horizontal distance of 2,000 ft from clouds must be maintained
- A vertical distance of 1,000 ft above and 500 ft below clouds must be maintained
- Visibility must be 5 statute miles or greater
- A horizontal distance of 1 statute mile from clouds must be maintained
- A vertical distance of 1,000 ft above and below clouds must be maintained.
Instrument Meteorological Conditions (IMC):
Periods of rain, low clouds, reduced visibility, or other inclement weather usually mean the onset of instrument meteorological conditions (IMC). IMC generally exist whenever visibility drops below 3 statute miles and the ceiling is less than 1,000 ft. During these times, pilots must fly under instrument flight rules (IFR) and thus, rely on flight instruments located inside the cockpit to tell them information about the flight situation (such as altitude, speed, and direction).