How The Existence of Weather Impacts Flight
Since aircraft operate in a three-dimensional environment, sailing in the ocean of air called the atmosphere, it is critical for pilots to understand how the weather affects flying through that ocean. From the very beginning of training, pilots learn how to read meteorological reports, interpret weather depiction charts, and analyze forecast discussions to create a mental picture of the conditions aloft.
Pilots don’t have to have a degree in meteorology, but the more a pilot knows how to use all the different weather information which is available, the safer the pilot will be when making the decision to take a flight. It’s an area in which a pilot should always be updating his or her education.
The Importance of Surface Wind
The most important element of weather is the wind. It’s so important to pilots that wind direction and speed is the first item on an airport’s hourly weather report. Gusty winds can make it more difficult to land, and the pilot must determine if the “gust factor” is too much to handle. Bigger, heavier aircraft are not affected as much by gusty winds as are lighter aircraft in the same way that a big ship isn’t affected as much by choppy water as small boats are.
The wind direction determines which runway to use at an airport. Aircraft must takeoff and land into the wind as much as possible. A headwind reduces the runway distance required to takeoff and land. A tailwind greatly increases the required distance, sometimes to the point of a runway being unusable.
A crosswind makes landing slightly more difficult, yet completely manageable for all but the most novice of pilots. Many aircraft have a crosswind limit, the point at which the control surfaces can’t counter the effect of the crosswind, and the pilot must select a facing a different direction, or go to another airport if there is only one runway.
The Jetstream and Mountain Wave Turbulence
As important as the surface wind is to pilots, the winds at different altitudes must be considered while flight planning. The atmosphere, being the dynamic environment it is, contains different layers and “rivers” of air, such as the jetstream. Due to terrain and other factors, surface winds are usually quite different in speed and direction from even only a few thousand feet up.
For example, a mountain ridge alongside an airport could cause swirling winds across the runway, while only 3000 feet above the ridge it could be perfectly smooth. On the other hand, tall mountains such as the Colorado Rocky Mountains create an effect called mountain wave turbulence, impacting flight conditions tens of thousands of feet above the range.
Mountain wave turbulence is similar to how rocks on the bottom of a river bed cause ripples on the surface. Tall mountains push up the air on their windward side, which in turn pushes up the air in the upper layers of the atmosphere, and then the air spills over the leeward side, creating a “wave” of turbulent air, resulting in extremely bumpy conditions for aircraft in the area.
Wind speed and direction aloft impact the length of time it takes to fly from one airport to another. Due to the jetstream, which flows from west to east, westbound flights must overcome a headwind, like a kayaker paddling upstream a river, whereas eastbound flights have a tailwind, giving them much higher groundspeed. This is why answering the question of an aircraft’s range much begin with asking winds are aloft. The same aircraft which can fly non-stop from California to Florida may have to make a fuel stop on the return trip.
Reflecting its importance to pilots, visibility is the next item on an airport’s weather report. Even pilots who are rated and authorized to fly in low visibility and among the clouds must see the runway at some point to land safely, and pilots operating under Visual Flight Rules (VFR) must have excellent visibility at all times, so the airport’s visibility factor is second only to wind on the weather readout.
An interesting aspect of aviation weather reporting is that wind is reported in knots or nautical miles per hour, yet visibility is measured in statute miles. When visibility drops low enough, optical sensors placed at the ends and middle of the runway measure the Runway Visual Range (RVR) in feet. For pilots landing in foggy, misty, or other low-visibility conditions, a difference of only 100 feet in the RVR value will dictate whether or not they can safely land.
Icing and De-Icing
Another huge impact of weather on flying is icing conditions. Icing takes place at zero degrees Celsius. It happens when visible moisture, usually in the form of clouds, turns into ice when the moisture meets the leading edges of an aircraft flying through it. The ice formations severely disrupt airflow over the wings and tail, meaning the aircraft must fly faster to stay airborne. Eventually, too much ice buildup will destroy the lifting ability of the airfoils. Pilots should always be on the lookout for ice even if their aircraft has anti-ice systems, as it can build quickly and lead to dangerous situations faster than one might think.
Modern jets have anti-ice systems which use hot air from the engines to heat the leading edges of the aircraft, preventing ice from forming. However, icing can still take place under severe conditions. Turboprop and some piston aircraft have pneumatic de-ice systems such as inflatable rubber “boots” which break off ice that has accumulated on the leading edges and propellers.
Other piston airplanes have “weeping wings,” a system which spreads anti-ice fluid on the wings, propellers, and tail, preventing ice build-up. The boots and weeping wings systems aren’t meant to allow an aircraft to remain in icing conditions for extended periods, only to escape from ice after an inadvertent encounter, or to penetrate icing conditions to get through to the other side, such as climbing through a cloud layer or flying an approach to land.