The Flight: Weather Phenomena: Airplane and Airline FAQ

Since clouds restrict visibility, predicting when a clear day will get foggy is an essential part of aviation weather forecasting. If the surface temperature is 50 degrees and the dewpoint is measured to be 49, further cooling of the air will cause fog to develop. As the sun reheats the air, the fog will dissipate.

TYPES OF CLOUDS

There are two types of clouds: cumulus and stratus clouds. Cumulus are the large, fluffy, beautiful puffballs. Because convective air is needed to form cumulus clouds, flying near or in them causes a choppy ride. Stratus are flat, layered clouds, which are usually smooth to fly through.

In grade school it becomes confusing when the cumulus and stratus clouds were given prefixes of alto and cirrus. No more. If a cumulus cloud is labeled altocumulus, the alto simply defines it as a higher altitude cumulus cloud. If a cloud is labeled cirrus, it means the cloud will be found at altitudes in excess of 25,000 feet. Nimbus is added after the cloud type to indicate some intensity of precipitation. A cumulonimbus cloud is a cumulus cloud carrying rain or snow. A pilot would try to fly around this. In the daytime it’s easy. At night pilots depend on their weather radar to “see” the clouds.

WEATHER RADAR

Weather radar signals are emitted from a 30-inch plate in the nose of the aircraft. It detects moisture in the air. Contrary to popular belief, radar can not see clouds, only the moisture contained within. The greater the moisture content, the easier the radar can “see the cloud.”

The weather radar antenna can be tilted 15 degrees up and down and nearly 90 degrees left and right. The newer radars are capable of seeing the weather up to distances of 320 miles in front of the flight path. Weather radar is more accurate at the higher altitudes, since closer to the ground lakes, rivers, oceans, and even buildings in a downtown area can clutter the scope. Weather radar can differentiate the intensity of the rain, snow, or ice by using different shades of green on a single color radar, and by using greens, yellows, and reds on the newer color CRT screens.

TURBULENCE

Turbulence is a major concern of most passengers, particularly the fearful ones. Pilots also dislike rough air, not because of any fear of the plane being damaged, but because we know it is discomforting for the passengers. Flight crews go to great lengths to avoid turbulence, but the occasional unforecasted area of rough air is difficult to avoid. All turbulence is reported to ATC and the airline’s own meteorological department to help the planes flying behind avoid the same bumps.

Turbulence is rated in terms of light, moderate, and severe. In light turbulence, passengers might feel a slight strain against their seat belts. In moderate turbulence, a definite strain will be felt. Severe turbulence can cause a more violent jarring against your seat belt and may even toss some unsecured object, like food service trays. Severe turbulence is extremely rare, and in many ways easier to forecast than lesser grades of chop. Any severe turbulence forecast will be issued and reissued as necessary in the form of a SIGMET (Significant Meteorological Condition).

Even the roughest turbulence seldom lasts a long time—a hard thing to convince passengers as they are being bumped around. Many times a change of route or altitude will lead to smooth air. Think of it like pilots do—turbulence is a tolerated nuisance. Most of the time the air is very smooth. Sometimes, like the ocean, the air can develop waves, similar to the rolling swells at sea. These swells can be small, medium, or large, but just like an oceanic cruise ship, the rocking motion doesn’t upset the aircraft, it only annoys the passengers.

AIR POCKETS

Among passengers who fear turbulence, there is a common fear that a plane can be flying level at 35,000 feet and suddenly fall. To use the ocean analogy again, a cruise ship can be sailing along over swells, bobbing up and down, but regardless of the intensity, the ship never plummets to the bottom of the sea. The same holds true in the air. In choppy air the plane can be bounced around, and it can feel as if it is falling. But it isn’t. People find it hard to believe that even in some of the worst turbulence, a change in altitude is hardly indicated on the altimeters.

WHAT CAUSES TURBULENCE

The cases of turbulence are many and varied. First, there is convective air, which is ordinarily responsible for the bit of choppy air when a plane is within 3000 to 4000 feet of the ground. This can be the result of the earth’s rotation, which creates friction between the ground and the atmosphere and results in changing winds. Buildings and mountains near airports can also alter wind conditions, causing choppy air. And in the desert, uneven heating of the earth’s surface can produce thermals, more rough air. But this, too, is often nothing more than light turbulence.

The second cause of turbulence is usually associated with clouds, air masses, and fronts—the type of weather discussed on the evening news. This is fairly predictable. Cumulus clouds can be full of bumpy air, and avoided. Whenever a weather front is crossed, flying from a warm to a cold front and vice versa, there will be a change in temperature and wind direction, and with that, possible turbulence. Before departure, the flight crew will have studied the various weather charts and determined how to avoid the fronts or cross them at their smoothest altitude and location.

During the flight, pilots look for other clues of impending choppy air. Rapid changes in the outside air temperature, changes in wind speed and/or direction, and changes in barometric pressure, can signal turbulence ahead.

CLEAR AIR TURBULENCE

Clear air turbulence (CAT) is different from the chop associated with clouds and fronts. On beautiful days, when the sky is as clear as can be, the conditions that give rise to CAT can be present. Rapid changes in pressure plotted on isobar charts, marked temperature fluctuations forecasted on isotherm charts, and dramatic changes in wind direction and velocity, especially around the boundaries of the jet stream, are all clues that clear air turbulence is a possibility. Because CAT forecasting is not yet an exact science, if any of the above conditions are present, flight crews exercise precaution and leave the seat-belt light on longer than may seem necessary.

THUNDERSTORMS AND ASSOCIATED TURBULENCE

Thunderstorms give rise to some of the worst turbulence and have to be avoided. Thunderstorms are caused by the mixing of hot and cold air that has become capable of carrying a great volume of moisture to very high altitudes. As the air is churned up and down, again and again, a thunderstorm is born. This very unstable gusty air can give rise to heavy rain showers, hail, lightning, and tornadoes. Individually, thunderstorms can be nasty. Grouped together, they are called a squall line and even worse.

Thunderstorms have definite life cycles. They begin as lower clouds and then build into the upper atmosphere. Their tops can rise above 50,000 feet, and during the storm’s building stage, some turbulence is expected, but it’s the mature thunderstorm that contains the severe weather and worst turbulence. As the convective air that gave birth to the thunderstorm dissipates, so does the violent air.

A thunderstorm near the airport during takeoff is not a threat to the flight’s safety. The plane will simply stay on the ground. If there’s a thunderstorm over the airport during approach, the plane will simply hold a safe distance until it moves away. In cruise, during the daytime, thunder storms are easily circumnavigated visually and by use of the weather radar. At night, pilots rely on the radar alone, but as mentioned, the heavier the storm, the better the radar picture. Above 20,000 feet the rule is to fly no closer than 20 miles to avoid the moderate and possibly severe turbulence. Flying on the upwind side of the direction of the storm’s movement will provide the smoothest ride and least risk of hail. Slowing the airspeed to a predetermined turbulent speed will help smooth out the bumps if any are present, just like a car would slow on a bumpy road.

THUNDERSTORMS AND LIGHTNING

Planes are equipped with white flashing anti-collision wing-tip strobe lights, which, when flying through clouds, are sometimes mistaken for lightning. There have been other times when static electricity builds on a plane and its sudden discharge is mistaken for a strike. In cruise, a plane will not fly closer than 20 miles to a thunderstorm. However, when a plane is within the vicinity, static electricity may build up on the plane, particularly on the nose, which is the area subject to the most air friction. Airplanes have static electricity wicks located on the wing tips and tail. These are points at which the static electricity is discharged. If the static electricity builds up faster than the wicks can steadily discharge it, a rapid discharge accompanied by a white flash might be seen. Anyone who has dried clothes in a dryer without using antistatic products knows about static electricity. Static electricity is not a hazard and does not affect any of the airborne equipment.

Just the word lightning sometimes evokes fear in air travelers. It shouldn’t. First, an airplane is not grounded, so there is nothing to attract lightning to it. Second, lightning strikes are extremely rare and highly unlikely occurrences. In the worst-case scenario of a plane that just happened to be in the path of lightning discharging from a thunderstorm to the ground, a freak strike could occur. Since the plane’s aluminum surface is a very good conductor, the electricity would quickly pass through the aircraft’s skin. The only result would be a very small burn mark.

As for the various systems on the plane, they would be unaffected. The hydraulic system would not be damaged by a lightning strike, nor the electrical system, which is insulated by surge protectors. The fuel system is usually the passengers’ biggest concern, and the possibility exists that lightning could put a tiny pinhole in the wing, and then fuel could very slowly drip out. The chance of this occurring is minute, and even in the worst-case possibility, a divert to a nearby airport is always an alternative. The hazard of a fire or explosion is statistically nil. Jet fuel, stored in the tanks, is not very flammable. It needs to be vaporized, heated to greater than 100 degrees, and mixed with oxygen to burn. A lightning strike would be scary, but it would not be dangerous.

HAIL

Hail is a hazard associated with thunderstorms, and every effort will be made to avoid it. The concern isn’t so much for structural damage as it is for the havoc hail might play with the outside paint job. Most hail is found between the months of April and June, and usually between two and ten P.M. The most severe hail often falls between the Continental Divide in the Rockies and the Mississippi River. Since hail is fairly short-lived, and falls in a narrow band, if a plane gets caught in a hailstorm, the pilot is apt to just push ahead and get it over with quickly rather than turn around and, perhaps, prolong the battering.

ICE

Passengers are often concerned about the buildup of ice on the wings. A buildup could disrupt the airflow over and under the wing, reducing the lift capability and adding excess weight. But ice in cruise is not as great a hazard as perceived. When the outside air is a little above 32 degrees Fahrenheit (zero degrees Celsius), the precipitation is rain. Several degrees below the freezing temperature the precipitation is already frozen as snow. Ice can only form on an aircraft if rain freezes on contact. Since the atmosphere gets colder the higher a plane climbs, there is only a small altitude range where the outside air temperature is actually at freezing. This altitude is called the freezing level. When operating near this freezing level with visible moisture present, the engine and wing anti-ice will be turned on to heat up the critical surfaces and prevent any ice buildup. In the very remote worst-case scenario, with the entire system inoperative and ice accumulating on the plane, a change in altitude away from this freezing level would provide a solution.

CONTRAILS

Nearly everyone has at one time or another looked up in the sky and noticed long white streaks trailing behind a high flying jet. Some last a few minutes, some a long time. These white streaks are called contrails. They are formed when heated hydrogen, a by-product of burned jet fuel, mixes with the atmospheric oxygen to make water. If the temperature is cold enough, the water will immediately freeze into ice crystals. These tiny ice crystals are what you actually see.

Severe weather can be frightening. Despite the fact that airplanes are constructed with the worst weather in mind, pilots develop a healthy respect for the weather from their very first flight. While it is not possible to avoid every little inconsequential bump, every effort is made to steer clear of all significant weather. One of the great advantages of air travel is that circumnavigating bad weather, even when it means flying 100 miles or more off course, adds only minutes to a passenger’s flight time.

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