Most people who have travelled in a passenger jet have felt the plane slow down and heard the following phrase, “We will be encountering some turbulence ahead, so please stay seated with your seat belt fastened.” The pilot slows the plane down to the planes rated maneuvering speed or below so as to not put too much stress on the airplane when it encounters the turbulence. Where does this turbulence come from and how do the pilots know about it?
There are four types of turbulence that can be encountered. They are Low-Level Turbulence, Clear Air Turbulence, Mountain Wave Turbulence, and Thunderstorm Turbulence. Low-Level Turbulence usually occurs at 15,000 feet or below and can be further broken down into mechanical, convective, frontal, and wake turbulence. Mechanical turbulence occurs when an object obstructs airflow such as buildings or trees; the air gets disturbed and can rise as high as 3,000 feet. When wind approaches perpendicular to hillsides, this is also referred to as mechanical turbulence. Convective turbulence is caused by the heating of the earth’s crust which causes warm air to rise as thermals. Gliders and birds use thermals to stay aloft for long periods of time. Frontal turbulence occurs in front of a fast moving cold front, the updrafts can raise really fast reaching speeds of 1,000 feet per minute. A front that moves over rough terrain will produce moderate to great turbulence. Wake turbulence comes from large, heavy planes operating at slow speeds. The wind comes off the wingtips, creating vortices that flow behind the aircraft and sink to the ground. Air traffic controllers warn of wake turbulence when planes land, but planes are usually separated by either miles or minutes depending on the runways and the sizes of the two aircraft involved. It is also important on takeoffs that planes be separated so no wake turbulence is encountered by the second departing plane.
Clear Air Turbulence usually happens above 15,000 feet, but can happen at any height. This turbulence occurs when one air mass over takes another slower mass of air causing a disturbance. Usually this takes place in the jet stream. Mountain Wave Turbulence is a product of airflow from the windward side of a mountain to the leeward side of the mountain. The air mixes and becomes unstable on the leeward side causing turbulence. When flying over mountains you will feel the ride start to get bumpy because of the disturbance in airflow. Thunderstorm Turbulence occurs when drafts of air change direction rapidly over a short distance. Wind shear, which is the sudden change in wind speed and or direction, can derive from Thunderstorm Turbulence. Wind shear has been responsible for air crashes in the past. One of the worst being at the Dallas/Ft. Worth airport in which an L-1011 aircraft slammed to the ground after encountering wind shear on its final approach, killing 134 of its 165 passengers and one person on the ground.
How do pilots know about turbulence in the area? They have many tools at their disposal. In the aircraft they may have radar which will point out thunderstorms which the pilots will go around. Air traffic controllers will advise pilots of the current conditions at or near the airport. Pilot Weather Reports, or Pireps, are a very good source of information from pilots who have just flown through an area. Reports called Sigmet and Convective Sigmet are reported to pilots in the presence of severe weather. Sigmets are used to warn pilots of weather hazards like icing. Convective Sigmets warn pilots of severe weather such as tornadoes, thunderstorms, and hail greater than ¾ of an inch in diameter.
The main cause of turbulence is the heating of the earth’s crust or Convective Turbulence. If you have ever flown into Phoenix, Arizona on a summer afternoon it calls for one bumpy landing. Although it is impossible to escape turbulence by flying at night or in the early morning hours the amount of turbulence encountered can be mitigated. The next time you are in a passenger plane and feel the plane slow down just before the announcement to stay seated with your seatbelt fastened, check outside the window and try to figure out what kind of turbulence you might encounter. If you are over a mountain range it could be mountain wave or mechanical turbulence, part of the Low-Level Turbulence. If at cruising altitude, usually around 35,000 feet, you may be experiencing Clear Air Turbulence. Either way your pilot will find smoother air ahead, so pry those white knuckles off the armrest and enjoy the rest of the flight.