Most fluids exhibit a property called turbulence and its velocity seems chaotic and appears to be random. Because air is also considered a fluid and in the free atmosphere an aircraft and its occupants can and may experience turbulence due to these fluid (air) movements. But most of the time a flight is relatively free of turbulence.
The aircraft responds to these motions and the crew and passengers feel bumpiness during their flight. The amount of bumpiness is based on a number of factors, aircraft type and weight, altitude, terrain and weather conditions.
To minimize the effect the pilot must have a comprehension of turbulence, its background and the effects it may have on the flight. On this page the role of atmospheric stability is investigated as this is a key factor for turbulence creation.
This is the resistance of the atmosphere against vertical motion. For example: if air is forced upwards and is colder (more dense) than its surrouding air it will return to its previous level. On the contrary, if air forced upwards remains warmer it will continue upward without any help. A good indicator of unstable air are cumulus clouds whereas stratus clouds are formed in stable air (with enough moisture).
Meteorologists measure stability by taking vertical temperature soundings compared to height. The stability is then determined by comparing the temperature with a parcel of air (unsaturated) if it was displaced vertically in the same pocket or layer of air.
The temperature variation with height is called lapse rate (LR) and in the international standard atmosphere it is 2 °C (3.5 °F) per thousand feet. A dry parcel of air has a dry adiabatic lapse rate (DALR) of 3 °C (5.4 °F) / 1000 ft. But if that parcel of air is saturated the so called moist adiabatic lapse rate is lower due to the release of latent heat. The amount of saturation will determine the eventual lapse rate (MALR).
From the above we can determine three stability situations: We have unstable air when the LR > DALR or MALR, stable air when LR < DALR or MALR and a conditionally unstable situation when LR = DALR or MALR. And this can vary with altitude or air layer.
The four main causes of turbulence are easy to define: convection, surface properties, wind shear and gravity waves.
For convection we need unstable air and it rises because its less dense (warmer). During this ascent the air forms bubble like eddies varying in size and strength. The air can rise to several thousand feet AGL and pilots of gliders will appreciate this. If the air doesn't contain enough moisture or the dew point is not reached when the convection stops, clouds will not form and this is called dry convection.
Remember that any wind will blow the convection and turbulence downwind, so look up-wind to find the source of the convective activity and turbulence.
As you approach for landing you will notice on a windy day that turbulence may increase due to surface roughness. This causes eddies of different size and shapes which depend on the wind speed and direction, air stability and surface features (even hills and small ridges).
Buildings, hangers, tower and wind turbines can cause moderate to even severe turbulence, depending on which aircraft you fly. This is what is called mechanical mixing and also referred to as mechanical turbulence.
Defined as a change in wind speed and or direction and can be made up in two components, horizontal and vertical. It is important to understand that wind is a vector because it has speed and direction. As a result wind shear can be the result from a change in either property.
An aircraft can experience wind shear while maintaining altitude and while changing altitude, the first is called horizontal wind shear and the latter vertical wind shear. Low level vertical wind shear is most common near the surface of the earth due to friction.
Low level horizontal wind shear is caused by the wind interaction with surface features as hills, buildings etc. High level horizontal wind shear finds its cause in mesoscale circulations as fronts, jet stream and the like.
When air is displaced vertically due to horizontal movement over hilly or rough terrain, it tends to get restored into its original location by gravity, especially so when the lapse rate is stable. The result is that these wave like oscillations move away from their origin. The properties of these waves (length, amplitude, speed and direction) are set by the environment and the way they are created.
Mountain waves, shearing gravity waves are some of the turbulence phenomena caused by gravity. Another occurs when two layers of air move with different speed over each other. In the shear/friction zone turbulence will occur and it develops into wave like action with a saw tooth pattern. With enough moisture you may even see this saw tooth when the air condenses due to the rising motion.