I'm not a pilot, so I always wonder when I'm flying why does it happen... Is it because of bad weather, or something else?
Air turbulence is very much the same as the currents of a river.
When a parcel or stream of air moves differently than the area around it, you get turbulence as you transition between them.
An example would be if you are outside on a windy day but stand behind a tree to "get out of the wind". If you step out from behind the tree, you will feel a sudden "jolt" as the wind hits you. If it's a particularly strong wind, it could even cause you to lose your balance for a second until you compensate for it. Well, these invisible air currents affect an aircraft in much the same way.
It isn't normally dangerous, and I think of it like driving down a bumpy road. Of course, it's usually even safer in an airplane because even if you hit a big "pothole" you don't have to worry about it throwing you off the road into a tree.
The actual movement of the air can be caused by several things:
- Wind hitting something (like a building or a mountain) and displacing it (just like rocks in a river).
- Air being heated by the ground which then causes it to rise.
- Weather activity like fronts, thunderstorms, or jetstreams.
- An aircraft moving through the air will cause turbulence of its own (known as wake turbulence).
What is turbulence?
random and stochastic
Each realization of a turbulent flow is unique and fluctuations in velocity are very irregular in space and time.
Turbulence cannot maintain itself and will decay into laminar flow without energy input from the environment (e.g. mean shear in the velocity field or buoyancy). Reducing viscosity in a flow does not remove dissipation, it just moves dissipation to ever decreasing scales.
A time-averaged scalar transported in a turbulent flow will have larger plume than that of a laminar flow. For example, if you injected a dye into a smooth flowing river and one into a turbulent flowing river, the dye plume would cover a larger area in the turbulent flow.
3D vortex dynamics play a very important role in turbulence and for most cases you cannot represent a turbulent flow as a 2D problem.
occuring on a wide range of scales
This is perhaps the most important distinction. When I say scale, I am referring to a characteristic length and velocity of an eddy, or swirl. Turbulence is produced at larger scales and the transfers its energy to smaller and smaller scales. At some (very small) scale, dissipation starts happening and the turbulence decays. A feature of a turbulent flow is energy at all scales, which basically means there are whirls of all sizes and it is the interaction between all of these swirls that gives turbulence.
How does it happen?
The main production of turbulence is through mechanical production and buoyant production. Turbulence happens because energy is being put into it via these production methods. Turbulence will decay on its own but will not stop until production ceases.
Mechanical production can be envisioned as a shear flow, or air of two different speeds interfacing. This can also be wind against a wall, or around some object. The velocity gradient at the periphery of the jetstream is a shear flow, and the boundary layer that we live in is a sheared environment on most days.
Buoyant production is through buoyancy. Relatively warm parcels rise and this is balanced by subsidence and if you follow these thermals and the descent between them and squint hard enough, you can envision eddies on the largest scales.
Both of these methods put energy into turbulence at large scales. For boundary layer flows, this can be on the order of a kilometer or more, typically the height of the boundary layer. These big eddies will have perturbations of their own and these smaller perturbations are the next smaller scale. Energy is cascaded downward until the sizes of the eddies are small enough that viscosity dominates over inertial forces and the energy is then lost from the turbulence as heating. The superpositioning of these eddies at all scales resembles a jumbled bumpy mess, and this is precisely how we experience turbulence.