Everyone that flies has occasionally experienced turbulence. This unexpected and unsettling movement of the plane is caused by several factors, but basically it is the result of changing movements in the air, which can happen very suddenly. Planes are very much at the mercy of changes in the atmosphere, and on any given flight are likely to encounter one or more of these conditions that produce a bumpy, and sometimes dangerous, ride. Thermal turbulence is one form.
Turbulence, while it may all seem the same when you are being bounced around in the air, actually results from four main conditions. Mechanical turbulence may occur when planes fly over large obstacles such as mountains and there is a change in wind speed from one side to another. Shear turbulence is caused by the different wind speeds that the plane passes through at various altitudes. And aerodynamic turbulence is the result of one airplane flying in the wake of another, much like a boat is rocked by the wake of another vessel.
Thermal turbulence, is caused by surface heating , or actually, by the columns of heated air that arise from warmer surfaces of the earth. When the plane passes through these heated columns, the plane will most likely experience turbulence. For instance, an airplane flying over cooler water may suddenly experience an updraft when it reaches land that is warmer, or a city, or valley, that gives off a great deal of heated air. Because the air becomes warmer during the afternoon, or the warmest parts of the day, instances of thermal turbulence are generally not as great during the evening hours when temperatures tend to become more moderate. Thermal turbulence increases when the temperature on the surface becomes more intense. In some instances, planes can actually experience both thermal and mechanical turbulence together.
Over all, the intensity of the turbulence produced is divided into four areas, from mild to extreme. In the milder situation, passengers may experience a slight movement of the plane that is barely noticeable. In the second category, anything not anchored down may be moved and walking around may be difficult. At the third level, passengers who are not secured and all other objects may be tossed around and walking is impossible. And, in the worst case scenario, even controlling the plane may be extremely difficult.
By and large, pilots are well aware of these conditions and passengers can be forewarned in many instances. It is always a good idea to keep buckled in whenever turbulence is expected.
http://aviationknowledge.wikidot.com/aviation:atmospheric-turbulence#toc0
