Turbulence is caused by uneven motion of the air around an airplane- eddies and gusts that hit the plane with varying amounts of force. There are two kinds of turbulence: mechanical and thermal. The two often occur together, adding to each other’s effects. Mechanical turbulence is caused by air moving along close to the surface that is disrupted by something rough, say, a mountain range. The rough ground scatters the air in little currents called eddies. Mechanical turbulence affects a friction layer of the atmosphere up to about 2,000 feet above the surface. Its intensity will vary based on the wind speed and how rough the surface of the ground is.
Thermal turbulence, on the other hand, extends much higher into the atmosphere. It occurs when rising thermals, columns of air warmed by the surface, clash with prevailing winds higher in the atmosphere. The intensity of thermals is dependent upon the surface temperature, as well as height, because temperature decreases with altitude. This is called the lapse rate, and temperature decreases about 3.5 degrees Fahrenheit for every thousand feet into the atmosphere. In the process of convection, warm, unstable air will rise, and cooler, stable air will fall. This is due to the fact that the warmer air has a lower density and will rise above the cooler air, which has a higher density. Thermal turbulence mainly happens in the middle of the day, when the ground has warmed up to its maximum temperature and the unstable hot air rises in convective currents. As the prevailing wind high above the surface hits the thermals, they act like obstacles, and the resulting air currents push in all directions. These eddies hit the airplane and are responsible for causing the bumps.
Thermal turbulence is also one of the main ways energy is exchanged between the surface and the winds in the atmosphere. Through convection, the process brings winds with higher velocity and momentum down near the surface. As a result, wind speeds near the surface are increased during the afternoon, when the lower levels of air are unstable, and decreased at night when the lower levels of air are more stable. Ultimately, thermal turbulence is dependent upon the intensity of the thermals and the speed of the prevailing winds. A faster prevailing wind and stronger thermals will result in more intense thermal turbulence, and a bumpy flight as more eddies and currents hit an airplane. You can expect more thermal turbulence in warmer regions during the afternoon, when convection currents are strongest.