What is the primary cause of mechanical turbulence?

Mechanical turbulence primarily results from friction between the air and the ground. As wind flows over the Earth's surface, it encounters various types of terrain, such as hills, buildings, and trees. This interaction disrupts the smooth flow of the air, leading to turbulent eddies and variations in wind speed and direction. The roughness of the surface increases this friction, enhancing the turbulence experienced near the ground.

In this context, temperature differences and wind speed are factors that can influence turbulence, but they are not the primary cause of mechanical turbulence specifically. Air mass collision refers more to atmospheric conditions at higher altitudes where different air masses meet, potentially causing turbulence, but again, it is separate from the mechanical turbulence caused by surface interactions. Thus, the emphasis on ground friction highlights why it is the correct answer in this context.