Prevailing winds don't actually blow diagonally. They blow in a generally horizontal direction, but there are two main factors that can make them appear to blow diagonally when we look at a map:

1. The Coriolis Effect: This is a crucial factor. It's caused by the rotation of the Earth, which deflects moving objects (like air) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection is strongest at the poles and weakest at the equator. Because of this, winds don't flow directly north or south, but are deflected eastward or westward, creating the apparent diagonal movement.

2. The uneven heating of the Earth: The Earth's surface is not uniformly heated, creating differences in air pressure. Air moves from high-pressure areas to low-pressure areas, creating the winds. However, the combination of air pressure differences and the Coriolis effect causes the winds to curve as they move. This curvature can again create the illusion of diagonal movement on a map.

Therefore, the apparent diagonal movement of prevailing winds is a combination of these two factors:

* The Coriolis effect causes a horizontal deflection.

* Uneven heating and pressure differences cause the wind to flow in a curved path.

It's important to note that the diagonal movement is only an apparent effect. The winds are still flowing horizontally, but their direction is modified by these factors.