This curvature is due to the Coriolis force. The Coriolis force is extremely important for wind directions as we will see later. The Coriolis force is always perpendicular to the direction of motion. Finally at the surface of the Earth friction of air with topographic features can change the direction and speed of the winds. To understand the weather that we get in the US we need to understand the concept of air masses.
All weather happens at the boundaries between air masses.
The boundaries between air masses are called fronts. Warm fronts have warm air moving faster than cold air and colliding with it. Here we tend to have light precipitation at most. Cold fronts form when cold air moves faster than warm air and collides with it. Here we tend to have high precipitation.