Found this, makes for interesting reading.
When you drive above speeds of 90 km/h aerodynamic forces come into effect on the vehicle, meaning that air passing over, under & around your car has an effect on its performance & handling.
On a windy day, cross winds can blow onto you and alter the way that air passing from the direction of travel holds your vehicle to the road. ie it can blow your vehicle off course because there is more wind from the sides than there is from the front, and your vehicle is more aerodynamic from the front than from the side.