What electromagnetic wave polarization does a Yagi antenna have when its elements are parallel to the Earth's surface?

The physical orientation of an antenna's radiating elements directly determines the orientation of the electric field it produces. When the elements of a Yagi antenna are mounted so that they are parallel to the ground, the electric field they generate is also parallel to the ground. Because an electric field that is parallel to the Earth's surface defines horizontal polarization, the resulting radio wave is horizontally polarized. To achieve vertical polarization, you would need to rotate the entire Yagi antenna 90 degrees so that its elements point straight up and down.