The Earth rotates once each day no matter where on the Earth you are standing. This means that its angular speed is constant and equal to 360 degrees every 24 hours. However, the speed of the planet at its surface must depend on how many kilometers the surface has to move to complete a rotation in 24 hours. Near the equator, it has to travel its full circumference at that latitude which is 2 x pi x radius. However, at any other latitude, the circumference of the Earth decreases by the cosine of the latitude so that you get 2 x pi x cos(latitude) x radius. You can see this with a piece of string and a globe of the Earth.