Several forces create this rotational wind movement. Pressure gradient force: The movement of winds from high to low pressure due to the pressure differential is known as the pressure gradient force. Atmospheric pressure is shown on maps by isobars, or imaginary lines connecting areas of equal pressure. The closer together the isobars, the higher the pressure gradient force, and the higher the wind velocity (see Figure a). Pressure Gradient Force Coriolis force: In the absence of all other forces, all winds move from high pressure to low pressure, crossing isobars at right angles, as shown by the purple arrow in Figure a. However, Earth's rotation creates the Coriolis effect, or Coriolis force, causing winds to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Thus, winds moving from high pressure to low pressure are forced to curve in a circular pattern. These circular winds, which blow more or less parallel to the isobars, are called isotrophic winds, as shown in Figure b. Pressure Gradient and Coriolis Force Surface friction: The Coriolis force mostly affects winds in the upper atmosphere, where no other major forces can affect them. In the lower atmosphere, the friction