The heating over the asphalt causes the air to expand, spreading the isobars. Therefore, at the 1 kilometer height, there is a higher pressure over the asphalt than over the grass. This pressure gradient induces the air to flow from the high pressure area toward the low pressure area. As the air leaves the local high, air from the surface moves upward to replace it. This leads to the development of a local low pressure area near the surface. The pressure gradient formed at the surface induces air to flow from the local surface high over the grass toward the local surface low over the asphalt. Again, air leaves the high pressure area, and so air from above (the local low pressure area aloft) flows downward to replace the missing air. Thus, a circulation is created with an east to west motion at the surface and a west to east motion aloft.