12. in middle and eastern United States. Frontal zones are formed on the eastern side of the continent. The lows move along the polar front to the east. The cold, heavier air tends to under-run the warm, lighter air. This is called the zone of contact or surface of discontinuity. There are opposite winds on the two sides of the front (east on cold northern side, west on warmer southern side). When the movement is toward the warm air mass, cold air replaces the warm air (cold front); when movement is toward the cold air mass, warm air replaces cold air (warm front). It is an active front when the warm air is forced upward over the cold air to the point where condensation may occur (relative motion between the two). 4. In a passive front, no clouds form and no precipitation occurs. In a warm front the cold air mass recedes as fast or faster than the warm air approaches. In a cold front, the reverse occurs. A warm front has cirrus clouds well in advance of the front. Then alto-stratus clouds, which can produce snow, follow, in turn followed by nimbus clouds that provide rain just in advance of the front. This is due to warm air moving more rapidly than the cold air beneath. Both cold and warm fronts can be moving at the same time. Fronts are produced by changes of winds, or changes of temperature gradients. Extreme local heating or cooling (large seasonal contrasts) produce thermally-caused secondary circulations. The larger the size of continents in mid-latitudes, the greater the seasonal circulation there will be.