Warm air is less dense than the cold air, so the warmer air mass rides up the back of the colder mass, often at a very shallow slope. As the warm air rises, it begins to cool and lose some of its moisture. Warm fronts often are accompanied by precipitation. A cold front occurs when warm air is displaced by colder air. The nose of the cold front is steeper and it forces the warm air to move upward.
Cold fronts move fast, about twice as fast as a warm front.
High Pressure and Low Pressure Atmospheric pressure is the force, or weight, of the atmosphere above us. Differences in heating and the motion of the atmosphere create the differences in pressure. Where cold air masses descend, they create high pressure regions. Where warm air masses are rising, they create low pressure systems. In the winter, the continents cool more than the oceans.