Unit 4-5: Fronts

What is a “front”? A front is the boundary between two air masses. The “surface” of a front always slopes.  The slope is due to the different densities of the air masses. The warmer and more humid air mass is less dense and will rise above the cooler and less humid air mass.

What is a “front”? Most fronts have a very gradual slope.  An average front slope is 1km of vertical height for every 100km of horizontal distance. Fronts are very important in studying and predicting weather.  Fronts indicate a change in weather.  Fronts almost always bring precipitation.

Types of fronts Cold Front  Cold air mass is moving and displaces a warmer air mass  Cold Front Symbol: Warm Front  Warm air mass is moving and displaces a colder air mass.  Warm Front Symbol:

Types of fronts Stationary front  Two air masses meet, but neither mass becomes displaced by the other.  Stationary Front Symbol: Occluded front  When a cold front moves faster than a warm front and overtakes it.  Occluded Front Symbol:

Warm Front Weather Warm air is rising and pushing cool air in front of it. As the warm air rises, the water in it starts to condense and form clouds. Depending on the cloud level, it may form:  Nimbostratus  Then altostratus  Then cirrostratus  Finally cirrus

Warm Front Weather These clouds may stretch up to 1500km from the base of the front. Nimbostratus clouds will cause precipitation.

Cold Front Weather Cold air pushes warm air forward and up.  This type of front has a much sharper slope. Because of the steeper slope, there is more vertical air movement.  Cumulus clouds form as a result.  Clouds can stretch from about km from the base of the front.

Cold Front Weather If the displaced air is humid enough, then:  Cumulonimbus clouds form.  Large amounts of precipitation. Cold fronts are indicative of:  Fast weather changes.  Sudden rain.  Sharp decrease in temperature.  Strong winds.

Cold Front Weather