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Fronts and Lows
Weather in the mid-latitudes can be quite changeable. One day might be
mild and sunny, the next cold and clear, the next snowy and windy. These
changes in weather result mostly from the movement of low-pressure
systems and their associated frontal systems.
What Is a Front?
20.2
KEY IDEA
The movements of fronts and
lows greatly influence the
weather at mid-latitudes.
KEY VOCABULARY
Air masses of different types don’t easily mix. The boundary that separates
opposing air masses is known as a front. The width of a front can range from
200 meters to 200 kilometers. It can be as high as 5 kilometers and as long as
2000 kilometers and can affect weather patterns in areas hundreds of
kilometers wide. Fronts are most common at mid-latitudes, where
southward-moving polar air masses and northward-moving tropical air
masses often meet.
The air masses on either side of a front may differ in temperature, in
humidity, or in both. At the front, the less-dense air mass is forced to rise
over the denser air mass. As a result, the front is roughly wedge shaped, as
shown in the illustration below, which depicts warm air being forced to rise
over advancing denser and colder air. You will learn about the
characteristics of different kinds of fronts later in this section.
Fronts can have steep slopes. The slope may range from about 1 over 50
to 1 over 300. A slope of 1 over 50 means that the frontal surface rises 1
kilometer for every 50 kilometers of horizontal distance. Such a slope is much
steeper than one that rises 1 kilometer for every 300 horizontal kilometers.
Fronts usually bring precipitation. At the frontal surface, the less-dense
air rises high into the troposphere. The air cools as it rises, and if the air is
humid enough, clouds and precipitation form.
• front
• cold front
• warm front
• occluded front
• stationary front
Front
Warm
air mass
Frontal
surface
Length
of front
Advancing
cold air mass
At this front, warm air is being forced to
rise over advancing denser cold air.
Chapter 20 Weather
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