tropical air masses from cold, polar air masses in each hemisphere. Thus, no one
air mass permanently holds a single position and daily weather is variable in most
middle latitude locations.
Even though the width of a front occupies a very narrow distance in contact
with Earth’s surface, it is important that the frontal zone extends up several kilo-
meters into the atmosphere. Air masses of different densities push on each other,
so there is mechanical lift along the frontaloft and this means that significant
cloudiness and precipitation might extend far away from its surface position.
Weathering mapping has internationally standardized symbols indicating posi-
tion of fronts and their directions of movement. A cold front is indicated by a
series of triangles attached to a line (the frontal position) and pointing in the direc-
tion of motion of the advancing cold air. A cold front is the situation where cold air
is pushing on the warm air. Because cold air is denser than warm air, cold air is
able to wedge in under the warm air and the warm air is forced to rise. A cold front
extends into the air, a typical rise over run is 1:50; that is, for each 50 m along
Earth’s surface, the front is another meter away from the surface. Compared to
other frontal types, cold fronts are usually faster (45 to 75 kph) and have steeper
rises over runs resulting in more dramatic uplifts of air, intense precipitation, and
severe weather. As a cold front approaches a weather station in the Northern
Hemisphere, the typical weather would be warm with high dewpoint temperatures,
lowering barometric pressure, winds from the southwest or west, and showery pre-
cipitation in unstable air. As the front passes, the barometric pressure would
steadily rise with temperatures cooling and the wind changing to northwest or
west, and the atmosphere stabilizing. The heaviest precipitation would be close
to the frontal zone and be over in a short amount of time.
A warm front represents the advance of warm air from lower right to upper left.
Because the warm air is less dense than the air it is replacing, the warm air will
overrun the cold air at the surface and rise. In this case, the surface speed of the
front is typically 15 to 40 kph. A warm front extends into the air at a gentler ratio
of 1:80. This means that there is rising air and precipitation considerablyaheadof
the surface position of the front. Precipitation tends to be steady and gentle for up
to a day before the passage of the front at the surface. Additionally, as the warm
front approaches, temperatures are cool, the barometric pressure drops, the winds
are southeast to south, and low clouds are common. As the front passes, the air
warms, the barometric pressure rises, the winds shift to the southwest or south,
and precipitation ceases.
An occluded front usually results from the interaction of cold and warm fronts.
Cold and warm fronts are attached to a common, large disturbance known as a
middle latitude cyclone. Over a number of days, the cold front progresses more
quickly than the warm front and all the warm air mass is raised from the surface.136 Fronts