A front occurs on the boundary line of an
air mass (the same quantity of temperature or vapor has a
lump of air. For example, a cold air mass, a warm air mass and so on.), and an
air mass. On the boundary line, it has a large difference of temperature, and a
wind suddenly changes in many cases. A front is classified into four kinds by
the influence of the air mass which collides mutually. They are a warm front
which the influence of a warm air mass moves to the cold air mass side
strongly, the cold front which the influence of a cold air mass moves to the
warm air mass side strongly, an occluded front made with development of an
extratropical cyclone, and stationary front with about the same influence of a
cold air mass and a warm air mass.
The
air which generally flows in from low latitude is warm, and the air which flows
in from high latitude is chill. Furthermore, in the Northern Hemisphere, since
an air lump generally flows the surroundings of a low pressure into the center
of a low pressure with the surroundings counterclockwise, as shown in Figure
No.1, a front is formed by warm air and chill air, in the beginning. An
occluded front will be formed, if this low pressure glows and the cold front
catches up with a warm front (Figure No.2).
No.2
・A warm front
It
is a front when the warm air mass is superior compared with a cold air mass. As
shown in Figure No.3, a warm air mass runs aground on a cold air mass. For this
reason, cirrus appears over the front of a direction of movement 1000 km or
more away from the front, and it becomes low clouds as a front is approached. Rain
and snow usually fall continuously from a front to about 300 km of fronts of a
direction of movement.
No.3
・A cold front
It
is a front when the cold air mass is superior compared with a warm air mass. A
cold air mass is hidden under a warm air mass, as shown in Figure No.4, and it
moves forward. For this reason, a warm air mass is raised compulsorily near a
front, a cumulonimbus cloud is made, and a strong rain falls in a short time. At
the time of passage of the cold front, temperature may fall rapidly, with a
gust by a direction of wind may change suddenly, and thunder may occur .
No.4
・An occluded front
It
generates, when an extratropical cyclone grows and the cold front catches up
with a warm front. When the cold air mass on the backside of the cold front
which caught up is colder than the cold air mass which is ahead of the preceded
warm front, it becomes an occluded front (Figure No.5) of a chill type, and an
occluded front which is warm types if warmer.
・A stationary front
A
stationary front is a front when the influence of a warm air mass and a cold
air mass matches, and stays in the almost same position. Therefore, it rains
for a long time (Figure No.6). A seasonal rain front is a stationary front.
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