Fronts and Frontal Depressions
3
FRONTS AND FRONTAL DEPRESSIONS Having discussed the differences in air mass types, we are now in a position to look at the synoptic scale weather systems that aff ect us on a daily basis. These occur in the form of fronts and frontal depressions, which are the (bad) weather makers, and are separated by the highs, which relieve us with good weather. First the fronts. At the boundary between two differ ent air masses, there is a line of sharp contrast in air uali ty. At least we refer to it as a line. !n fact there will be an area of some mi"ing, but this is so narrow in comparison to the hundreds of miles of the air mass that it may as well be a li ne. This we call a fr ont. #ur most consistent local e"ample of this is the polar front, separating the polar easterlies from the westerlies. !n theory, if both air masses were either stationary or moving at the same speed, there wou ld be a more or less vertical boundary or front separating them. $ut in reality one air mass is generally moving faster and overtakes another. %hen a cold air mass overtakes a warmer air mass (a cold front) the colder air (being denser) will find its way beneath the warmer air , forcing the warmer air aloft. %hen warm air overtakes cold ( a warm front) the warm air slides up over the cold. %e have called this frontal lifting. %a rm fronts are fairly gradually sloped, while cold fronts tend to be fairly abrupt. %a rm fronts are indicated on weather maps as lines with half circles pointing toward the colder air, and cold fronts are indicated by lines with triangles pointing toward the warmer air. %hy are fronts sl oped& %e mustn't forget f riction. The air at the surf ace is slowed by fr iction, while the air aloft is free of this drag, only being slowed by the drag of the air below it . o the farther we go aloft the faster the air is moving. Thus the vertical cross section shows the air at the surface lagging behind the air aloft. The average slope of a warm front is about *+. The rising warm air (which is probably full of moisture) is cooled adiabatically and condenses, generating clouds and precipitation. At the high, leading edge of the front are high, wispy cirrus clouds (the first indicators of the approach of the warm front). These may be over - mi les ( km) ahead of the surface front. As the front comes closer , the front also lowers, and so do the clouds. The high cirrus first begin to thicken until the who le sky is covered with a thin, high veil of s, which then lowers to As, and on down to . At some point precipitat ion will begin and then you have /s. The and /s clouds will be about - mi., or 0 km., ahead of the surface front . Another visual clue to the approach of a warm front is the behavior of aircraft contrails. 1emember, they are actually skinny clouds of condensed water vapor from the 2et engines. This moisture will evaporate uickly in dry air, but will linger longer in air with high hu midity (high vapor pressure). o if contrails linger a long while, that is a good indication that the air aloft is humid, and that air could n ot be that humid unless it was fairly warm. 301996937.doc
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Transcript of Fronts and Frontal Depressions
7/25/2019 Fronts and Frontal Depressions
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/ote that this whole procession of cloud development and precipitation is dependent on
moisture, so a warm front that is leading a very dry air mass might pass unnoticed.
%arm fronts typically move rather slowly, which coupled with their gradual slope means that
they produce steady rain, dri33le, fog, showers, and clouds over a period of days. There are
combinations of unstable warm air that can produce warm fronts with b clouds, but this is nottypical. %hen this does occur, the cloud progression will be from c to Ac to u and b.
!n the 4 northeast, a warm front is normally the leading edge of a parcel of air from the m5 airmass from the 6ulf of 7e"ico area.
%hen cold air overtakes warmer air, the resulting cold front is also sloped, but is generally much
steeper than a warm front. Typically the slope of a cold front is about *. 1emembering thatthe cold air is forcing its way under the warm air we can see that the tendency would be to form
a wedge that is the reverse of the warm front, but when friction slows the lower air we end up
with a profile that is uite steep and somewhat rounded.
old fronts typically move a good deal faster than warm fronts, and it is this combination of
speed and steepness of the frontal passage that accounts for its more intense nature. old frontsare fast and vigorous. They produce essentially the same amount of lifting as a warm front, but
because it happens more abruptly, the precipitation is heavy and brief.
The approach of a cold front is seen on the hori3on as a dark, towering band of b clouds. %ith
its approach and passage we feel a substantial and abrupt temperature drop, wind shift, and
usually a downpour. This is where violent weather such as sualls, tornadoes and microbursts
may occur.
After the passage of a cold front the weather will be dominated by subsiding, cold air, bringing
colder, clearer weather to the area. !n the 4 northeast, the passage of a cold front generally brings the return of the c5 air mass.
%hen the air masses on both sides of a front are moving parallel to each other, the front itselfwill not move. !n this case we have a stationary front, which is indicated on a weather map as a
line with the half circles on one side and the triangles on the other side. !f any overrunning
occurs, some cloud development and precipitation may occur, in much the same manner as a
warm front.
The final type of front is the occluded front. !n this case, a cold front (remember they move
faster than warm fronts) overtakes a warm front. /ow we have the cold air from behindovertaking the warm air to the point where it meets the cold air in front of the warm. %hat
happens ne"t depends on whether the cold air behind is more cold or less cold than the cold air
ahead.
8ooking at figure 9: we can follow this process. First we have a typical frontal depression
with the warm front leading a parcel of warm air and meeting colder air ahead, followed be a
cold front at the leading edge of the following cold air mass. as the cold front catches up to the
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warm front, a point is reached where the cold front meets the cold air ahead, having completely
lifted the warm air off the ground. /ow we have an occluded front aloft, and at the surface we
have a cold front meeting cold air.
!f the cold air ahead is not as cold as the cold air behind, the cold front will push under the cold
air ahead. This is known as a cold9type occlusion, or simply a cold occlusion. This is the mostcommon type of occlusion east of the 1ockies where the very cold c5 air is overtaking the
slightly less cold air over the tates.
!f the cold air ahead is colder than the cool air behind, the cool air behind will be forced up and
over the cold air ahead. This known as a warm9type occlusion, or simply a warm occlusion, and
is more common west of the 1ockies. Here the relatively mild m5 air from the 5acific overtakes
the relatively colder air over the land.
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