Geography. FRONTS

1. Chapter 8 Lecture
© 2015 Pearson Education, Inc.
Weather
Part 2
Fronts
Geosystems
9th Edition

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Part 2 Fronts
• In this section:
• Cold fronts
– Squall line
– Berg winds
– Coastal lows
• Warm fronts
• Occluded fronts
• Stationary fronts

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1. Cold Front
• A cold front occurs where a large mass of cold air meets a
mass of warmer air, and the cold air advances on the
warmer air.
• The cold air undercuts the warm air pushing it upwards.
• Cumulonimbus clouds form a well-defined line along the
boundary between the air masses.

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1. Cold Front
• As the cold front passes, the air temperature may become
noticeably cooler, with temperatures dropping by 5 °C or
more within the first hour.
• Rain, gusty winds, and, sometimes, thunderstorms
occur with the passage of the cold front.
• On a synoptic chart, a cold front is represented by a solid
line with blue triangles along the front pointing towards the
warmer air and in the direction of movement.

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1. Cold Front
• Cold air is more dense than warm air
• Cold air forces warm air up.
• Days before arrival of a cold front – cirrus clouds, shifting
winds, T°drops, pressure drops
• Up to 400 km wide
• Precipitation is behind the cold front and often heavy
precipitation and thunderstorms

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Fronts
• Video: Cold warm occluded stationary – types of weather
fronts
• Duration: 3:07
• https://www.youtube.com/watch?v=-ez4QoQLnZ8
• Video: What exactly is a cold front?
• Duration: 3:53
• https://www.youtube.com/watch?v=YJCW4zuoMM0
• Video: Weather fronts explained
• Duration: 1:52
• https://www.youtube.com/watch?v=naarbGHoAGU

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Squall Line
• Fast advancing cold front can cause violent lifting creating
a squall line slightly ahead of the front creating high winds
and intense storms.
• Squall lines generally form along or ahead of cold fronts
and can produce severe weather in the form of heavy
rainfall, strong winds, large hail, and frequent lightning.
• Video: What is a squall?
• Duration: 1:51
• https://www.youtube.com/watch?v=JKR910Dyc2c

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Squall Line

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Cold Front

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Berg Winds
• Berg winds are warm, dry, gusty winds that blow from
the SA plateau towards the coast in winter.
• In winter, when there is a strong HP cell over the interior
and a LP cell at the coast (coastal LP), wind spirals
downwards and outwards around the HP.
• As the wind descends from the plateau to the coast, it is
heated by compression, arriving at the coast as a hot,
dry wind (sometimes over 35 ºC) and can last for two or
three days.
• Berg winds precede Coastal Low Pressures
• Video: South African Berg Winds
• Duration: 2:58
• https://www.youtube.com/watch?v=Tt4mqc8XVAM

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Coastal Lows – South Africa
• When a depression and its
attendant cold front passes over
South Africa, there is a
predictable sequence of
weather events throughout the
country.
• Map1 shows warm Berg Winds
over the KZN coast.
• Ahead of the coastal low (L1),
the weather along the KZN
coast is fine, but behind it
onshore, foggy weather is likely
to be experienced.
• Because a cold front has
invaded from the Atlantic, the
weather over the Western Cape
will be cold and wet.

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Coastal Lows – South Africa
• The subsequent maps trace the
advance of the cold front across SA
over the next few days.
• Map 2 shows the weather about one
or two days later.
• By this stage, the Berg winds over
KZN have been replaced by cold,
southerly air flows which may bring
snow to the Drakensberg and Cape
mountains.
• If the cold air moves inland, as it often
happens in winter, the Free State, and
the northern provinces will experience
a cold snap for a few days.
• The same airflow will cause south-
easterly winds to blow over Cape
Town.

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Coastal Lows – South Africa
• A day or two later (Map 3), the front has moved well to
the east and the Berg wind cycle has started again on
the west coast.
• While warm northeasters blow offshore over the west
coast, those on the east coast may experience cold
weather due to the passing front.

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Coastal Lows – South Africa
• The cycle is completed when the Berg winds and a
coastal low have reached the southeast coast (Map 4)
and a new cold front approaches the Cape.
• Some fronts are very cold, bringing wet weather and
even snow to the Cape, where others are poorly
developed and only bring a slight drop in temperature.

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Coastal Lows – South Africa
• Develops with Berg Winds.
• Imagine cells of low pressure as slow
moving shallow spirals of air, about 200 –
500 km wide, and with the spiral centred on
the coast.
• Starting on the west coast, the spiral moves
over a period of six to eight days around
SA’s coast as far as northern KZN, and then
disappears.
• Often, the coastal migration is squeezed in
a few hundred kilometres ahead of an
advancing cold front.
• When a coastal low advances from west to
east, the weather along the coast is
predictable – typified first by a period of
clear dry weather as the air ahead of the low
is moving offshore.
• This is followed 6 to 18 hours later by
cooler, foggy, and rainy weather as the LP
pushes sea air onshore.

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Coastal Lows – South Africa

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2. Warm Front
• A warm front is defined as the transition zone where a
warm air mass is replacing a cold air mass.
• The air behind a warm front is warmer and more
moist than the air ahead of it.
• When a warm front passes through, the air becomes
noticeably warmer and more humid than it was
before.

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2. Warm Front
• The leading edge of an advancing warm air mass is
unable to displace cooler, passive air which is dense
along the surface.
• Instead the warm air moves up and over cold air
creating a temperature inversion
• This can be 1000 km wide

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2. Warm Front
• First before the warm front arrives the pressure in
area starts to steadily decrease and temperatures
remain cool.
• The winds tend to blow north to northeast in the
southern hemisphere.
• The precipitation is normally rain, sleet, or snow.
Common cloud types that appear - stratus, cumulus,
and nimbus clouds.
• The dew point also rises steadily
• While the front is passing through a region
temperatures start to warm rapidly.
• The atmospheric pressure in the area that was
dropping starts to level off.

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2. Warm Front
• The winds become variable and precipitation turns
into a light drizzle.
• Clouds are mostly stratus type clouds formations.
• The dew point then starts to level off.
• Heavy precipitation occurs ahead of the warm front
• Lighter, wide-spread precipitation for a longer period of
time follows the front, thereafter clear skies
Symbolically, a warm front is represented by a solid
line with semicircles pointing towards the colder air
and in the direction of movement.
• If warmer air is replacing colder air, then the front
should be analysed as a warm front.
• If colder air is replacing warmer air, then the front
should be analysed as a cold front.

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2. Warm Front

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3. Occluded Front
• When a cold front
overtakes a warm front
• The cold air mass from the
cold front meets the cool air
that was ahead of the warm
front.
• The warm air rises as these
air masses come together.
• Occluded fronts usually
form around areas of low
atmospheric pressure.

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Occlusion (Southern hemisphere)
• When 2 distinctly different air masses meet, they mix very slowly
and begin to form a barrier between them.
• When the cold air meets the warm subtropical air, a front forms
between them.
• The front does not remain a simple boundary at points along it,
some of the warmer air pushes against it, ‘denting’ it and rising over
the colder, heavier air.
• Where a ‘dent’ occurs, depressions (LP cells) develop and the air
starts to move in a circular pattern.
• The warm air rises and spirals toward the centre, advancing
poleward (south), while the cold air moves equatorward (north) by
pushing under the warm air.
• As the rising air is lifted and cooled, it gives rise to clouds along both
the warm and cold fronts.
• Unsettled, rainy and even stormy weather will accompany the
depression as it moves eastwards with the frontal system.

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Occlusion

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3. Occluded Front
• There is often precipitation along an occluded front
from cumulonimbus or nimbostratus clouds.
• Wind changes direction as the front passes and the
temperature changes too.
• The temperature may warm or cool.
• After the front passes, the sky is usually clearer and
the air is drier.
• Video: What are Weather Fronts? Warm Front, Cold
front? | Weather Wise
• Duration: 2:18
• https://www.youtube.com/watch?v=5Cnx5Bzctas

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3. Occluded Front
• An occluded front is a composite of two frontal systems
that merge as a result of occlusion.
• Cold fronts generally move faster than warm fronts
(sometimes double the speed)
• As a result, a cold front will overtake an existing warm
front.
• an occluded front forms as the three air masses meet.

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3. Occluded Front
• When the cold air pushes
underneath the warm air, it
lifts the warm air up from
the ground, which makes it
hidden, or "occluded.“
• These fronts are
symbolized on a weather
map by a purple line with
both semicircles and
triangles on it.

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3. Occluded Front

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4. Stationary Front
• A stationary front forms when a cold front or warm front
stops moving.
• This happens when two masses of air are pushing
against each other but neither is powerful enough to
move the other.
• Winds blowing parallel to the front instead of
perpendicular can help it stay in place.
• A stationary front may last for days.
• If the wind direction changes the front will start moving
again, becoming either a cold or warm front. Or the
front may break apart.
• .

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4. Stationary Front
• Because a stationary front marks the boundary between
two air masses, there are often differences in air
temperature and wind on opposite sides of it.
• The weather is often cloudy along a stationary front and
rain or snow often falls, especially if the front is in an area
of low atmospheric pressure.
• On a weather map, a stationary front is shown as
alternating red semicircles and blue triangles
• The blue triangles point in one direction and the red
semicircles point in the opposite direction.

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4. Stationary Front

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End of Part 2
• Online class test this week Thursday 20
April covering Parts 1 and 2 