This document provides an overview of fronts and mid-latitude cyclones. It discusses the different types of fronts including warm fronts, cold fronts, stationary fronts and occluded fronts. It describes how mid-latitude cyclones form along polar fronts and the associated weather patterns. Key features of mid-latitude cyclones include the progression from warm to cold to occluded fronts as the cyclone evolves over time, bringing changing weather conditions to affected regions.

1. Fronts and Mid-latitude
Cyclones
ENVI 1400 : Lecture 4

2. ENVI 1400 : Meteorology and Forecasting : lecture 42
Fronts
The boundary between two
different air masses is called a
front.
It is a region of significant
horizontal gradients in
temperature or humidity.
Typically 100 to 200 km wide –
very sharp transitions are
uncommon.
Fronts are a dominant feature of
mid-latitudes. In particular fronts
associated with low pressure
systems (mid-latitude cyclones,
extra-tropical cyclones,
depressions).
The movement of fronts is
responsible for much of the day-
to-day variability in weather
conditions.
Northwest Europe receives many
different air mass types, with
frequent frontal passages –
results in very variable weather.

3. ENVI 1400 : Meteorology and Forecasting : lecture 43
Warm Front
warm air
cool air
movement
of front
cool air
warm air
nimbo-stratus
alto-stratus
cirro-stratus
cirrus
~300 km ~500 km
• Warm air flows up over denser cold
air
• Inclination of frontal surface is very
shallow: 0.5 to 1°
• Approach of front signalled by high
cirrus or cirrostratus, cloud base
lowering as surface front
approaches.
• Rain starts ahead of surface front,
is widespread and persistent
• Skies clear quickly after passage of
surface front
~10km

4. ENVI 1400 : Meteorology and Forecasting : lecture 44
Cumulo-
nimbus
Cold Front
cold air
warm air
movement
of front
cold air
warm air
~70 km ~200 km
• Dense cold air pushes forward into
warmer air, which is forced upward
• Steeper than warm front: ~2°
• Deep convective clouds form above
surface front, heavy rain in narrow band
along surface front
• Behind front cloud base lifts, eventually
clearing
• Near the surface the cold air
may surge forward, producing
a very steep frontal zone
~10km

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Stationary Fronts
• There is no fundamental difference
between the air masses either side
of warm and cold fronts – the front
is defined by the direction of
motion
• When the boundary between air
masses does not move it is called
a stationary front
• Note that the wind speed is not
zero – the air individual masses
still move, but the boundary
between them does not
cold air
warm air

6. ENVI 1400 : Meteorology and Forecasting : lecture 46
Occluded Fronts
movement
of front
• In general cold fronts move faster
than warm fronts, and may thus
catch up with a warm front ahead –
the result is an occluded front
• There are two types of occluded
fronts: warm and cold, depending
on whether the air behind the cold
front is warmer or cooler than the
air ahead of the warm front
• Cold occlusions are the more
common type in the UK
• Occlusion is part of the cycle of
frontal development and decay
within mid-latitude low pressure
systems

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cold air
warm air
cool air
Warm Occlusion
• In both warm and cold occlusions,
the wedge of warm air is
associated with layered clouds,
and frequently with precipitation
• Precipitation can be heavy if warm
moist air is forced up rapidly by the
occlusion

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cold air
warm air
cool air
Cold Occlusion

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Mid-latitude Cyclones
• Low pressure systems are a
characteristic feature of mid-
latitude temperate zones
• They form in well defined
zones associated with the
polar front – which provides a
strong temperature gradient –
and convergent flow resulting
from the global circulation
31-08-2000

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31-08-2000 : 1310 UTC

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• Low pressure forms at surface
over polar front due to
divergence aloft
• As rotation around initial low
starts, a ‘wave’ develops on the
polar front
• Friction effects cause surface
flow around low to converge
• Mass balance: inward flow
compensated by large-scale
lifting ⇒ cooling ⇒ cloud
formationcloud

13. ENVI 1400 : Meteorology and Forecasting : lecture 413
• Surface low is maintained (or deepens) due to
divergence aloft exceeding convergence at surface
• Flow is super-geostrophic: cold sector air pushes
cold front forward; warm sector air flows up warm
front – warm front moves slower than cold
• Cold front overtakes warm front
to form an occlusion, which
works out from centre
• Depression usually achieves
maximum intensity 12-24 hours
after the start of occlusion

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• Low starts to weaken as
inflowing air ‘fills up’ the low
pressure
• Low continues to weaken,
clouds break up

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A
BA
B

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A
BA
B

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Ana-Fronts
• Air is rising with respect to both
frontal surfaces
• Clouds are multi-layered and
deep, extending throughout the
troposphere
tropopause
cold
warm
cold

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Kata-Fronts
tropopause
Sc
Sc
subsidence inversion
• Air aloft in the warm sector is sinking
relative to the fronts
• Restricts formation of medium & high-
level clouds. Frontal cloud is mainly
thick stratocumulus, it’s depth limited
by the subsidence inversion
• Precipitation is mostly light rain or
drizzle.
cold
warm
cold

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Ana-cold fronts may occur
with kata-warm fronts, and
vice-versa.
Forecasting the extent of
rain associated with fronts is
complicated
– Most fronts are not ana- or
kata- along whole length, or
at all levels within the
troposphere
Some general guidance
may be obtained from
charts of vertical velocity (eg
from NCEP)
For short-term forecasts
(periods of hours) &
‘nowcasts’, rainfall radar
provide the best estimates
of rainfall.

22. ENVI 1400 : Meteorology and Forecasting : lecture 422

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L
L
500mb surface height (dm)

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upper wind
A
B
C
D

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Crossed-Winds Rule
If an observer stands with their back to the surface wind
and estimates the direction of the upper-level winds from
motion of high-level clouds, they can a) estimate their
position within a low pressure system, and hence b)
make a rough forecast:
– If upper wind from your LEFT (position A), the weather is likely to
deteriorate
– If upper wind from you RIGHT (position B), the weather is likely
to improve
– If upper wind is BEHIND or AHEAD of you (positions C, D), there
is likely to be little change in the weather

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0°
30°
60°
Polar Front
Mid-latitude
Jet Stream
Tropical
jet

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30
60
80
Major Frontal Zones Northern Hemisphere Winter
Atlantic
Polar Front
Pacific Polar Front
Canadian
Arctic Front
Atlantic/Asiatic
Arctic Front
Mediterranean
Front

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500 hPa height (m), and temperature anomaly (°C)