1) The document discusses fronts and mid-latitude cyclones, including the formation and characteristics of warm fronts, cold fronts, stationary fronts, and occluded fronts. 2) Mid-latitude cyclones form in zones along the polar front where there are strong temperature gradients and converging winds. They develop through the formation and movement of different air mass fronts. 3) The movement and interaction of fronts within mid-latitude cyclones is responsible for much of the day-to-day variability in weather conditions in mid-latitude regions like Northwest Europe.

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

2. ENVI 1400 : Meteorology and Forecasting : lecture 4 2
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 4 3
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
~10
km

4. ENVI 1400 : Meteorology and Forecasting : lecture 4 4
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
~10
km

5. ENVI 1400 : Meteorology and Forecasting : lecture 4 5
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 4 6
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

7. ENVI 1400 : Meteorology and Forecasting : lecture 4 7
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

8. ENVI 1400 : Meteorology and Forecasting : lecture 4 8
cold air
warm air
cool air
Cold Occlusion

9. ENVI 1400 : Meteorology and Forecasting : lecture 4 9
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

10. ENVI 1400 : Meteorology and Forecasting : lecture 4 10

11. ENVI 1400 : Meteorology and Forecasting : lecture 4 11
31-08-2000 : 1310 UTC

12. ENVI 1400 : Meteorology and Forecasting : lecture 4 12
• 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
formation
cloud

13. ENVI 1400 : Meteorology and Forecasting : lecture 4 13
• 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

14. ENVI 1400 : Meteorology and Forecasting : lecture 4 14
• Low starts to weaken as
inflowing air ‘fills up’ the low
pressure
• Low continues to weaken,
clouds break up

15. ENVI 1400 : Meteorology and Forecasting : lecture 4 15

16. ENVI 1400 : Meteorology and Forecasting : lecture 4 16
A
B
A
B

17. ENVI 1400 : Meteorology and Forecasting : lecture 4 17
A
B
A
B

18. ENVI 1400 : Meteorology and Forecasting : lecture 4 18

19. ENVI 1400 : Meteorology and Forecasting : lecture 4 19
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

20. ENVI 1400 : Meteorology and Forecasting : lecture 4 20
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

21. ENVI 1400 : Meteorology and Forecasting : lecture 4 21
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 4 22

23. ENVI 1400 : Meteorology and Forecasting : lecture 4 23
L
L
500mb surface height (dm)

24. ENVI 1400 : Meteorology and Forecasting : lecture 4 24
upper wind
A
B
C
D

25. ENVI 1400 : Meteorology and Forecasting : lecture 4 25
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

26. ENVI 1400 : Meteorology and Forecasting : lecture 4 26
0°
30°
60°
Polar Front
Mid-latitude
Jet Stream
Tropical
jet

27. ENVI 1400 : Meteorology and Forecasting : lecture 4 27
30
60
80
Major Frontal Zones Northern Hemisphere Winter
Atlantic
Polar Front
Pacific Polar Front
Canadian
Arctic Front
Atlantic/Asiatic
Arctic Front
Mediterranean
Front

28. ENVI 1400 : Meteorology and Forecasting : lecture 4 28
500 hPa height (m), and temperature anomaly (C)