1. Anticyclones Questions

5. BRITISH ISLES
JUNE 2013

6. June 2013

14. BRITISH ISLES
JUNE 2012

24. BRITISH ISLES
JAN 2011

34. OTHERS

35. Describe and explain the weather
associated with a winter anticyclone in
the British Isles. (8 marks)

36. Describe and explain the weather associated with a
winter anticyclone in the British Isles. (8 marks)
Describe Explain
Clear skies Descending air, Less RH
Calm / light breezes Stable descending air, shallow
pressure gradient
Cold days Angle incidence, solar
footprint, daylight hours
Very cold nights No insulation, cold ground,
long night time
Morning fogs, slow to burn off Temp diff between land and
air, fog blocks insolation

37. Describe and explain the weather associated with a
winter anticyclone in the British Isles. (8 marks)
Level 1 (1-4 marks) (mid-point 3)
• Simple statements of characteristics/causes, lacking
detail or sophistication of either characteristics or
processes.
Level 2 (5-8 marks) (mid-point 6)
• Detailed statements of characteristics and
explanation, possibly with some use of supportive
material. The answer progresses through the level as
more is added at this level. Full mark answers show
breadth of knowledge and understanding.

38. Explain the characteristic features of the weather
associated with a winter anticyclone in the British
Isles. (8 marks)
Winter weather
In winter anticyclones result in:
• Cold daytime temperatures – below freezing to a maximum of 5C;
• Very cold night-time temperatures – below freezing with frosts;
• Clear skies by day and night generally. Low level cloud may linger and radiation fogs
(caused by rapid heat loss at night) may remain in low lying areas;
• High levels of atmospheric pollution in urban areas, caused by a combination of
subsiding air and lack of wind. Pollutants are trapped by a temperature inversion
(when air at altitude is marginally warmer than air at lower levels). Some
anticyclones are described as being “ blocking ” . Such anticyclones establish
themselves over Britain and NW Europe and remain stationary for many days.
Depressions which would normally travel across the British Isles on a westerly air
stream are steered around the upper edge of the high, and away from the area.
Extreme weather conditions are then produced, with dry and freezing weather in
winter.

39. Explain the characteristic features of the weather
associated with a winter anticyclone in the British
Isles. (8 marks)
Reasons include:
• Anticyclones move slowly and may remain stationary over an area for several days
and weeks
• The air in an anticyclone subsides, that is falls from above. Therefore, this air warms
as it falls, producing a decrease in the relative humidity of the air. This in turn leads
to a lack of cloud development, and dry conditions
• Isobars are usually far apart, and therefore there is little pressure difference between
the centre and edges of the anticyclone
• Fog may form due to ground temperatures falling below freezing, resulting in a drop
in temperatures and with it an increase in relative humidity. The air in contact with
the ground will cool first leading to condensation, provided the air is now saturated.

40. Explain the variations of the
weather experienced
under anticyclone conditions

41. Explain the variations of the weather experienced
under anticyclone conditions (8 marks)
Variation Explain
Clear skies Both times if year descending air, Less RH
Calm / light breezes Stable descending air, shallow pressure
gradient
Day time temps – quantify Angle of incidence, insolation footprint,
daylight hours
Night time temps – quantify Both cases no insulation BUT summer –
ground stored heat released to sustain
higher temp.
Fogs / Mist – thickness,
duration
Freezing air can hold very little moisture so
reach saturation far quicker = @ winter

42. Explain the variations of the weather
experienced under anticyclone conditions
• Basic characteristics
– Descending air mass due to convergence at
altitude
– Air gradually moves away from the centre of
the High in clockwise direction.
– Winds are light – low pressure gradient.

43. Explain the variations of the weather
experienced under anticyclone conditions
Despite these consistent characteristics, the resultant weather varies daily and
seasonally.
SUMMER TIME
Daytime
•Insolation heats ground, heats air above – high temps can reach 30oC after
prolonged heating
•Warm air means greater evaporation and reduce relative humidity so clear skies.
•Coastal regions may experience onshore breezes in later parts of the day (diagram
to demonstrate)
Night time
•Lack of cloud cover allows warm air to rise and temperatures drop
•Will still remain in the teens – the ground is warmer to a greater depth so is still
able to keep the air above it warmer during night time.
•However, relative humidity is likely to rise (why?) and this can cause light mist
(advection – why?) and dew on the ground (where & why?)

44. Explain the variations of the weather
experienced under anticyclone conditions
WINTER TIME
Daytime
•Due to low angle of incidence, insolation is spread over larger area so ground does not
heat up very much – possibly only a few degrees above freezing.
•Clear skies still as colder air is not going to cause as much evaporation so there is less
moisture in the atmosphere.
Night time
•Temperatures drop dramatically as the little heat generated from insolation is quickly
lost.
•The ground has not been heated to any depth so cooling effect is more dramatic.
•Due to air cooling, will reach saturation point resulting in radiation fog – especially in
valleys.
•Fog may linger for many hours in the morning as the fog absorbs some of the insolation
but also reflects it preventing it from reaching and heating the ground which is then
unable to heat the air above it (currently saturated) thus clearing the fog.
•Sea mist present near coastlines due to advection fog – temp difference between land
and sea (due to diff specific heat capacity – energy needed to heat by 1oC)