Anticyclones Questions

JUNE 2013

BRITISH ISLES

June 2013

JUNE 2012

BRITISH ISLES

JAN 2011

BRITISH ISLES

OTHERS

Describe and explain the weather associated with a winter anticyclone in

the British Isles. (8 marks)

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

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, lackingdetail or sophistication of either characteristics orprocesses.

Level 2 (5-8 marks) (mid-point 6)

• Detailed statements of characteristics andexplanation, possibly with some use of supportivematerial. The answer progresses through the level asmore is added at this level. Full mark answers showbreadth of knowledge and understanding.

Explain the characteristic features of the weather

associated with a winter anticyclone in the British

Isles. (8 marks)

Winter weatherIn 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 ofsubsiding air and lack of wind. Pollutants are trapped by a temperature inversion(when air at altitude is marginally warmer than air at lower levels). Someanticyclones are described as being “ blocking ” . Such anticyclones establishthemselves over Britain and NW Europe and remain stationary for many days.Depressions which would normally travel across the British Isles on a westerly airstream 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 inwinter.

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 warmsas it falls, producing a decrease in the relative humidity of the air. This in turn leadsto a lack of cloud development, and dry conditions

• Isobars are usually far apart, and therefore there is little pressure difference betweenthe centre and edges of the anticyclone

• Fog may form due to ground temperatures falling below freezing, resulting in a dropin temperatures and with it an increase in relative humidity. The air in contact withthe ground will cool first leading to condensation, provided the air is now saturated.

Explain the variations of the weather experienced

under anticyclone conditions

Explain the variations of the weather experiencedunder 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

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.

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?)

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)