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Easterly winds bring dry weather
Low pressure systems influence and are influenced by westerly winds – bring storms particularly in
autumn
Fog:
‘Cloud on the ground’ – official visibility less than 1000m – realistically its 200m
Requirements:
High humidity – relative humidity at 100% (double that of surrounding air)
Temperature no more than 2.5 degrees above dew point
Available hygroscopic/ condensation nuclei
2 types:
Radiation:
Stationary air cooled by a ground surface that cools, usually overnight – occurs fastest when no
cloud-cover to contain emitted long-wave terrestrial radiation
Cold surface common in mountainous areas i.e. snow reflects solar radiation
Advection:
Common in coastal areas – warm air above ocean meets cold air above land – NAD warms air above
it, especially in winter due to different specific heat capacities – air has 100% relative humidity over
ocean – cooled by contact with cold ground + air over land – condenses forming ‘cloud on the
ground’
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Factors affecting climate:
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UKs position:
Warm tropical air to south, cold polar air to the north
Sub-polar jet stream can migrate to being over the UK
Surrounded by water – NAD on west coast
Latitude:
Insolation more direct at equator and tropics than at poles – insolation more diffused at poles
Higher latitude – colder it is
Variations in day length minimal at tropics (12hrs each) and greater at poles (times of 24hr day and
night)
Angle of incidence looks at height of sun during day – midday sun at tropics high all year
More Northerly parts of UK cooler than the South – may be insignificant due to small length of UK
Altitude:
Temp decreases with distance from sea level – why mountains in Africa can have snow
Adiabatic lapse rate – temperature change with height for a parcel of air that has no exchange of
heat or moisture with the air surrounding it
3 types:
 Environmental Lapse Rate (ELR) – change in temperature with height in the atmosphere –
0.64 degrees per 100m – varies with season – higher loss rate in summer
 Dry Adiabatic Lapse Rate (DALR) – air has not become saturated by cooling to its dew point
(temp when air is saturated with moisture) to relative humidity is less than 100% - 1 degree
per 100m
 Saturated Adiabatic Lapse Rate (SALR) – lower than DALR as when air reaches 100%
humidity condensation creates latent heat, and as condensation occurs more at higher
altitudes, this reduces the effect the altitude has – 0.5 degrees per 100m
Mountainous regions e.g. Scottish highlands and Pennines, cooler than low land areas e.g. Fen lands
Isobars close together = strong winds
Formation:
 Origin – form when cold polar air and warmer tropical air meet - colder denser air mass
undercuts warmer one at a wave like kink in polar front
 Maturity – precipitation forms from rising air cooling to dew point. Low pressure centre
created from rising air. Wind blows anti-clockwise into depression – increase wind strength
as this air rises, making pressure gradient larger
 Occlusion – cold front catches up to slower warm front, forcing warm sector up which cools
it, thus losing its identity – occluded front forms (when the two fronts meet. Winds fade as
pressure difference drops as filling in of low pressure occurs – depression fades away
Conditions:
UK experiences around 100 depressions a year – last from 12 hours to 3 days
Wind direction rotates from SE to NW
Conditions experienced across the UK in the order following
Approach:
Pressure falling, SE wind, winds building, cool temperature, Cirrus clouds (high + thin), no rain
Warm front:
Pressure fall slows, SW wind, strong winds, temperature rising, Nimbostratus clouds (low + thick),
continuous heavy rain
Warm sector:
Pressure troughs, SW wind, winds drop, warm, stratus clouds (low + thick), drizzle
Cold front:
Pressure rises, NW wind, strong gusty winds, temperature falls rapidly, cumulonimbus clouds
(towering + storm), heavy rain + hail/ sleet
Cold sector:
Pressure rises, NW wind, winds drop, cold temperature, clouds clear, heavy showers with sunny
intervals
After this, conditions go back to normal + moderate
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C/S: St Jude’s Storm, ’13:
Formed on 26th October, dissipated 31st
965mb lowest pressure
Atlantic Ocean 3-5 degrees hotter than normal – increased evaporation + humidity
High pressure difference between Iceland and the Azores – jet stream strong from West to East –
240mph across Atlantic, 180mph across Channel
Low pressure system south of Greenland + tropical storm Lorenzo collided 1000miles SW of UK –
blown quickly across Atlantic
Hundreds of trees blown over – 4 people died as a result
660,000+ homes lost power
99mph winds on Isle of Wight
53mm of rainfall in Cardiff – localised flooding
Widespread transport disruptions including double-decker bus blown over on Suffolk
Crane collapsed on Cabinet Office, London
Port of Rotterdam closed for shipping on 28th
Tropical Climates:
ITCZ migrates to Tropic of Cancer in our summer + Tropic of Capricorn in our winter – explains rain
throughout much of year in equatorial zone but distinct wet + dry seasons in tropical areas