What is a Front?
A front marks the boundary between two air masses of different density
How and why do mid-latitude depressions form?
Depressions originate at the POLAR FRONT - where the Polar Easterlies (cold air mass from the Poles) blow parallel to the Westerlies (warm air mass from the tropics). The depression begins when the cold air begins to undercut the warm air.
A wave develops on the Polar Front. The warmer, less dense air is forced to rise in a spiral movement (due to Coriolis Effect). This means there is less air at the surface creating lower pressure. Winds begin to blow around it.
Formation of a Depression:
 Dense, moist, cold pm air meets light, moist, warm tm air
 Warm air is forced to rise over the cold air
 Rotational forces of the Earth cause the rising air to twist
 Twisting vortex creates a wave at ground level in the polar front
 Warm front is at the leading edge of the depression
 Behind this is the warm sector
 Following on is the cold front
 As the depression moves east (due to Rossby waves), the cold front catches up with and overtakes the warm front, creating an occluded front
Pressure
Temperature
Clouds/Rainfall
Wind
Other weather conditions
Winter
High
Very cold during the day time, maximum of 5 degrees
None
Calm
Frost & Fog
Summer
High
Very warm, above 25 degrees during the day and at night no less than 15 degrees
None, may trigger thunderstorms
Calm
Heat waves, potential for drought
Blocking Anticyclone: An Anticyclone which remains stationary for days
Anticyclone Fog Type 1: Adevection Fog
 Forms when a mass of warm air passes over a cooler surface.
 Air is cooled to its dew point and condensation occurs.
 Common around coastal areas and over the sea in summer (areas are called a fret or haar)
 As the fog moves inland it is warmed and evaporates as the dew point rises again.
Why is there no precipitation during an Anticyclone?
Air is sinking towards the ground – as it sinks it warms and expands.
This results in the air becoming less saturated as warm air can hold more moisture than cool air.
Therefore less cloud and less rainfall as humidity levels fall
Anticyclone Fog Type 2: Radiation Fog
 Radiation fogs at night (cloudless skies)
 Air is cooled quickly by contact with the ground so condensation occurs as the dew point falls– common in winter – longer period without solar radiation.
 Causes a temperature inversion in valleys
Weather Characteristics associated with Depressions
1) Rainfall
 Warm front: As the warm Tm air rises over the cold Pm air, it cools via uplift and contact to dew point, there is condensation and rainfall. The rainfall intensifies as the WF approaches
 Warm sector: rainfall will stop
 Cold front: The cold air undercuts the warm air and causes more rapid uplift, therefore higher intensity and shorter duration rainfall
2) Temperature
As the warm air passes there will be a sudden increase in temperature. Once the CF has passed it become much colder
3) Winds
Veer from S/SE to SW to NW, becoming very strong as CF passes. Anticlockwise circulation into the centre
4) Clouds
Thicken and lower towards the WF, become scattered in the warm sector and become towering at the CF
5) Pressure
Decreases steadily to the centre of the depression
Depressions – Characteristics:
 Low pressure <1000mb
 Steep pressure gradient from the outside to the centre
 Usually move rapidly W to E across UK
 Strong winds blow inwards in anticlockwise manner
The origins of depressions that affect the UK are found between Greenland and Iceland
– the Icelandic Low.
The depressions follow the pattern created by Rossby waves. They form in the
‘troughs’ as air rushes in from the ‘ridges’ (high pressure).
Why does the temperature difference occur between winter and summer?
In summer the clear skies allows more solar radiation during the day and the heat to escape at night.
In winter cloudless skies means the ground cools quickly at night cooling the air above it – water vapour in this air can freeze forming a frost or fog.
Why do Anticyclones provide the potential conditions for thunderstorms?
On very hot summer days – hot air may rise quickly and then cool – creating large clouds that bring thunderstorms
What type of rain is this called?
Problems caused by Anticyclone weather conditions:
Winter – damage to crops, increase in road traffic accidents, and cancellation of sporting events
Summer – drought, poor air quality, water shortages, heat related deaths
“France’s largest mortuary –set up in an empty food warehouse near Paris – has begun to take in the first victims of the country’s heat wave. The disused fruit and vegetable warehouse is being used as emergency accommodation as part of the
Government’s bid to cope with the growing numbers who have perished in the hot temperatures.” – Paris encountered harsher conditions due to continentality!
16 th August 2003
In 2003 – much of main land Europe was affected by a severe heat wave – Fires swept across France, Portugal and Spain (over 35,000 died from the effects of heat, water shortages.