The Marston Vale is subject to temperature inversion where the local topography creates a ‘bowl’ like feature https://www.theguardian.com/news/2016/feb/29/weatherwatch-temperature-inversion-mist-moisture-pollution-high-ground. This was demonstrated during the activity of the Brickworks in the area where visible emissions (which happened to be coloured brick-orange) were being ‘dragged down’ towards ground level. Inversion layers are areas where the normal decrease in air temperature with increasing altitude is reversed – and air above the ground is warmer than the air below it.
Inversion layers are significant to meteorology because they block atmospheric flow – which in turn – causes the air over an area experiencing an inversion to become stable.
Areas experiencing inversions with heavy pollution are prone to unhealthy air and an increase in smog when an inversion is present because they trap pollutants at ground level instead of circulating them away.
Air not subject to a temperature inversion event will decrease at a rate of 3.5°F for every 1000 feet up into the atmosphere. When this normal cycle is present it is considered an unstable air mass – and air constantly flows between the warm and cool areas. As such the air is better able to mix and spread around pollutants.
During an inversion event temperatures increase with increasing altitude. The warm inversion layer then acts as a cap and stops atmospheric mixing. This is why inversion layers are called stable air masses. They occur most often when a warm – less dense air mass moves over a dense – cold air mass. This can happen – for example – when the air near the ground rapidly loses its heat on a clear night. In this situation the ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day.
As in the case of the Marston Vale – topography can also play a role in creating a temperature inversion since it can sometimes cause cold air to flow from high ridges down into valleys. This cold air then pushes under the warmer air rising from the valley, creating the inversion.
Rookery South is in a basin – low lying with a ridge of upland surrounding it. Rookery South lies 38m (124.7 ft) above sea level
The height of the stack at 105m (344.5ft) (of which some will be below ground level in the pit) make it lower than the highest points along the rim of the ‘bowl’ at Cranfield of 112m and Lidlington at 130m.
Bedfordshire Geology Group neither supports nor opposes the Covanta proposals but have allowed BACI to reproduce this topography map from “The Mapping of Landscapes, Geology and Soils of Bedfordshire & Cambridgeshire” by Timothy Farewell, Peter Friend, Martin Whiteley and Joanna Zawadzka. http://www.bedfordshiregeologygroup.org.uk/landforms.html
During low pressure depressions coming across from the South West – the air will be moving both upwards and in an anti-clockwise direction- but due to the height of the stack in comparison to the ridge around the Vale – it is unlikely the emissions would be lifted enough to clear the ridge.
During a high pressure system weather system – an anti cyclone – air moves slowly in a clockwise direction and instead of uplifting it descends from the Troposphere to the ground compressing as it does so – and the air pressure increases with this compression.
A resident requested information from the Met Office regarding the number of high pressure system days for the 12 months from the 1st June 2010 to the 31st May 2011 – as evidence for the planning stage hearings – which were as follows:
- Greater than or equal to 1016.0 hPa: 233 days
- Greater than or equal to 1024.0 hPa: 89 days
- Greater than or equal to 1032 hPa: 24 days
During these high pressure systems – sometimes a layer of warmer air slides over the top of the descending air where it can act like a blanket and hold down the air and its pollutants. This is what is referred to as a temperature inversion event.