On 11 December 2012 at 1237 utc, the NOAA-19 Advanced Very High Resolution Radiometer (AVHRR) captured an image over the UK, and a false-colour composite, from channels 1, 2 and 4, is shown in Figure 1. It shows a large swathe of low-lying fog across the Midlands and central Wales, typically below 250m in altitude as indicated by the protrusion of the Cotswold escarpment, Bredon Hill and the Malvern Hills through the fog bank. To the north and west of London the extent of the fog is restricted by the Chilterns, with fingers of fog extending southsoutheast through the valleys.
To the north of the fog the isolated plumes from power stations can be observed. These include Rocksavage (RS) and Fiddlers Ferry (FF) on Merseyside, Ferrybridge (F), Egg-borough (E) and Drax (D) in Yorkshire, and West Burton (WB), Cottam (C), Staythorpe (S), Ratcliffe (Ra) and Rugeley (Ru) along the River Trent. The plume from Didcot power station (Di) is just discernible in the southern part of the fog bank. Research into the formation and development of power station plumes includes Hanna (1967; 1972), and, with plume-induced precipitation, Kramer et al. (1976).
A few hours earlier (1030 utc) the Landsat-7 satellite made an overpass. The Landsat series of satellites, although principally designed for mapping of the Earth's surface, can provide useful fine-scale information on weather-related phenomena. The Enhanced Thematic Mapper (ETM+) instrument provides observations across the visible, near infrared (at 30m resolution) and thermal infrared (at 60m resolution) sections of the electromagnetic spectrum. In addition, a panchromatic channel provides imagery at 15m resolution. Despite the failure of a scan-line correction mirror in 2003, the sensor is still capable of providing detailed imagery.
Two of the images captured by the Landsat-7 ETM+ are illustrated. Figure 2(a) is that of the panchromatic band and shows the Fiddlers Ferry power station (53°37’N, 2°68’W) near Widnes. The white plume of condensate from the cooling towers is clearly evident, as is the shadow cast by the plume. Careful examination of the shadows in the imagery shows five separate plumes close to the power station itself. A smaller plume and shadow is seen in the lower part of the image and is caused by the condensate from the Rocksavage power plant. It can be deduced from the profile of the shadows that winds were light near the surface and increased with altitude. Because of the low sun angle, the shadow from the plume is large, extending more than 7km from the power station. Since the direction of the plume (about 240°) and the azimuth of the sun (148°) at the time and date of the imagery are essentially perpendicular, the height of the plume can be calculated using the elevation of the sun (8.8°): it works out at about 1100m. From the AVHRR imagery, the observed temperature of the plume is −5.5°C, although it could be lower since the plume is smaller than the 1.1km resolution of the imagery. Nevertheless, this compares well with data from the 1100 utc Herstmonceux radiosonde ascent which shows a temperature of −4.5°C at 1132m. The thermal infrared image (Figure 2(b)) uses the true radiometric readings: white is warm and black is cold. The River Mersey, its estuary, the Manchester Ship Canal and lakes close to the power station are evident by their relative warmth (about 4.5°C), while the plume from the power station is colder with a temperature of about −5.5°C. The shadow of the plume can clearly be seen as a thermal imprint: the surface temperature in the region of the shadow is about 2 degC lower than that of the surrounding region. Interestingly, close to the plume there appears to be a slight warming at the surface, particularly on the northwestern side. Although this could be due to surface features, such as the urban area of Widnes, it is apparent near the plume and is not observed over the larger urban area of Liverpool. Possible causes of this warming could be the reduction in the net outgoing radiation due to the plume aloft, and/or increased solar insolation due to scattering of solar radiation off the plume.