Haze and smoke over the Po Valley

Visible satellite imagery is susceptible to light scattering from smoke and dust particles, especially when looking with some component towards the Sun (Mie scattering). Smoke and dust particles, which are of roughly the same size as the wavelength of the incident solar radiation, are the dominant sources of Mie scattering. This type of scattering usually takes place in the lower atmosphere, in the so-called Planetary Boundary Layer (PBL), where the pollutants are frequently trapped below a temperature inversion.

In contrast to the scattering from molecules (Rayleigh scattering), which has no directional dependence and is strongest for the short (blue) wavelengths, the scattering caused by dust and smoke particles can affect longer wavelength radiation and most of the incident radiation is scattered in a forward direction. The effect in visible satellite imagery is a hazy image when looking in a direction close to the Sun. In Europe, this effect can be best observed in the early morning and late evening hours when the azimuth and zenith angles between the satellite and the Sun are the largest.

One example of haze in the Po Valley can be seen below in the early morning Meteosat-8 images from 30 July 2004 (05:45 UTC). In the HRV image, and even more pronounced in the RGB colour composite, the industrial area in the Western Po Valley looks more hazy when compared to the more clear Alpine areas. The haze is less visible at 8:15 UTC when the relative angle between the Sun and the satellite is smaller and the forward scattering plays a smaller role (compare the area around the Lago di Garda by toggling between the two HRV images).

To reduce the haze effect, a longer wavelength channel such as the near infrared at NIR1.6 should be selected. In the RGB composite image, it is also interesting to note the greenish area to the west of Milan which represents the area covered by rice fields (Provinces of Vercelli and Novara in northern Italy, between the Dora Baltea and Ticino rivers, see also 26 April 2001 and 4 August 2003).

Another interesting phenomena that can bee seen in the images below (i.e. in the animation of the HRV channel) is the dark smoke coming from a fire at a plant storing toxic refuse located close to Milan. The fire, which broke out at about 5:45 UTC, caused the temporary closure of the A8 Milan-Varese motorway. As it was not initially clear whether the smoke contained toxic gases or not, Meteosat-8 images were essential to monitor the dispersion of the smoke plume and to issue early warnings to the local population.