What are the various types of satellite imagery available?
There are four principal types of satellite imagery used in operational and research meteorology. Each has its advantages and disadvantages. Many examples of each type can be found at meteorology related web-sites.
1. Visible Imagery (VIS)
Images obtained using reflected sunlight at visible wavelengths, in the range 0.4 to 1.1 micrometres. Visible imagery is displayed in such a way that high reflectance objects, e.g. dense cirrus from CB clusters, fresh snow, nimbostratus etc., are displayed as white, and low reflectance objects, e.g. much of the earth's surface, is dark grey or black. There are grey shades to indicate different levels of albedo (or reflectivity). Very dependent upon angle of incident sunshine, and of course, not available at night, though some military/research satellite sensors can utilise reflected moonlight to detect cloud.
2. InfraRed (IR)
These images are obtained by sensing the intensity of the 'heat' emissions of the earth, and the atmosphere/atmospheric constituents, at IR wavelengths in the range 10-12 micrometres. The earth, and its components, radiate across a wide spectrum of wavelengths, but for many of these, the atmospheric gases, of which water vapour is an important constituent, absorb a significant proportion of such radiation. Thus so-called 'windows' need to be chosen to allow the satellite sensors to detect such radiation unhindered, and the 10-12 micrometre band is one such. IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are white. This convention was chosen so that the output would correspond with that from the VIS channels, but there is no need to follow this scheme - indeed in operational meteorology, colour slicing is frequently used whereby different colours are assigned to various temperature ranges, thus rendering the cooling/warming of cloud tops (and thus the development/decay) easy to appreciate: warming/darkening of the imagery with time indicates descent and decay; cooling/whitening images imply ascent and development.
3. Water Vapour (WV)
This imagery is derived from emissions in the atmosphere clustered around a wavelength of 6.7 micrometre. In contrast to the IR channel, this wavelength undergoes strong absorption by WV in the atmosphere (i.e. this is not a 'window'), and so can be used to infer vertical distribution and concentration of WV - an important atmospheric constituent. WV imagery uses the radiation absorbed and re-emitted by water vapour in the troposphere. If the upper troposphere is moist, WV emissions will be dominated by radiance from these higher levels, swamping emissions from warmer/lower layers; this radiation is conventionally shown white. If the upper troposphere is dry, then the sum of the radiation is biased towards lower altitude WV bands: it is warmer/less intense radiation, and this is displayed as a shade of grey, or even black. WV imagery is very important in the study of cyclogenesis, often being displayed as a time-sequence.
4. 'Channel 3' (CH3)
Imagery from a specific wavelength of 3.7 micrometre, lies in the overlap region of the electro-magnetic spectrum between solar and earth-based/terrestrial radiation. It is sometimes referred to as 'near infrared' (NIR). CH3 images use a mixture of back-scattered solar radiation plus radiation emitted by the earth and atmosphere. It is used in fog/very low cloud studies. Interpretation is sometimes complex, especially in the presence of other tropospheric clouds.