METEOSAT (MOP, MTP)

Name	Spectral Range in Micrometres	Primary Use
VIS	0.5 - 0.9	Daytime cloud-mapping and albedo determination
IR	10.5 - 12.5	Day and night cloud-mapping and determination of surface temperature
WV	5.7 - 7.1	WV features for synoptic purposes; WV content in upper troposphere UTH (Upper Tropospheric Humidity)

VIS - VISible: 0.5 - 0.9 micrometre

The satellite receives that part of the sun radiation which is reflected by the earth surface or by cloudiness. It is a so-called window channel which means that radiation is not significantly absorbed by the gases in the troposphere; this means in other words that it is possible to look as though through a window downward to the radiating surface which can be earth surface or cloud tops.

Different grey shades in this VIS channel represent different values of albedo:

Bright values	High albedo	Thick clouds Land surfaces
Grey values	Low albedo	Thin translucent clouds
Dark values	Almost no reflection	Water and vegetation

IR - InfraRed: 10.5 - 12.5 micrometre

The satellite receives radiation which is emitted by the earth and the clouds because of their temperature. IR is a window channel, as is VIS, and with the same consequences.

Different grey shades in the IR channel represent different temperatures of the radiating surface which can be the earth surface or the cloud tops. The grey shades presented in the IR images are reversed (white becomes black) to get a similar appearance to the VIS image.

Bright values	Low temperatures	Cold (high) cloud tops
Grey values	Middle temperatures	Middle cloud tops
Dark values	Warm temperatures	Hot land or sea surface

WV - Water Vapour: centred at 6.3 micrometre

The satellite receives radiation which is emitted by the water vapour in the upper troposphere (approximately above 600 hPa up to 300 hPa). The WV channel represents an absorption measurement of the WV absorption band. This means that radiation from below 600 hPa is absorbed by the WV in the layer above which according to the radiation laws (Kirchhoff) becomes a radiating surface itself.

Different grey shades in the WV channel represent different WV content above 600 hPa:

Bright values	Liquid water	Clouds
Grey values	Water vapour	Water vapour with varying intensity
Dark values	Very low water vapour	Dry upper troposphere

Comparison of the three channels IR, VIS and WV: 19 October 1998/12.00 UTC

Looking at the IR image of 12.00 UTC (top left image) the following main synoptic to mesoscale cloud configurations can be discriminated:

A	A cloud band of a Cold Front extending from north Africa across the Alpine region to the Ukraine representing a Cold Front with a Wave feature over the Alps
B	A cloud band, a huge cloud shield and a distinct cloud spiral over the Atlantic representing a frontal system consisting of Cold Front, Warm Front and Occlusion
C	A smaller scale cloud spiral extending from Sweden across Denmark to the Netherlands representing a well developed Comma system
D	Large areas of cellular cloudiness with different vertical extension representing cold air streaming over warmer ground

Some specific areas will be selected and compared in the three channels IR, VIS and WV (compare Artificial and Combination Channels - Meteosat combination VIS + IR )

A1	The easternmost part of the frontal cloud band A located over the Ukraine IR: band feature, dark grey with some brighter spots and fibres superimposed VIS: band feature, very bright, some texture and darker stripes WV: grey WV band with distinct gradient to a black stripe on the rearward edge Conclusion: Frontal cloud band with lower cloudiness in general but some cells embedded; high fibres at the anticyclonic jet side superimposed casting a shadow on the lower cloud ceiling jet axis parallel to northern cloud boundary with very dry air on the cyclonic side
A2	The Wave area over north Italy - Switzerland - west Austria and south Germany IR: anticyclonically curved cloud bulge, very bright and rather smooth texture VIS: anticyclonic curvature, dark grey especially over Germany, partly translucent WV: anticyclonically curved cloud bulge, very bright; rearward boundary very distinct, black stripe especially in the southern part behind the cloud feature Conclusion: Wave or at least Wave - like bulge with a high smooth cloud shield superimposed; very dry air behind support Wave process
A3	Two mesoscale cells MCSs over the western Mediterranean (40.5N/03E and 41N/06E) IR: oval shaped, very bright, sharp upstream edge, rather fibrous downstream VIS: oval shaped, very bright upstream, grey and fibrous downstream WV: oval shaped, very bright, partly embedded within dry air, partly embedded within frontal cloud band Conclusion: Thunderstom complexes and MCSs with the thick cloudiness in the upstream part and a cirrus shield downstream; different synpotic environment for the two cells
B1	Cloud shield over the Atlantic IR: cloud shield, very bright, smooth, sharp edges at the leading boundary (east of 60N/30W), rather fibrous at the downstream boundaries (west of Ireland) VIS: cloud shield, grey, becoming translucent to the downstream boundaries WV: very large extended WV shield; very bright where cloudiness exists but grey to the south, south-east; dark area to the north, north-east of the WV shield over Scotland Conclusion: Huge area of humid air in the upper layer; partly cloudiness in the Warm Front Shield but humidity extends far to the south with some cloud fibres embedded; very dry air to the north extending from south Iceland across Scotland into the North Sea
B2	Cloud band over the Atlantic (36N/35W - 50N/30W) IR: band feature, bright, ragged texture with different grey shades, brighter grey shades to the rearward edge of the band VIS: band feature, bright in the leading part of the cloud band, ragged texture, grey to the rearward edge of the cloud band WV: band feature, ragged, bright areas similar to IR; grey in the leading part Conclusion: Cold Front cloud band with multilayered cloudiness and high humidity supply in the upper layer; rising of cloud tops from the leading to the rearward edge, similar to Ana front model
B3	Cloud spiral west of the cloud shield (around 57N/36W) IR: band feature, bright but clearly less than cloud shield to the east, ragged texture VIS: band feature, bright, ragged texture WV: band feature, bright but clearly less than cloud shield to the east, ragged texture very pronounced spiral configuration to the west Conclusion: Typical Occlusion cloud band of the cold conveyor belt type; cloud spiral of low centre well developed; probably cut-off of separate Comma
D1	Cloud cells west of the comma feature and the Norwegian coast: IR: cells of different size, irregularly distributed, bright VIS: cells, very bright WV: brighter cells embedded in larger area of light grey shades Conclusion: Thick cold air cells with large humidity supply in the higher layer; additional rising to merely convective effects
D2	Cloud cells west, north-west of Scotland and north Ireland IR: cells, regularly distributed (open cells), grey to dark grey, transition to dense structure more to the north-west VIS: cells, regularly distributed (open cells), bright, dense cloud part to the north-west very bright WV: no cells visible at all; dark grey stripe Conclusion: Typical cold air cloudiness of the open cell type; no humidity supply from above
-	Additional WV features over north Spain - the Pyrenees - France: IR: some grey cellular structure over land and sea, irregularly distributed, orographic Stau at Pyrenees VIS: some white cellular structures over lands and sea, irregularly distributed, small cells at Pyrenees WV: two distinct band features in grey to dark grey, small black stripe separating the two bands, very dark dry air at the rear over north Spain Conclusion: Cold Air Cloudiness in lower layers but completely different process in upper layer: very early stage of Upper Level Low development possible

The advantage of the geostationary satellite Meteosat is the high temporal frequency of a large part of the earth. NOAA images are available at different points of time.

NOAA AVHRR

Channel Number	Spectral Range in Micrometres	Name	Present on NOAA	Primary use
1	0.58 - 0.68	VIS 1	12, 14, 15	Daytime cloud-mapping and albedo determination
2	0.725 - 1.1	VIS 2	12, 14, 15	Daytime land sea delineation and cloud-mapping and above sea
3A	1.158 - 1.164	NIR 1.6	15	Discrimination between snow, ice and cloud, aerosol determination
3B	3.55 - 3.93	NIR 3.7	12, 14, 15	Night time cloud detection
4	10.3 - 11.3	IR 4	12, 14, 15	Day and night cloud-mapping and determination of surface temperature
5	11.5 - 12.5	IR 5	12, 14, 15	Day and night cloud-mapping and determination of surface temperature

VIS 1 - VISible 1: 0.58 - 0.68 micrometre

The satellite receives that part of the solar spectrum which is reflected by the underlying surface, which can be land, sea or cloud. It is a so-called window channel which means that radiation is not significantly absorbed by the gases in the troposphere; this means in other words that it is possible to look as though through a window downward to the radiating surface which can be earth surface or cloud tops.

Different grey shades in this VIS 1 channel represent different values of albedo:

Bright values	High albedo	Thick clouds Land surfaces
Grey values	Low albedo	Thin translucent clouds
Dark values	Almost no reflection	Water and vegetation

VIS 2 - VISible 2: 0.725 - 1.1 micrometre

The satellite receives that part of the solar spectrum which is reflected by the underlying surface, which can be land, sea or cloud. It is a so-called window channel which means that radiation is not significantly absorbed by the gases in the troposphere; this means in other words that it is possible to look as though through a window downward to the radiating surface which can be earth surface or cloud tops.

Different grey shades in this VIS 2 channel represent different values of the reflected radiation:

Bright values	High reflectivity	Thick clouds Land surfaces
Grey values	Low reflectivity	Semi-transparent clouds
Dark values	Almost no reflection	Water and vegetation

NIR 3 - Near InfraRed 3: 1.58 - 1.64 micrometre

This IR channel is located in that part of the spectrum where reflected sunlight still plays a role during daytime. On the other hand, hot surfaces, like forest fires, stubble burning after harvesting, volcanoes, etc, have their largest radiation contribution around this wavelength. Finally, water droplets of a size comparable to the wavelength can be abundant in the atmosphere, especially in fog layers, causing strong reflection of sunlight at this wavelength. In other words, the NIR 3.7 channel is partly a thermal emission channel and partly a reflectivity sensing channel. This implies that great care is needed with the interpretation of its measured values.

Different grey shades in this NIR 3.7 channel represent different values of brightness temperature:

Bright values	Low temperature	Cold cloud tops
Grey values	Intermediate temperature	Medium and low clouds
Dark values	High temperature, high reflectivity	Land and sea surface, top of fog layer

IR 4 - InfraRed 4: 10.3 - 11.3 micrometre

Being located in one of the atmospheric window regions of the thermal part of the spectrum, this channel IR 4 measures the emitted radiation from the highest optically dense surface in the earth-atmosphere system. Since these surfaces (land, sea, clouds) are not perfect black body radiators, the brightness temperature observed can differ from the temperature of the emitting surface with the emissivity of that surface.

Different grey shades in this IR 4 channel represent different values of brightness temperature:

Bright values	Low temperature	Cold cloud tops
Grey values	Intermediate temperature	Medium and low clouds
Dark values	High temperature	Land and sea surface

IR 5 - InfraRed: 11.5 - 12.5 micrometre

Being located in one of the atmospheric window regions of the thermal part of the spectrum, this channel IR 5 measures the emitted radiation from the highest optically dense surface in the earth-atmosphere system. Since these surfaces (land, sea, clouds) are not perfect black body radiators, the brightness temperature observed can differ from the temperature of the emitting surface with the emissivity of that surface.

Different grey shades in this IR 5 channel represent different values of brightness temperature:

Bright values	Low temperature	Cold cloud tops
Grey values	Intermediate temperature	Medium and low clouds
Dark values	High temperature	Land and sea surface

Examples of the five AVHRR channels

Area to be examined:

In western upper corner	Great Britain
In eastern upper corner	Baltic Sea
Below in western corner	north-eastern Spain
Below in eastern corner	Balkans

AVHRR - VISible channel 1

VIS 1

Uniform smooth anticyclonically curved cloud band or shield or developing Wave. (Size of the picture area is too small to say anything about the width of the cloud shield!) Brightest and most uniform area is extending from western France across Switzerland to Bavaria. North of Adriatic Sea and north of Austria and Hungary cloudiness thinner or partly translucent. Edge of cloud band from France to Bavaria quite sharp. From Bavaria eastwards edge fibrous and indistinct. North and west of large cloud band partly cloudless area, where cloudiness more grey with narrow shadows on the western side of cloud patches. River beds and some valleys can be seen. Maybe "morning fog" over rivers and valleys? Higher clouds seen over the area extending from Scotland across Denmark to Baltic countries. Over Scotland and North Sea open Cu clouds. A cloud band extending from the Netherlands across North Sea to Norway and becoming broader. Cloudiness in band lumpy especially at the western edge. From Denmark northwards large cloud patches with smooth texture. Probably large Cu cells? Over Baltic countries closed Cu cloudiness. Further north Cu clouds are smaller and lower and are arranged in parallel lines (Cloud Streets). Over southern Baltic ragged Cu form cloudiness with embedded white Cu cells. Conclusion: Perhaps Wave or at least Wave - like band in the lower part of the picture. Over North Sea typical Cold Air Cloudiness of the open cells. The south-north oriented cloud band has Cu form cloudiness.

AVHRR - VISible channel 2

VIS 2

Wave - like band somewhat darker but cloudiness eastwards as bright as in VIS 1. Dark spots below in the right corner of the picture not found in VIS 1. They represent obviously small cloudless holes in the cloud cover. All other features in the large cloud band are seen as in VIS 1 image. River beds with thin fog cannot be found. Shadows of the cloud patches over France and Germany are seen better. No differences in the northern part of picture. Conclusion: Perhaps Wave or at least Wave - like band in the lower part of the picture. Over North Sea typical Cold Air Cloudiness of the open cells. The south-north oriented cloud band has Cu form cloudiness.

AVHRR - Near InfraRed channel 3

It should be kept in mind that the south-eastern part of the picture is illuminated by the sun more than the northern part. Bright white anticyclonically curved cloud band or wave extending from south-west to Bavaria having quite uniform, smooth texture. In particular, the edge at the north side from Bavaria eastwards is vague and fibrous. The white cloud areas represent cold high clouds composed of ice crystals.

Differences compared to VIS channels and combination 1,2,4 image (compare Artificial and Combination Channels - AVHRR combination channel 1 + 2 + 4 ):

NIR 3

Cloud band not so widespread. East of Bavaria and westernmost Austria no uniform white cloud band as in other channels. Over Hungary and eastern Austria a separate white cloud patch, but then downwind almost black with some small white spots. Lower clouds over France and Germany are darker than land surface, dark grey or almost black. Land is light grey. High Cu clouds in the right upper corner are seen as white embedded in darker lower clouds. Also near the coastline of France in the English Channel can be found white (high) Cu clouds. Over the Baltic whitest Cu clouds just north of Poland; otherwise the clouds over Baltic countries are very dark. Conclusion: White cloud band or wave area is superimposed by Ci clouds. Downwind there are lower clouds. Highest Cu clouds in the upper part of the picture are seen white and lower clouds much darker as well as low clouds over land areas.

AVHRR - InfraRed channel 4

Large wave-like cloud band brightest and distinctly fibrous from southern France to Bavaria and further eastwards darker and lumpy appearance. No differences found from channel 4 and channel 5 (compare channel 5 below), otherwise comments as above with channel combination 3, 4 and 5 (compare Artificial and Combination Channels - AVHRR combination channel 3 + 4 + 5 )

AVHRR - InfraRed channel 4

Large wave-like cloud band brightest and distinctly fibrous from southern France to Bavaria and further eastwards darker and lumpy appearance. No differences found from channel 4 and channel 5 (compare channel 4 above), otherwise comments as above with channel combination 3, 4 and 5 (compare Artificial and Combination Channels - AVHRR combination channel 3 + 4 + 5 )