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1

Satellite imagery can be in one of the following two formats:

Analog and digital

2

Analog:

which data is displayed in a pictorial or photograph‐type format, independent of what type of sensor was used to collect the data and how the data were collected

interpretation and identification of targets in this imagery is performed manually or visually, ie by human interpreter

3

Digital

data is represented in a computer as arrays of pixels, with each pixel corresponding to a digital number, representing the brightness level of that pixel in the image
When remote sensing data are available in digital format,digital processing and analysismay be performed using a computer.

4

Both analogue and digital imagery can be displayed as

black and white (also called monochrome) images, or as

color images by combining different channels or bands representing different wavelengths.

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 Visible Imagery (VIS)

Images obtained using reflected sunlight at visible wavelengths, 0.4 to 0.7 um

 

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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 black

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There are grey shades to indicate

 different levels of albedo (or reflectivity)

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Visible imagery is not available

at night

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Infra Red (IR)These images are obtained by measuring

 the intensity of the thermal emissions from the earth and the atmosphere, at IR wavelengths in the range 10‐12 um

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This so‐called 'window' need to be

chosen to allow the satellite sensors to detect such radiation unhindered, and the 10‐12 um band is one such.

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For example, the GOES (8‐11) sensors use 

the atmospheric infrared window centered at 10.7 micrometers

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For example, the GOES (8‐11) sensors use the atmospheric infrared window
centered at 10.7 micrometers

at this wavelenght

 energy radiated by the earth's surface and clouds is not significantly attenuated by atmospheric gases.

13

For example, the GOES (8‐11) sensors use the atmospheric infrared window centered at 10.7 micrometers.

At this wavelength, energy radiated by the earth's surface and clouds is not significantly attenuated by atmospheric gases

in this channel

most surfaces and cloud types have an emissivity close to 1.0, with a notable exception being thin cirrus.

14

For example, the GOES (8‐11) sensors use the atmospheric infrared window centered at 10.7 micrometers.

In this channel, most surfaces and cloud types have an emissivity close to 1.0, with a notable exception being thin cirrus.

therefore

the brightness temperature sensed by the satellite is close to actual surface skin or cloud top temperature for other scenes.

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IR imagery is so presented that warm/high intensity emissions are

dark grey or even black

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IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are

white

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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

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Color slicing is also frequently used whereby

different colors 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

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 Water Vapor (WV)

This imagery is derived from

emissions in the atmosphere clustered around a wavelength of 6.7 um

20

The infrared water vapor channel on board GOES‐8to‐11 is located at 6.7 micrometers where the earth's

emitted spectrum is highly attenuated by water molecules. Thus, this channel senses radiation from the mid‐and upper‐ levels of the atmosphere, from both water vapor and clouds.

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IR channel, this wavelength undergoes

strong absorption by WV in the atmosphere (i.e. this isnota 'window'), and so can also be used to infer vertical distribution and concentration of WV ‐ an important atmospheric constituent

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WV imagery uses

the radiation absorbed and re‐emitted by water vapor in the troposphere.

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If the upper troposphere is moist

WV emissions will be dominated by radiance from these higher levels is conventionally shown white.

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If the upper troposphere is dry

then the sum of the radiation is biased towards lower altitude WV bands: and this is displayed as a shade of grey, or even black.

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Because water vapor is transported by

atmospheric circulations

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Because water vapor is transported by atmospheric circulations, it allows

the detection of features in the mesoscale flow as well as hemispheric patterns

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WV imagery is also very important in the study of 

cyclogenesis, often being displayed as a time‐sequence.

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 Near‐IR (Shortwave IR)

Imagery from a specific wavelength of 3.9um, lies in the overlap region of the electromagnetic (EM) spectrum betweensolar and terrestrial radiation.

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Near‐IR (Shortwave IR)

Radiation in this wavelength region is 

not significantly attenuated by the earth's atmosphere.

30

Near‐IR (Shortwave IR)

these images uses

a mixture of reflected solar radiation plus radiation emitted by the earth and atmosphere.