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Thermal Infrared 10-12 µm

Thermal Infrared (IR) images show the temperature of the land, the sea or the tops of the clouds above them. Warm temperatures (0-30° C) generally mean land or sea without cloud cover. As the temperature decreases it implies that clouds are getting higher and denser. Very cold temperatures mean that cloud tops are very high, which can imply strong convective storm activity.

IR imagery is derived from emission from the Earth and its atmosphere at thermal-infrared wavelengths (10-12 µm) and provides information on the temperature of the underlying surface or cloud. However, since the emitted radiation must traverse the Earth's atmosphere before reaching the satellite, it is modified during passage by atmospheric absorption and re-emission.

The conventional displaying of IR images in black and white is to present them so they are consistant with the appearance of visible images by having the clouds appear in white shades against the darker background of the Earth. Since the temperature normally decreases with height, the IR radiation with the lowest intensity is emitted by the highest and coldest clouds and these appear whitest. This is convenient but is the reverse of the procedure used for VIS images where the lowest reflectivities appear black.

Quantitative measurements of the temperature of an emitting surface needs to take account of absorption and emission within the window. However for qualitative interpretation the atmosphere can normally be considered as transparent in the window region. The only exception to this is in the very warm, high dew point air in the tropics where imagery of cloudless air may show patterns of grey shades that are related to the humidity distribution.
[Source Bader et al, 1995]

IR images are available 24 hours per day because temperatures can always be measured, regardless of day or night. (This is in contrast to Visible images which are only available during the day. See below.)

The temperatures can be represented in a grey-scale (black is no-cloud, and increasing white means higher colder clouds), or in a colour scheme (dark-blue for land/sea and low cloud, through various colours for mid temperatures to very light shades for very cold high clouds). See below for the colour scheme we use.

Colours

In colour Infrared IR satellite images only, artificial colours have been added to indicate different temperatures. Temperatures at the base of each colour band are:

° Centigrade

-68	Very pale orange (off-white)
-61	Pale orange
-55	Orange
-47	BRIGHT BRICK-RED
-33	Dull reddish-green
-27	Dull green (slightly reddish)
-20	BRIGHT LIGHT GREEN
-12	Medium green (bluish)
-6	Dark Green (blue/green)
+1	Light Blue
+8	Medium Blue
+15	Dark Blue
+22	Dark grey/blue
>22	Black

The two main colours that seem to be the most obvious are
(1) bright brick red (-55 to -47)
(2) bright light green (-27 to -20)

IR Sensors

IR imagery is available on:

MTSAT: IR1: 10.3-11.3 IR2: 11.5-12.5 µm

NOAA 15 Channel 4 10.3-11.3 µm	NOAA 15 Channel 5 11.5-12.5 µm
NOAA 14 Channel 4 10.3-11.3 µm	NOAA Channel 5 11.5-12.5 µm

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