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The main limitation of an infra-red image observation is:
  • A
    It is not intended to identify thick and rather high clouds.
  • B
    The infra-red image is based on ground observation combined with satellite returns.
  • C
    It is difficult (sometimes impossible) to spot fog and/or very thin clouds such as Cirrus.
  • D
    The infra-red image is unusable at night or in conditions of very low sunlight.

Refer to figures.

SATELLITE IMAGERY

Cloud detection.
There are two methods of producing the weather picture; visual photography and infrared.

Visual Images. Although visual photography may be easy to interpret, it suffers the disadvantage of not being available continuously, due to lack of sunlight at night. Clouds will appear white, the land grey and the sea black
- Visible satellite image show solid clouds (such as CBs) as bright white and wispy clouds (thin or faint cloud - such as cirrus) as translucent whitish layers.

Infrared (IR). Infrared images have the advantage of being available for 24 hours a day and the shading of the picture will be more or less the same by day and by night. Cold (high) cloud will give a white image, lower cloud a somewhat darker one, whilst warm land will give a dark image => If the clouds near the surface are approx. same temperature as the land surface it can be difficult to distinguish the clouds from land.


To get a clear picture of the weather you should analyse both the IR and visible images.
  • If both images are bright in the same area => very likely that you can find a very high and thick cloud with high convective activity over that area.
  • If you get cloud returns on a visible imagery but not on an IR image => most likely a low cloud
  • If you get a white return on an IR imagery, along with a translucent whitish layer on a visible imagery => it is most likely a high wispy cloud - a thin layer of cirrostratus.

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