# 3a21

3a21, also known as *False color*, is a visible-light enhancement typically used on the Metop and METEOR-M satellites.

The name 3a21 comes from the usage of AVHRR channel 3a (1.61 µm) in the red position, 2 (0.63 µm) in the green position, and 1 (0.862 µm) in the blue position.

### Appearance

Land appears green (vegetated areas) or yellowish orange (deserts), while sea appears blue. Snow and ice appear cyan, including sea ice.

Clouds appear white if composed of water in the liquid phase, and cyan if composed of ice particles.

### Intended usage

General purpose imagery and ice detection at daytime.

### Limitations

Cannot usually be used on NOAA POES satellites as they do not usually transmit channel 3b.
Hard to discriminate ice clouds and snow/ice on the ground.
Cannot discriminate between cirrus clouds and fog.