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The following is classified as a reason for an obscuration:
  • A
  • B
  • C
  • D

Refer to figure.

ICAO Annex 3

Chapter 4
a) Precipitation

Drizzle DZ
Rain RA
Snow SN
Snow Grains SG
Ice Pellets PL
Hail GR

Small hail and/or snow pellets GS

c) Obscurations (hydrometeors)

Fog FG
Mist BR

d) Obscurations (lithometeors)

Sand SA
Dust (widespread) DU
Haze HZ
Smoke FU
Volcanic Ash VA

=> An obscuration is any phenomena in the atmosphere, other than precipitation, that reduces the horizontal visibility. Within the given options, dust is the only phenomenon which causes obscuration. The remaining options are precipitation.

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