Misty envelope: A satellite image shows fog (in red) shrouded the Pearl River Delta and neighbouring waters.

Hong Kong was shrouded in fog as a moist maritime airstream affected the region, the Hong Kong Observatory says, adding the lowest visibility on Tuesday morning was just 200 metres.

The observatory said it is possible to locate the fog from space with the use of a weather satellite.

The observatory receives and processes satellite images from the Earth Observing Satellites to monitor fog around Hong Kong.

The sensors onboard the satellite take images of the earth using a number of infrared channels. After special processing, the fog's location can be highlighted in colour.

A few examples of fog episodes in 2006 can be found in the satellite gallery on the observatory's website.