Measuring Visibility
Visibility is the way we describe how far we can see, and it is measured at an official weather station by reference to a number of known landmarks at different places. Three or four landmarks at say 100 m, 1000 m, 2 km, and 10 km will allow the visibility to be described as 'thick fog', 'fog', 'poor visibility' and 'good visibility'. Fog is present if the visibility is less than 1 km.

What can I use?
Find a map of your local area which shows at least 50 km on either side of your weather station. Work out the scale of the map, and mark the distances above off on a ruler. You can then see what on the map could be used as a landmark - usually church steeples, hills, buildings or even large trees. A pair of binoculars will help to see the furthest away landmarks.