The distance a meteorologist can see is called "the visibility". Oddly enough, it bears little relation to how far anybody else might see. It is carefully defined as "the furthest horizontal distance at which a person of normal sight can distinguish and identify an object for what it is known to be, in normal conditions of daylight illumination" - a concept not at all as simple as it seems.
The definition is framed to exclude extraneous matters from the reckoning - irrelevant variables such as the eyesight of the observer, the amount of light, if any, that may be present at the time, and the degree of contrast that may exist between an object and its background.
It tries to isolate, as the most important factor, the "turbidity", or transparency, of the atmosphere.
The air is never completely clear. Rays of light heading towards an observer from some distant object undergo an attenuation caused by impurities or water droplets suspended in the atmosphere, and it is this attenuation that is being assessed.
Visibility may be rendered less than perfect by either mist or haze. Haze is a reduction in visibility caused by tiny solid particles suspended in the air - little motes of dust or smoke, that come from a great variety of sources - desert sands or particles of arid soil from afar, ash from some volcanic eruption in a distant land, or more commonly in this part of the world, domestic or industrial pollution.
In the case of mist, on the other hand, the obscurity is caused by droplets of water, and these will not normally be present in the atmosphere unless the relative humidity is high - perhaps 90 per cent or greater.
If the water droplets are sufficiently numerous or large enough to bring the visibility below 1,000 metres, weather people call the resulting murk a "fog". Sometimes, to add to the confusion, people refer to a light drizzle as a "mist", a gaffe that will have any thinking meteorologist recoil in horror.
At the other extreme, it is reckoned that even if the air were perfectly clear, the maximum visibility would still be only about 150 miles - because the molecules of the air itself attenuate the light.
In practice, it is unusual to be able to see clearly a distance of more than 40 miles. The term "visibility unlimited" in the shipping forecast, for example, should not be taken literally; it just means that visibility is something in excess of 50 miles or so, and will not be a limiting factor for any conceivable activity that might be undertaken.