Photo of Bluebell Woods in Sussex Photo of Bluebell Woods in Sussex

Why is visibility so good in the mountains?

Great visibility in cold dry air in the Himalayan mountains

Many visitors to cold mountains are struck by the amazing visibility.

The visibility is so good that it can trick the brain into thinking things are much closer than they actually are. A summit that is 15 miles away appears to be 8 miles away, for example. But why does that happen?

It really is sometimes very clear in cold regions. The reason is ironically because the visibility in these places is often terrible. Let me explain.

Temperature, Humidity and Visibility

Warm air can hold more water vapour than cold air. Water vapour is water in gas state and it is transparent. However it does reduce visibility when we look through lots of it.

When air is too cold for water to be a gas, it condenses into a liquid – it forms a cloud, but if we are standing in that cloud, we call it fog.

What this means is that every time we look through air we are looking through water in either gas or water form – the air is never perfectly dry. If it is in liquid form then visibility is terrible: we’re in mist or fog.

However, if the water is in gas form then the air is transparent, but over many miles we will notice the effect of the vapour in reduced visibility – the more humid (ie. the higher the water vapour percentage) the worse the visibility.

In cold regions the air can’t hold much water as gas before turning to cloud. In warm regions it can.

This means that in cold regions you get two options: very bad visibility (wet or moist air) or very good visibility (very dry air).

In warm regions you get three options: very bad visibility (very wet air), OK visibility (fairly moist air) or very good visibility (dry air).

This why, in cold regions we only ever see mountains through clear dry air. At all other times they are hidden in cloud or fog.

The pictures on this page show this contrast in the Himalayas, Hong Kong and the UK.

OK visibility in warm moist air in Hong Kong
Bad visibility in cool moist air in the UK

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The Secret World of Weather – The Book

Image Credit.