7 mountain weather phenomena: a quick intro, courtesy of Tristan Gooley

The weather forecast for the week wasn’t good when we arrived at our cottage in Glencoe before Christmas. It was going to rain heavily all week, and there was really only one suitable day for a good hill walk (see my previous post).

As happened on previous trips, we spent the first day of our holiday in Fort William, shopping for outdoor gear and browsing the shelves of the Highland Bookshop for rainy day reading. While wandering around the nature section of the latter, my eyes chanced upon the title The Secret World of Weather by Tristan Gooley. It seemed an apt choice for reading by the fireside in a cosy cottage as the driving rain hammered on the windows.

I read another of Tristan’s books, The Walker’s Guide to Outdoor Clues & Signs, a few years ago, and found it an interesting read that opened my eyes to many aspects of the natural world that I’d not paid attention to before.

It was a somewhat quirky read, written in what I can best describe as a stream-of-consciousness style. Although the chapters were loosely structured into themes, the text jumped around from subheading to subheading in paragraphs of observation, scientific explanation and personal anecdote. Tristan lives in the Sussex Downs and runs courses on ‘natural navigation’ (using nature, rather than technology, to navigate), and the book is as much a memoir as a guide in the traditional sense. As befits a trainer, he does, however, write in a breezy, plain English style that I find appealing, and his books are highly readable (I think I’ve remarked before how the field of nature writing seems to be plagued by a fine writing style that can blow like a gale over many reader’s heads).

The Secret World of Weather is written in much the same style, so I knew what to expect. Its cover features a monotone line drawing of a cow sitting nonchalantly in a field while clouds and lightning thunder overhead and leaves whiz past its line of vision. This led my father to ask if I was reading a humorous book. I had to explain that that’s his real name, and it’s a book to be taken seriously.

The book is all about microclimates: that is, weather caused by local geography and landmarks, rather than weather caused by larger systems. As Tristan remarks in the opening pages, this is the weather that we actually experience and it can often be very different to the one we read about in the weather forecast.

As with The Walker’s Guide to Outdoor Clues & Signs, I learned about many things that I’d never noticed before, and many more things that I’d noticed but couldn’t explain.

While the book is UK-centric, the concepts he describes are universal and can be experienced all over the world. In this post, I outline 7 simple phenomena that many of us will have experienced in the mountains, and why they happen.

1. Lens clouds

Also known as lenticular clouds, and by the Latin name altocumulus lenticularis, lens clouds occur when winds hit the summit of a mountain, then ride up and over the summit in what is known as a mountain wave. As it rises over the mountain and cools, the air condenses to form a cloud. These are isolated clouds above a mountaintop, sometimes in the shape of a lens or flying saucer.

Lens clouds commonly form above the summit on its downwind side, but if the dew point (i.e. the point when air condenses into water) occurs lower down, then the cloud can form over the summit of the mountain.

I’ve seen this happen a lot in the Himalayas while sitting at base camp waiting for a weather window. Like its close relation, the banner cloud (see below), a lenticular cloud hanging over the summit is an indication that the summit is getting battered by high winds, and it was a good decision to remain in camp.

2. Banner clouds

You will only ever see banner clouds over proper mountain-shaped mountains with pointy summits; never over rolling hills. When high winds strike such a mountain, they hit a solid wall and the air is compressed on the upwind side. The air can’t escape, and this means there is a corresponding area of low pressure on the downwind side of the summit. Here the air cools and forms clouds in the shape of banners streaming off the summit. These banners indicate the direction of the wind as plainly as any flag waving on its pole.

Perhaps the most famous example of banner clouds are those which stream off Everest’s north-east ridge from base camp on the Tibetan side of the mountain. Indeed, I stared at these banner clouds for weeks in 2012, wondering if they would ever fade from view and give me a chance to climb to the summit.

3. Rotor clouds

The third type of cloud that forms when winds pass the summit of a mountain is the rotor cloud. It’s something I must confess I’ve never paid close attention to until now, but now I’m aware of it, I’ve been scanning through my photos trying to find examples.

Whenever winds strike an object such as a summit, it causes turbulence. Once the wind has passed the summit, the turbulence can sometimes cause the wind to blow back towards the mountain again. The rotor cloud is formed by a swirling vortex of wind on the downwind side of the mountain. Strange shapes are sometimes formed as the rotating wind cools enough for the air to condense.

If you see an unusual, isolated cloud downwind of a mountain summit, then it’s probably a rotor cloud. There are several candidates in this photo of mine of Muztag Ata in western China.

4. Gap winds

A few years ago, Edita and I did a backpacking trip around the rim of Ennerdale in the Lake District, across all the high summits. When we descended into the col between Green Gable and Great Gable, the wind was so strong that I wondered if we’d be able to stand up if we continued to the summit of Great Gable.

But when we reached the summit, the wind wasn’t as strong. The reason for this is that the col had been subject to a gap wind. Whenever winds are forced through a narrow gap, they accelerate. The same phenomenon affects water. You will notice that whenever a river narrows, the water flows more quickly. It’s why you see prayer flags bedecking every high pass in the Himalayas; the wind is stronger and the locals believe more prayers will be blown to the mountain gods. And it’s why it’s sometimes windier down in a col than up on a summit.

5. Foehn winds

When the wind blows moist air up towards mountain peaks, the air expands, cools and condenses to form rain and snow. After it has passed the mountains, the wind descends again, where it warms and absorbs moisture. This descending wind on the downwind side of mountains is called a foehn wind, and it causes the phenomenon known as a rain shadow.

Many mountain ranges experience quite different weather on their two opposing sides depending on the direction of the prevailing winds. In Patagonia, in the southern part of South America, westerly winds come from the Pacific. The western side in Chile is extremely wet and the land is forested; on the eastern side in Argentina, it is much drier and the land is desert. The same is the case in the Himalayas. Winds come from the Bay of Bengal in the south-east, deposit rain and snow over Nepal, then suck up moisture in the dry desert regions of Tibet on the northern side.

6. Mountain breezes

The mountain breeze is the highland equivalent of the sea breeze, which blows in from the sea during the afternoon on sunny days. When the sun is beating down, land warms more quickly than the sea. By mid-morning, the land has warmed sufficiently to cause a large pressure difference between land and sea. This causes a wind to blow in from the sea to the land. The wind is actually circular. Once on land, the air warms, rises up, cools again, then flows out to sea to replace the air which flowed in on the sea breeze.

A mountain breeze works in the same way. If high ground faces the sun, the ground warms up, creating a pressure difference, and causing a wind to blow from the cooler land down below. Once on the warmer hillside, the air rises, cools and flows back out again. If you’re standing on a hillside in the sun, you may therefore feel a breeze blowing towards you from down below.

7. Summit winds

It seems natural for the wind to increase as we climb higher and for it to be highest at the summit, but why is this?

As we have seen with the example of the gap wind on the col, it isn’t always the case. This is because the wind is always influenced by local factors closer to the ground. As we’ve seen with some of the examples above, the wind you experience may be flowing in a completely different direction to the prevailing wind that blows without the influence of these local factors.

The reason the wind is stronger on the summit is actually because it’s a truer reflection of the main wind that presenters tell us about during a weather forecast: the wind that is uninhibited by local terrain. The winds that we feel lower down have often been deadened by bouncing off surfaces and passing through patches of warmer or cooler air.

When wind blows along a ridge, whether into your face or at your back, it’s much more constant than wind that blows across the ridge. Side winds on a ridge will be subject to much greater fluctuations and therefore much more gusty.

To summarise, I highly recommend Tristan Gooley’s books to any active, outdoor enthusiast. I can guarantee you will read something that will cause you to look at future hikes and climbs with fresh eyes. While The Secret World of Weather may not be my favourite gooley, it’s at least my third favourite.