This article was written by Andreas Dewanto, a Physics Instructor at the Faculty of Science, National University of Singapore. He is an avid mountaineer with MIR and has a strong interest in the science behind mountaineering.

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Weather is a determining factor in mountaineering. You can be the fittest, most experienced and skillful mountaineer in the world, and yet, you may end up in the wrong place, at the wrong time, and you could die because of exposure (e.g check on the net what happened to Mt. Everest XP in summer 1996). You may start with a sunny clear blue-sky in the morning, and end up in blizzard within the next 2-3 hours. Moral of the lesson is this: in the mountain, weather can shift very fast. So always be constantly vigilant (e.g. keep all your warm clothing within reach, check your barometer constantly, check up the distant sky to see if any cloud forming up, etc).

Why is that so? Well, let’s start with some physics background: like any cycles in nature, weather derives its energy from the sun. The sun heats up the ocean and the land. The heating up of ocean causes water vapor to form what we know as cloud. the land (because of it’s lower heat capacity) heats up faster than the ocean, causing pressure difference/gradient between the land and the ocean, with the land having lower pressure as compare to the ocean. As a result, there will be mass of air moving from the higher pressure region to lower pressure region (giving rise to a phenomenon we know as “wind”), bringing the cloud with it.

What happens when the cloud meets the mountain range? Well, the wind will simply try blowing over the mountain range, pushing the cloud to gain altitude along with the wind. The face of the mountain will of course define the course of the wind, and once it gets over the peak, it will blow in a projectile-like trajectory again at the other side of the range (think of a cart that jumps off the rail at the top for going up so fast uphill; or water spurting out of a slightly-elevated hose-pipe). The gale, however, can be so violent that it can “shred” the cloud that it’s carrying apart or blow the snow-powder on the mountain top off, giving rise to what I dub as “chimney” effect (see attached picture, at the cloud on top of the peak of Mt. Ama Dablam)… so if you see something like that at your destination peak, be prepare for super strong wind at the peak.

However, not all the water vapor (read: cloud) is able to get over the mountain top. As it gain height, temperature drops (I’ll explain in another occasion why temperature drops as altitude increases). As temperature drops, the vapor will condense into water droplets, giving rise to a phenomenon we call “rain”. Still, as temperature drops some more due to increasing heights, liquid will precipitate into solid, giving rise to a phenomenon we call “snow”. Thus, if you experience a wet trek at lower altitude, you may expect a lot of fresh snow at higher altitude. The point where you reach the snow-line (i.e. the altitude where snows start to form) is the time to don your crampon, and the adventure goes on ^_^

Now, I have mentioned earlier that wind moves due to pressure gradient. And when it moves, it carries cloud with it… and too much cloud translates into all sort of problems: rain, snow, white-out (that is when you enter the cloud/fog so thick that you can only see white everywhere… you soon becomes disoriented and may end up being lost, falling over the cliff, etc)

So the keypoint is this: if you’re entering a region of low pressure, it may imply of an impending doom because that means all the clouds are coming in to your way. For that reason, it’s important for a mountaineer to carry a barometer (that is, to check the local pressure) in the expedition. When the barometric trend is going down, there’s a possibility that bad weather is gathering up. When the trend is going up, well, it’s time to move. From personal experience, most of the time I find that when my barometer watch telling me the pressure is going down on certain day, the weather on that day is still good… and only about 12-24 hours later the weather turns for the worse. Similarly, sometimes it is a havoc outside, yet my watch indicates an increasing pressure, which means I’ll expect a better weather tomorrow or the day after tomorrow.

If you are in a big expedition, equipped with the most sophisticated gadget that communicates directly with satellite and weather station, you may have a weather map with you (such as the weather map over Australia below) which very much looks like a topographic map, except that the contour-lines are telling you the points of same pressure, instead of altitude. Hence you can imagine that such map is a dynamic map, i.e. the map may look different at different time.

The region with the lowest pressure is what we call as “the eye of the storm”, which surprisingly can be a moment of calm. Thus, if let’s say the weather is bad now… then suddenly the wind calms down and the sky clears up, but your barometer tells you that pressure is still going down or has reach rock bottom the for the past few hours but no increase of pressure detected yet… don’t quickly venture out too far yet, because likely that the weather will turn bad again before it’s getting better afterward. However, sometimes climbers make use of this opportunity to do things, like refresher training, short exploration around the camp-site, or supply mission (i.e. a day trip of going up to higher base-camp with load, dump the load, and go back down to lower base-camp… this is used as a way to acclimatize and preparing oneself to move-up to higher camp afterward), or attempt a summit. The problem is that, unless you have those support from sophisticated weather station, you may not know for sure how long this time-window would last… you may be in the middle of summit attempt or supply mission, when the storm starts again, the you’re going GG larh =P.

Hence, even if the weather today is good, but if you detect a region of lower pressure coming toward you, prepare for the worse, and don’t do anything reckless (e.g. launch an alpine climb that may take a few days of round trip). Vice versa, if you know a region of higher pressure is coming, despite the fact that now it’s blizzard outside, get ready to move in the next 12-24 hours. In other words, by paying attention to how the weather map evolves, you can plan the day ahead and have an enjoyable adventure out there. Legend says that experienced climbers can feel the approaching storm thru their skin or from the smell in the air.

You may be wondering: if the clouds are coming from the ocean, how come in places like, say Island Peak (in Khumbu region, where Mt. Everest is), which is thousands of miles away from the ocean, weather can still change very fast? By the way, it’s not just a hypothetical case, coz indeed that was happened to us when we did our expedition last year. You can see in the picture above how clear the weather was… in fact, it’s too good for our own good… it was super hot… and see the picture below, within the next hour there we were, trapped in the endless fog (a situation we call “white-out” in mountaineering term, literally you just see white wherever you turn your sight), and it’s started snowing some more. Where the hell the fog came from?

Well, the water vapor must not always come from ocean, must it? It may come from anything from mud-puddle, pond, lake, river… even the boiling kettle at your kitchen… and glacier!!! Now, that explains something! It is all make sense: why suddenly the cloud/fog can form up so fast and appear from no where in a seemingly good day. And that’s because, when you are up at the snow, you’re standing right in middle of a giant water reservoir.Oh, shucks!!

In fact, when the day is too good… hot and sunny to start with… there’s higher possibility for the weather to turn for the worse faster. Where as when it’s just cloudy and so so, it may probably stay like that for quite sometime. When it’s hot, sunny AND windless… GG!!! Beware of storm!!! Because likely you’re standing at the region of lowest pressure (so low that the air just stay still) and all the bad things are on the way toward you.

Now talking about “cloud”, there many way to categorize clouds. As I’m not an expert in this field, I’m not pretty sure myself what the technical differences between them. But to the very least, amateour mountaineers like us just need to be able to distinguish the so-called “cumulus cloud” and “cirrus cloud”. For that matters, Wikipedia has nice pictures to illustrate both of them (check: http://en.wikipedia.org/wiki/Cloud).

I’m pretty sure that you should have seen those type of clouds in your life, although you probably do not what it’s called. Cumulus cloud refers to those low altitude fluffy-kind of clouds, associated with the huge amount of vapor that they carry (and they are going to dump somewhere). Thus if you see such kind of cloud shrouding a peak at the distance, chances is that it’s raining or snowing up there. Whereas cirrus cloud looks like strayed fibers, which is a result of clouds being shredded apart by wind. Naturally you’ll find cirrus cloud being at high altitude when the wind is generally much stronger. Thus, associated with cirrus cloud is strong wind.