Clouds make for some of the best mountain photos. They pour through valleys, wrap around peaks and create spectacular scenes. But when you’re inside that cloud, the mountain becomes a very different place.
Two recent Search and Rescue incidents demonstrated why. On the West Lion, a hiker entered dense cloud, could no longer see the route and ended up climbing the wrong terrain. Days later on Mount Habrich, another hiker became disoriented in thick cloud, lost the trail and descended the wrong side of the mountain.
In both cases, cloud also affected the rescue. The helicopters we commonly use for mountain rescue need visual reference to the terrain. A pilot can’t simply fly toward your GPS coordinates through cloud wrapped around cliffs, ridges and trees. When we can’t fly, we walk, turning what might have been a quick helicopter response into hours of ground travel.
So how do you know whether cloud is going to be a problem?
START WITH CLOUD BASE
For mountain travel, “cloudy” isn’t particularly useful. We want to know where the bottom of the cloud is relative to where we’re going.
We commonly use Windy. Put the forecast point on the mountain, not in the town below, select Meteogram and look at the cloud layers and cloud base hour by hour. If you’re climbing to 1,700 metres and the forecast is showing low cloud at 500m through the period you expect to be on the mountain, that should raise a red flag.
COMPARE THE MODELS
The forecast you see in an app comes from a weather model. These models divide the atmosphere into a three-dimensional grid and calculate how temperature, humidity, pressure, wind and moisture are expected to change. From that predicted atmosphere, they estimate where clouds will form.
Windy lets you switch between several models or use Compare forecasts. We’re looking for agreement in the overall story: Do the models put cloud on the mountain? Do they show it lifting or dropping at roughly the same time? Do they agree on precipitation and wind?
When several models broadly agree, our confidence in the forecast increases. When they’re all over the place, be suspicious of any single answer. Our favourite continues to be ECMWF, but we still compare it with the others. Lots of disagreement means lots of forecast uncertainty, and for a consequential mountain objective, that’s useful information.
NOW LOOK AT WHAT IS ACTUALLY HAPPENING
This is where Squamish has an unusually useful tool. Most mountains don’t have a weather station sitting high on a ridge, but the Sea to Sky Gondola operates one at Exit Gully on Habrich Ridge, around 1,550 metres.
The station measures temperature, dew point, wind speed, gusts and wind direction, and there’s also a webcam. This gives us something quite rare: the ability to compare the model forecast with measurements taken high on the mountain and then actually look at the mountain.
WHAT IS DEW POINT?
Dew point tells us how close the air is to forming cloud or fog. It is the temperature to which air must cool before it becomes saturated with water vapour. When the air temperature reaches the dew point, relative humidity reaches about 100% and moisture can begin condensing into tiny droplets. Those suspended droplets are what make clouds and fog.
The important number is the difference between temperature and dew point. A large gap means the air is relatively dry. As the two numbers move closer together, the air approaches saturation and cloud or fog becomes increasingly likely.
On SpotWx, you can actually watch the temperature and dew-point lines converge on the graph. When those lines touch, the air is at or very near saturation - which means clouds.
There’s also a useful calculation for estimating how far rising air would have to travel before reaching saturation; you need to know temperature and dew point.
(Temperature − Dew Point) × 125 = approximate condensation level in metres above the observation point
If the temperature is 9°C and the dew point is 7°C:
9 − 7 = 2°C
2 × 125 = approximately 250 metres
Now let’s put all of this together using Habrich Ridge.
FORECAST → COMPARE → ACTUAL → LOOK
On September 7, the models were forecasting very moist air and low cloud around Mount Habrich, although they differed on the exact temperatures and timing.
At 9 p.m., the Exit Gully weather station actually measured:
Temperature: 7.227°C
Dew point: 7.220°C
The difference was just 0.007°C — effectively zero. The air at the station was essentially saturated = cloud.
Six minutes later, the webcam showed exactly why we care about those numbers: Exit Gully was inside cloud.
That is the workflow we use when evaluating mountain weather:
FORECAST: Where is the cloud predicted to be relative to our route?
COMPARE: Do several weather models tell roughly the same story?
ACTUAL: What are weather stations measuring right now?
LOOK: What do the webcams and the mountain itself show us?
Exit Gully gives us an unusually good source of high-elevation actual weather for the Habrich area. For other objectives, we may have to rely on valley weather stations, airport observations and whatever mountain webcams are available. Always remember that a weather station measures conditions at that particular location.
WHEN “DEW” YOU TURN AROUND?
Sometimes you don’t need the math.
You arrive at the beginning of a two-hour scramble and the mountain above you has disappeared into cloud. The route requires you to see the terrain, identify landmarks and make route-finding decisions.
That is a completely valid reason to turn around.
A route that is straightforward in clear weather can become remarkably difficult when you can’t see the next three metres, let alone the ridge you’re supposed to follow. GPS can tell you where you are, but it misses micro-terrain features that can still hurt you.
Before committing to consequential terrain, ask yourself: Can I safely navigate this route if I can’t see it? If the answer is no, don’t enter the cloud in the first place.
Cloud can contribute to a navigation mistake, turn that mistake into an accident, and then prevent the helicopter from reaching you.
Poor visibility is a mountain hazard. Treat it like one.