Why does mountain weather change so suddenly?
― The mountain itself is a slope that makes clouds
The valley below is under blue sky, yet the mountaintop is wrapped in white cloud and cold rain is falling. This happens all the time on mountains. The reason isn't just "because it's high up." A mountain is a giant ramp that forces wind upward. The lifted air expands and cools, and the water vapour it can no longer hold turns into droplets. On top of that, sun-warmed slopes pull air up from the valley, so cloud tends to build the further into the afternoon you go.
You leave the station in the morning under a cloudless sky. The forecast said "fine." You start the climb in short sleeves, warm enough to work up a sweat.
Past noon, though, you reach the ridge and suddenly find yourself wrapped in white mist. The wind picks up, and rain starts spattering against you. You're soaked, your sweat turns cold, and the heat from a moment ago feels like a different day entirely.
Look down at the valley, and the town is still bathed in sunshine. Same day, same hour. Why is it so different up on the mountain?
There are two main reasons
When wind hits a mountain, it can't dodge around it, so it climbs the slope instead. As the air rises it expands and cools, and the water vapour it was carrying turns into droplets. Moist wind that would do nothing at all over flat ground turns to cloud the moment it meets a mountain.
When morning sun warms a slope, the air above it grows lighter and rises. Wind flows up from the valley to replace it. That's why cloud can build in the afternoon even after a clear morning.
Both are ways of moving air upward. Let's look at each in turn.
Air that's forced upward cools just from rising
The higher air goes, the lower the surrounding pressure. With less pressure squeezing it, a parcel of air expands. Expanding takes work — the air pushes against its surroundings — and that work is paid for out of its own heat. So even with no flame anywhere near it, air cools simply by rising. This is called adiabatic cooling.
The amount of water vapour air can hold shrinks as it cools. At some point, the cooling air can't hold any more, and the excess vapour turns into fine droplets, which is what we see as white cloud. The height at which cloud starts to form is set almost entirely by how moist the air is.
A mountain keeps up this lifting all day without a break. As long as the wind is blowing, the mountain acts as a slope pushing air upward. That's why cloud can sit stubbornly on a mountaintop even while the valley below is sunny. Take a look at Figure 1.
Why does cloud tend to build later in the day?
In the morning, a slope faces the sun head-on. As the ground warms, the air above it warms too, grows lighter, and starts climbing the slope. Air is drawn up from the valley to fill the gap. This flow is called a valley wind.
Valley winds are said to strengthen gradually from morning and peak in the early afternoon. In other words, the afternoon is when lifting by wind and lifting by sunlight overlap on a mountain. That's the source of the change from clear blue morning sky to sudden cloud and rain once midday has passed.
The mountaineering advice to "start early, finish early" exists to dodge this window. After dark, the pattern reverses: a mountain wind flows down from the cooling slopes into the valley.
Weather forecasts are, in most cases, aimed at the towns down in the valley. Once you climb to 1,000 or 2,000 metres, the temperature, wind and cloud are a different world. Before heading into the mountains, it's standard practice to check information and forecasts specific to mountain weather and to the particular range you're entering.
Air that dropped its rain on the way up has less water vapour left when it descends the far side. Descending air is compressed and warms, so a warm, dry wind can blow on the far side of a range. This is called the foehn phenomenon, and it's a cause of sudden temperature spikes in areas beyond a mountain range (for more, see Why does wind that has crossed a mountain suddenly turn hot?).
So what should you actually do?
- Walk the mountain in the morningCloud tends to build in the afternoon. It's safer to already be heading down by midday.
- Carry a jacket and rain gear even when it's sunnyThe mountaintop can be more than 10°C colder. Getting wet and hit by wind strips body heat fast.
- If cloud suddenly increases, turn back"Just a bit further" is dangerous. Deciding sooner leaves you more routes to choose from.
On mountains, hypothermia — the drop in body temperature caused by wet skin and wind — is said to occur even in summer. If you notice uncontrollable shivering or unclear responses, move somewhere sheltered from the wind, change out of wet clothing, and warm the body. If the sky darkens and thunder sounds approach, stay away from ridges and tall trees. The standard advice is to leave open ground and take shelter in a sturdy building such as a mountain hut. If someone becomes unable to move, or their consciousness is unclear, call 119 without hesitation. In the mountains, some situations may be handled through 110 instead. What's written here is a general guide — the best action depends on the situation. Follow the instructions of the fire department, police and local authorities.
Summary
Mountain weather changes so readily because the mountain itself is a machine for moving air upward. Wind hitting it gets lifted by the slope; sunlight hitting it gets pulled up from the valley. The lifted air expands, cools, and its water vapour turns to droplets, forming cloud. Air unlike the valley's is being made continuously on the mountain above. That's how the same day, the same hour, can be sunny in town and rainy on the mountain.
Mountain weather isn't fickle.
It's the result of a slope — the mountain — continuously lifting air.
For how clouds manage to float despite being made of water, see "Clouds are made of water, so why don't they fall?"; for how afternoon cloud grows so tall, see "Why do thunderclouds (cumulonimbus) grow so incredibly tall?"; for how wet skin and wind cool the body, see "Why does hypothermia happen even on days that "aren't that cold"?"; and for how altitude alone affects the body, see "Why does altitude sickness happen just from gaining elevation?" A larger-scale example of air being lifted by a mountain and dropping snow is explained in "Why does Japan's Sea of Japan coast get so much snow?"
- On a clear day, look out at distant mountains twice — once in the morning, once in the afternoon. The ridgeline is often crisp in the morning, yet by afternoon the summit alone is often capped in cloud.
- Take a ropeway or similar up in elevation, and compare thermometer readings at the bottom and the top. You can feel for yourself the rule of thumb that temperature drops by roughly 0.6°C for every 100 metres of elevation.
- After using a hand pump to put air in a bicycle tyre, touch the pump's barrel. Compressed air warms up. The same warming you feel with your palm is exactly what happens to air descending a mountain.
Only make mountain observations on maintained trails and with proper equipment.
Want to go deeper? ― Terms, formulas, and how this maps to the curriculumFrom middle-school science to university-level specialist subjects, each level is labelled
- MSCovered in middle-school science
- HSCovered in high-school "Earth Science Basics / Physics Basics"
- HS+High-school advanced content, or textbook sidebar material
- Univ.Not taught in high school — university-level specialist content (meteorology, atmospheric dynamics)
- ResearchNot even taught as settled fact at university — an active research question
MSTerms: this phenomenon has names
- Adiabatic cooling: the temperature drop that occurs when a parcel of air rises and expands without exchanging heat with its surroundings. This is where cloud formation on mountains starts.
- Orographic lifting: wind hitting a mountain or hill and being forced up along the slope. Most mountain cloud is born this way.
- Lifting condensation level: the height at which lifted air can no longer hold its water vapour and cloud starts to form. The dashed line in Figure 1 marks this.
- Valley wind and mountain wind: wind blowing up from the valley to the mountain during the day is a valley wind; wind blowing down from the mountain to the valley at night is a mountain wind.
MSHSWorking it out: how cold is a 2,000-metre summit, really?
The temperature of lifted air can be roughly estimated from its altitude. Here we use rough figures and take it all the way through to how cold it actually feels. The symbols used, and their units, are in the table below.
| Symbol h | Meaning: elevation difference from foot to summit / unit: metres |
| Symbol t | Meaning: temperature drop from the elevation difference / unit: °C |
| Symbol w | Meaning: further drop in felt temperature from wind / unit: °C |
| Symbol T0, Tsummit | Meaning: temperature at the foot and at the summit / unit: °C |
| Symbol Γ | Meaning: lapse rate (temperature drop per 100 metres) / unit: °C |
| Symbol k, V | Meaning: felt-temperature drop per 1 m/s of wind (°C), and wind speed (m/s) |
| In symbols | Tsummit = T0 − Γ × h (felt temperature = Tsummit − k × V) |
| In words | Summit temperature = foot temperature − lapse rate × elevation difference. Felt temperature = summit temperature − (felt-temperature drop per 1 m/s of wind) × wind speed |
| Where it comes from | The first formula comes from the fact that pressure is lower at altitude, so lifted air expands and cools by using its own heat to do that work (adiabatic cooling, from the first law of thermodynamics). Γ is roughly 0.6°C per 100 metres in an average atmosphere. The second is an empirical rule summarising how wind strips away the warm air layer at the body's surface. |
| Temperature at the foot | taken as 25°C |
| Elevation difference from foot to summit | taken as 2,000 metres |
| Temperature drop per 100 metres of elevation gain | taken as roughly 0.6°C |
| Wind speed at the summit | taken as 10 metres per second |
| Drop in felt temperature per 1 m/s increase in wind speed | taken as roughly 1°C |
| How many 100-metre units is the elevation difference | 2000 ÷ 100 = 20 |
| Temperature drop from elevation | 20 × 0.6 = 12 |
| Summit temperature | 25 − 12 = 13 |
| Felt-temperature drop from wind | 10 × 1 = 10 |
| Felt temperature including wind | 13 − 10 = 3 |
Even on a 25°C summer day at the foot, the summit ends up around 13°C. Add a strong wind, and it can feel more like 3°C — about as cold as a winter morning. If a body that climbed up in short sleeves is soaked with sweat and rain, heat is stripped away even faster. This buildup is why hypothermia is said to strike even in summer.
HSHS+Dry air and moist air cool at different rates
HSAir with no cloud forming in it cools at a fixed rate of roughly 1.0°C per 100 metres of rise. This is because the energy spent expanding shows up directly as a drop in temperature.
HS+Once cloud starts to form, though, the cooling slows down. As water vapour turns to droplets it releases heat, and that heat warms the air back up. The cooling rate in this case is said to be roughly 0.4 to 0.5°C per 100 metres. It's this reheating that lets air inside a cloud keep climbing while staying warmer than its surroundings.
Univ.What decides whether air goes over a mountain or around it
Air hitting a mountain doesn't always climb the slope. When the atmosphere is stable and the mountain is tall, it becomes easier for the air to flow sideways around it than to rise, producing flow that skirts the mountain. This dividing line is captured by the mountain Froude number, a dimensionless value found by dividing wind speed by (buoyancy oscillation frequency representing atmospheric stability, i.e. the Brunt–Väisälä frequency) × mountain height. Roughly speaking, a small value means the flow goes around; a large value means it goes over. Air that crosses a mountain can also oscillate up and down like a wave, producing regularly spaced cloud bands or strong downslope winds on the lee side. This is called a mountain wave, and it's covered in atmospheric dynamics textbooks.
📖 For the derivation and further reading: Lapse rate (Japanese Wikipedia) ／ Froude number (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How to build fine-scale terrain into forecasts. Numerical forecasts represent the ground as a grid, and ridges and valleys smaller than the grid get smoothed away. This is thought to be one reason mountain wind and cloud can't be fully reproduced.
- Predicting when and where cloud will form. Cloud born from valley winds is affected by even slight differences in ground moisture and vegetation. Pinning down the time and place it forms remains a difficult problem.
- Too few observation points on mountains. Observation stations at high elevation are hard to maintain, so real temperature and wind data are scarce. The material available for verification is itself limited.
In other words, even the content of this article is "the best explanation given what's currently known." As observation and calculation get finer, some parts may need to be rewritten.
How this maps to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science ― Weather and its changes | How rising air forms cloud, and the relationship between elevation and temperature |
| HS | Earth Science Basics ― Atmosphere and ocean / Physics Basics ― Heat and work | Adiabatic cooling and the calculation of summit temperature |
| HS+ | Earth Science advanced ― Latent heat and moist adiabatic processes | The section on why dry and moist air cool at different rates |
| Univ. | Meteorology / atmospheric dynamics (terrain and flow) | What separates flow that crosses a mountain from flow that goes around it |
| Research | Local meteorology / higher-resolution numerical forecasting | The "what's still not fully understood" section |
| ― | Everyday relevance | Moving in the morning, and turning back when cloud increases |
- Ogura Yoshimitsu, General Meteorology (2nd revised edition), University of Tokyo Press (東京大学出版会)
- Meteorological Society of Japan (日本気象学会), ed., Encyclopedia of Meteorological Science, Tokyo Shoseki (東京書籍)
- Japan Meteorological Agency (気象庁), "Forecast terminology ― wind, cloud, local winds," JMA website
- Inokuma Takayuki, The Complete Guide to Mountain Weather, Yama-kei Publishers (山と溪谷社)
- National Police Agency (警察庁), "Overview of mountain accidents," NPA website
※This article is a general-audience science explainer. The figures given are approximations meant to help illustrate how things work. For decisions about heading into the mountains or acting in severe weather, follow announcements from the Japan Meteorological Agency and instructions from fire, police and local authorities.