The two most common ways a volcano day goes wrong have nothing to do with volcanoes. They are the cold, which people did not expect because they were in the tropics yesterday, and the altitude, which people did not respect because they did not have to work for it.
Both are entirely predictable. Both are arithmetic.
The arithmetic of getting colder
Air cools as you go up, at a rate that is remarkably consistent: roughly 6.5 °C for every thousand metres. That is the standard figure used in aviation, and it holds well enough in practice that you can plan with it.
Put a number on it and the tropics stop being tropical. If it is 28 °C at the beach in Bali, then at the top of Batur at 1,717 metres it is around 17. That is before the wind and before four in the morning, which is when you will be standing there.
The same sum on the bigger mountains produces figures people simply do not believe until they are shivering.
| Where | Altitude | Base station reads | Estimated at the top |
|---|---|---|---|
| Mount Batur, Bali | 1,717 m | sea level, tropical | about 13 °C at dawn in August |
| Kawah Ijen, Java | about 2,386 m | Banyuwangi, near sea level | about 8 °C at night in August |
| Penanjakan, Bromo viewpoint | 2,770 m | Malang, 590 m | about 4 °C, and in practice colder |
| Santa Ana, El Salvador | 2,381 m | 1,771 m | about 7 °C in January |
| Haleakala, Maui | 3,055 m | summit station | around freezing in December, wet and windy |
| Acatenango base camp | about 3,600 m | Antigua, 1,545 m | about minus 3 °C in January |
| Acatenango summit | 3,976 m | Antigua, 1,545 m | about minus 5 °C in January |
| Mount Rinjani summit | 3,726 m | Mataram, sea level | about minus 4 °C at dawn in August |
| Cotopaxi refuge | 4,864 m | Quito, 2,800 m | about minus 5 °C |
| Mount Fuji summit | 3,776 m | summit station | 3.8 °C mean minimum in August |
Look at that Fuji row again. August, the warmest month, on the summit: the average overnight minimum is 3.8 °C, and the coldest August night on record there is minus 4.3. This is the mountain people climb in summer in trainers.
Why calderas are colder than the arithmetic says
There is a correction to make, and it applies to exactly the places where sunrise tours go.
On a clear, still night the ground radiates its heat away to the sky. The air touching it cools, becomes denser, and does what dense air does: it flows downhill and collects at the bottom. In an enclosed basin it has nowhere to go, so it sits there and keeps cooling. Meteorologists call it cold air pooling.
A caldera is an enclosed basin. So is the sand sea inside the Tengger caldera at Bromo, where frost is regularly reported at around 2,300 metres on dry-season mornings, while the simple altitude sum predicts something like seven degrees.
Two consequences worth carrying with you. The dry season is the cold season at altitude, because clear skies are exactly what allows the ground to radiate away its heat. And the bottom of a crater at dawn is colder than the rim, which is counterintuitive and reverses everything you know about mountains.
The dangerous kind of easy
Altitude you did not earn is the altitude that hurts
On an ordinary mountain you climb for hours, and the effort is itself a warning system: you feel the thinning air gradually while your body has time to react.
Volcanoes break that. You can drive from sea level to 3,055 metres on Maui in about two hours, and to 4,207 metres on Mauna Kea in about the same. Cotopaxi's car park is at 4,600. The walk to the refuge above it takes forty-five minutes and is the shortest itinerary we have scored, and it happens where there is a little over half the oxygen you are used to.
The agency managing Mauna Kea publishes advice that most visitors never see. It recommends a minimum thirty-minute stop at the visitor centre at 2,804 metres before going higher, and advises against going above it at all if you are under thirteen, pregnant, have heart or respiratory conditions, or have been scuba diving within the previous 24 hours.
That last one is worth reading twice, because it describes a completely ordinary Hawaii itinerary: diving in the morning, stargazing in the evening.
What altitude sickness actually feels like
Mild altitude sickness is common above about 2,500 metres and it does not feel dramatic. It feels like a hangover: headache, nausea, no appetite, poor sleep, and being unreasonably out of breath on small efforts. Most people who get it above 3,000 metres describe exactly that, and most of them are fine.
The rules that matter are simple and not negotiable. Do not go higher if you feel worse. Going down fixes it, and going down a few hundred metres usually fixes it quickly. Alcohol and altitude are a poor combination, and dehydration makes everything worse, which is awkward because thin dry air dehydrates you faster than you notice.
Fitness does not protect you. This surprises strong walkers repeatedly, and the physiology is indifferent to how fast you can run.
The serious forms are rare at the altitudes in this catalogue but they exist above roughly 3,500 metres, and the trip that most deserves respect is Acatenango, not because of the climb but because you sleep at around 3,600 metres, often having arrived in the country a day or two earlier.
The layers that solve it
The practical fix for cold is boring and effective. Three layers, and the middle one is the one people leave behind.
Next to the skin, anything that is not cotton. You will sweat on the climb and then stand still in the wind at the top, and wet cotton at 4 °C is genuinely dangerous rather than merely unpleasant.
The insulating layer, which is the fleece or light down jacket that felt absurd when you packed it in Bali. It is the difference between watching a sunrise and enduring one.
Wind and rain, on top. Wind matters more than the thermometer: still air at 2 °C is comfortable in a fleece, and the same temperature with 30 km/h of wind is not.
Add gloves and something over your ears, both of which weigh nothing and are what people actually regret. And carry the layers rather than wearing them on the way up, because arriving at the top wet with sweat is the most common way to get properly cold.
If you do end up renting a jumper at the trailhead at four in the morning for more than it would have cost new, you will have joined a very large club.
Quick answers
How cold is it at the top of Acatenango?
Base camp sits at roughly 3,600 metres and calculated dawn temperatures in the dry season are around minus 3 °C, with the summit a couple of degrees colder. Clear dry-season nights are the coldest, which is also the season you want for seeing Fuego. Most operators supply a tent and a sleeping bag; assume you supply the warmth.
Do I need to worry about altitude on Bromo or Batur?
Not really. Batur tops out at 1,717 metres and Bromo's viewpoint at 2,770, both well below the range where altitude sickness is common. Cold is the real issue on both. Rinjani, Acatenango, Fuji, Teide and anything in Ecuador are a different conversation.
Can I take altitude medication?
Preventive medication exists and is prescribed for high-altitude trekking, and whether it is appropriate for you is a question for a doctor rather than a travel site. What we can say is that no medication substitutes for going down when you feel worse.
Why do I feel worse at night?
Breathing slows during sleep, oxygen saturation drops further, and altitude symptoms are typically worse in the small hours. It is why sleeping height matters more than the highest point you reached during the day, and why Acatenango is harder than its statistics suggest.
Is Mauna Kea safe to visit?
For most people, with the published precautions: stop at the visitor station for at least half an hour, do not go up if you fall into the listed groups, and remember that the summit closes to visitors from shortly after sunset to shortly before sunrise. It is not a casual detour, and the road above the visitor centre requires a four-wheel drive.
The volcanoes where none of this applies
Not every volcano is at altitude. Lanzarote, Santorini, the Azores and Hawai'i's park all sit low enough that the only thing you need to think about is a jacket.
Data verified on. Summit temperatures are calculated from national weather service normals at the nearest station using a 6.5 C per 1,000 m lapse rate, except Fuji and Haleakala which have summit stations. Editorial Policy