Tropical rainforest and humid heat

For sweat to cool you, it has to evaporate, and for that the air needs “room” for more water vapour. In high humidity that room is almost used up. That is why a humid 30 degrees often feels far more draining than a dry 30 degrees, and heat illness can occur at temperatures that would still be quite bearable in dry air. Weather services therefore sometimes combine temperature and humidity into a “feels-like temperature” or heat index.

Constant wetness has a second effect: skin that never properly dries becomes soft and sensitive. Chafing, blisters and small wounds heal less well and can become infected. The feet suffer most when they spend days in wet shoes. Gear also goes mouldy, rusts or loses its insulation if it never dries.

Near the equator the day is about twelve hours long all year, and dusk is short. Under the dense canopy it gets dark even earlier. Heavy downpours often arrive suddenly, and water from large areas collects quickly in streams and rivers – sometimes even though it is not raining where you are.

Preparation starts at home: travel-medicine advice well before the trip covers vaccinations, protection against mosquitoes and how to handle drinking water at your destination. Which measures are needed depends heavily on the region; those questions are answered by a specialist service, not a general text.

On the ground, pace is decisive. The body needs a few days to adjust to humid heat. Walk more slowly in the first days, put strenuous sections in the cooler early hours and plan breaks in the shade. Drink regularly and do not rely on thirst alone. Treat water from streams or rain before drinking it, for example by boiling it; a filter on its own does not make it safe.

A proven habit is the night set: by day you walk in clothing that will get wet anyway. One set of dry clothing stays in a waterproof bag and is worn only in camp and for sleeping. In the evening you dry your feet, check them for sore spots and let them air. That way at least the night stays restful.

Heat illness is the biggest immediate danger in humid heat. Severe exhaustion, headache, nausea and dizziness are signals to stop exerting yourself at once and find shade. Confusion, vomiting or reduced consciousness are an emergency: call the local emergency number (112 in the EU), move the person into the shade and cool them.

After heavy rain streams and rivers can rise very quickly. Never camp right on the bank, in gullies or hollows; choose a raised spot. Look up as well: dead trees and branches fall frequently in the damp forest. A hammock with a mosquito net and tarp keeps you off the wet ground and protects you from mosquitoes and crawling creatures.

In many tropical regions mosquitoes transmit diseases such as malaria or dengue. Long, light-coloured clothing, insect repellent and a net for sleeping are the basic protection; anything beyond that you sort out before the trip. Only leave the paths with a local guide – navigation is difficult in dense forest, and plants and animals are not something to identify on your own.

Humid heat is not only found in the tropics. Central Europe also has muggy summer days when the body struggles to cool down. The same principles apply then: less exertion, seek shade, drink regularly and pay special attention to children, older people and the sick.

In the rainforest water is often plentiful but not automatically drinkable. Streams can contain pathogens even when they look clear. The chain of collecting, pre-filtering if needed and then disinfecting, for example by boiling, remains important there too.

Travel-medicine references such as the CDC Yellow Book and the Wilderness Medical Society's guidelines describe why high humidity hinders heat loss. But they do not replace a personal consultation with a travel-medicine service that knows your health and your exact destination.