Light signals at night: SOS and flashing

In darkness the eye has little else to see, so every light source stands out. But there are many fixed lights – houses, roads, vehicles. What sets a signal apart is movement and rhythm. A light that flashes regularly or moves looks like a message.

SOS comes from Morse code, which was formerly used for radio telegraphy. Three short signals stand for S, three long ones for O. For the distress sign, the letters are given one after the other without a pause: ··· ––– ···. SOS is listed in the international rules for distress signals at sea as a light or sound signal and is known worldwide.

Not every observer can read Morse code. That does not matter: a regularly repeated pattern is still recognised as a human signal. Light signals in groups of three or the alpine distress signal (six signals per minute) are also understood. What counts is rhythm and repetition.

Find a spot with a clear view towards where help may come from – a knoll, a clearing, a shore. Nobody can see you from a hollow or behind dense bushes. But only go there if you can do so without danger; at night the terrain is much harder to judge.

Point the lamp at possible observers – lights of vehicles, houses, boats, the direction of an engine noise – or sweep it slowly across the horizon. Many lamps have a strobe or SOS mode: it is conspicuous and saves battery. Try out before the trip how to switch it on, so you do not have to fumble in the dark.

Rationing energy is important. Signal at intervals, and especially when you hear or see something. Cold weakens batteries; keep spare batteries warm, for example in an inside pocket. If you also need the lamp for walking and working, think carefully about how much you can spare for signalling.

Never shine powerful lamps directly into the cockpit of a helicopter or aircraft, and never point a laser at an aircraft. Crews often fly at night with night-vision devices that strongly amplify bright light – they can be dazzled by it. Pointing lasers at aircraft is illegal in many countries.

As soon as you have been spotted – for instance because a helicopter is flying towards you or circling overhead – stop flashing in its direction. Instead point the light at the ground or at yourself, so the crew can see your position without being dazzled, and follow their instructions.

SOS and other distress signals are only for genuine distress. Giving them for fun risks rescuers turning out for nothing and can lead to costs or penalties in some countries. And if your phone has signal, call the local emergency number first (112 in the EU).

Even without a headlamp there are options. A phone's torch function works, but uses battery you may need for an emergency call. Vehicle headlights or hazard lights can also be used to signal – but the vehicle battery should not run completely flat, or the engine will not start again.

A light stick swung in a circle on a cord creates a large, clearly visible ring of light. That is more conspicuous than a small, steady point of light. In fog and heavy rain, however, light signals are severely limited; then a whistle and shouting often help more.

Practise the SOS rhythm at home, for example with a torch against a wall or with the light switch: short-short-short, long-long-long, short-short-short, then a pause. Count in your head as you go – a short signal is about 'one', a long one about 'one-two-three'. After a few rounds the rhythm sticks.

Practise outdoors in a way that nobody could take for a real emergency – so not at night towards houses, boats or roads. Use the opportunity to check your lamp's modes too: how do you switch on strobe mode, how long does the battery last, where are the spare batteries?