Rare Natural Phenomena Quiz

Answer: It reveals high veils of cirrus cloud, which often come ahead of a weather front. The halo comes from light refracted by ice crystals in cirrus, very high clouds. These veils often arrive ahead of a warm front. So the old saying isn't entirely wrong, though it is not infallible.

Answer: The walker sees his own huge shadow cast on the mist, surrounded by a coloured halo. With the Sun behind you, above a sea of clouds, you see your own enormous shadow copying your every move. The phenomenon is named after the Brocken, a German peak often shrouded in mist and already linked to witch legends.

Answer: Its droplets, still liquid below 0 °C, freeze instantly on contact with surfaces. These 'supercooled' droplets need only the slightest surface to turn into ice. The rime builds up mainly on the windward side, sometimes forming long white feathers. On roads and aircraft wings, the phenomenon is particularly dangerous.

Answer: A microalga and pigment-producing bacteria thrive in their very salty water. The microalga Dunaliella salina makes red pigments to protect itself from intense sunlight, and salt-loving microbes deepen the colour. At Lake Retba, workers harvest salt by hand from canoes, smearing their skin with shea butter to protect it.

Answer: The 'Sardine Run', a migration of billions of sardines. Between May and July, huge shoals of sardines move up the coast, drawn by cooler waters. Predators herd them into tight 'bait balls', attacked from all sides. In terms of biomass, the migration is sometimes compared with the great migrations of the Serengeti.

Answer: Upwards, from the top of thunderclouds into the stratosphere. These cones of blue light shoot from the tops of cumulonimbus at high speed and can reach 40 to 50 km in altitude. They are very hard to see from the ground, hence the value of observations from space.

Answer: Glider pilots. This tube-shaped cloud can stretch for hundreds of kilometres and move at more than 50 km/h at daybreak. Glider and microlight pilots ride the rising air at the front of the roll, like a surfer on a wave, around the small town of Burketown.

Answer: From Arabic, in Sudan, where it evokes a blowing wind. A storm's downdrafts lift sand and form a front that can reach several kilometres high. Common in Sudan and the Sahara, these dust walls also hit Arizona, where the word has been adopted. Visibility drops to a few metres.

Answer: A layer of wet, sticky snow on top of an icy crust, and just the right wind. The wind lifts a small chunk of snow that rolls and wraps around itself, leaving a track behind. The underlying layer must be smooth so the snow doesn't stick to the ground. These rollers, often hollow in the middle, are rare and short-lived.

Answer: Nyiragongo. In its crater, a sea of molten lava has been churning for decades. Its very fluid lava is particularly dangerous: in the eruptions of 1977, 2002 and 2021 it raced down the slopes towards the city of Goma.