# Snow in the Atacama Desert

A convincing argument can be made that the Atacama Desert in Chile and Argentina is the driest place on Earth. The average rainfall is one millimetre per year and some weather monitoring stations have never detected rain. This week eighty centimetres of snow fell in the region.

This is what the desert normally looks like:

And this is how it looks now:

The South Pacific is on the left of the image and Chile and Argentina on the right.

It’s difficult to differentiate between cloud and snow in the true-colour image above, but the false-colour image below makes the difference more obvious.

Areas covered in snow are bright white in the visible part of the spectrum and all the visible light detected by the camera in the second image is mapped to the red channel of the image. Ice is very absorbent in the shortwave infrared region that is mapped to the green and blue channels and therefore ice appears bright red.

# A public service announcement regarding paper snowflakes

As Christmas draws nearer decorations are starting to appear in windows, and some of those decorations are paper snowflakes. The problem is that most paper snowflakes are wrong.

Real snowflakes demonstrate hexagonal (sixfold) symmetry; each snowflake is composed of one part, repeated six times, with a separation angle of 60°.

The “standard” method of creating paper snowflakes, folding a piece of paper in half and then in half again, creates snowflakes with tetragonal (fourfold) symmetry, each piece separated by 90°.

How to Make A Genuine Paper Snowflake

1.) Get a square piece of paper and fold it in half diagonally twice, across both sets of corners.

2.) Using the centre of the piece of paper as a marker, fold the paper into equal thirds, and trim off the top to create an equilateral or isoceles triangle.

4.) You should now have a folded triangle of paper. Cut your pattern into the paper, making sure that it is symmetric (if it’s not symmetric you’ll end up with three-fold symmetry). If you cut off the edges of the triangle but not the centre you’ll end up with the desired “spokey” effect.

Almost every day I walk past this yellow grit container. I’ve never really paid attention to it before, but it’s certainly in the news at the moment.

The term “grit” is a bit of a misnomer and an ambiguous term. The “grit” used on roads is actually rock salt, known properly as halite.

Halite is the mineral form of sodium chloride, the same salt that’s used to season food. Most of the UK’s halite comes from Winsford Rock Salt Mine in Cheshire in the North West. You can see the yellow gritting vehicles at the bottom of the map below.

Gritting roads is all about melting points. Water begins to freeze below 4°C, so if the ambient temperature is anywhere below 4°C any water on the roads will turn to ice, which is very slippery and cannot be pushed away by tyre tread.

Spreading salt on the roads causes it to mix with the water to produce brine. The saltier the brine, the lower the melting point. For example: a mixture of 30g of salt per 1000g of water lowers the melting point to −1.6°C. Now imagine the ambient temperature is 0°C: any pure water on the road will freeze but the salty water will remain a liquid.

Unfortunately there is a limit to salt’s usefulness. To make the melting point as low as possible you must add as much salt as possible and supplies are limited. Even a completely saturated solution, a totally impractical measure using a huge amount of salt (23.3% salt by weight) only has a melting point of −21.1°C, making the use of rock salt pointless below this temperature. Realistically it would be impractical and very difficult for councils to use rock salt to cope with temperatures much below −10°C.

# Satellite view of snow-covered UK from space

This is what the UK looks like at the moment, covered in snow. Clicking on the image will load a high resolution (250m per pixel) version (3400×4400 px, 3.02MB).

The imagery comes from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) satellite’s Rapid Response System which provides near real-time imagery of the Earth’s surface.