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For about 10% of the population, including the entirety of Australia, it is now Summer.
But for the other 90% living in the Northern Hemisphere, including most of Asia, all of
Europe, and all of Central and North America, it's time for winter. Temperatures drop,
snow falls from the sky, and a guy in a red suit breaks into homes all over the globe
and we're all weirdly okay with this? But even though most of us associate winter with
cold, the Earth is actually at its closest to the Sun during late December and early
January. So how's that possible? Well, in case you didn't know, the Earth's orbit is
not perfectly spherical, it has a slight oval shape. Which means that there are points in
the Earth's orbit at which it is closer or farther away from the Sun. The most distant
point is known as aphelion and occurs around the 4th of July. The closest point is known
as perihelion and occurs around the 3rd of January. So while that makes a lot of sense
for the Southern Hemisphere it doesn't make much sense to the other half of the globe.
That is, until you account for the 23.4° tilt of the Earth which is what gives us seasons
to begin with. Also known as axial tilt or obliquity. And if we look at the Sun-Earth
system from afar we find that the Earth's axis maintains the same orientation regardless
of its position in its orbit around the Sun. And that's the key to the paradox. When the
Earth is closing in on the Sun during December the Northern Hemisphere is turned away from
the Sun giving us a cooler climate even though the Earth is now at its closest to the sun.
Meanwhile the Southern Hemisphere is directed towards the Sun and thus gets a warmer climate.
When the Earth is moving away from the Sun again the opposite occurs. In summary, the
tilt of our planet has a much greater significance compared to the variations in distance to the Sun.
Rough estimates show that around 28,000 km³ of snow falls down on the Earth ever single
year. It's more or less impossible to imagine what the number really means but this is what
only 1 km³ looks like compared to the city of New York. 28,000 km³ of snow also contains
roughly one septillion snowflakes. That's a 1 with twenty four 0's. And as the saying
goes: "No two snowflakes are alike." But is that really true? This adage begins with a
man known as Wilson Bentley. In 1885, Bentley became one of the first photographers to capture
an image of a single isolated snowflake and for the rest of his life he was obsessed with
snowflakes. Even gaining the nickname The Snowflake Man and taking over 5000 photos
before his death in 1931. He caught the snowflakes on a piece of black velvet which allowed him
to capture an image before the snowflakes melted or sublimated. And even with his limited
technology at the time, he was so good at this that hardly anybody bothered to take
photographs of snowflakes for the next century or so. In 1925 he wrote:
"Every crystal was a masterpiece of design, and no one design was ever repeated.
When a snowflake melted, that design was forever lost."
The thing is, two snowflakes can definitely look alike if
they happen to be exposed to the exact same conditions. These conditions includes humidity,
temperature, wind speeds and so on. This claim can also be demonstrated in a controlled environment
like a lab for example. Shown here in a video from snowcrystals.com in which a pair of snowflakes
grows to look very much alike. Of course, nature is not a controlled environment and
thus two of the same is a very unlikely occurrence. However in 1988 scientist Nancy Knight did
actually find two tiny ice crystals in a snowstorm that looked quite similar. So maybe a more
accurate saying would be something like: "No two snowflakes are identical."
According to Guinness World Records, the largest snowflake ever recorded measured in at a staggering
38 cm. Though the truthfulness of this record is questionable at best as this supposedly
happened back in 1887 which means there's no corroborating evidence to support this
claim. And it is a quite fantastical claim to make given that most reports with more
substantial evidence usually range between 5-12 cm. I mean can you imagine seeing half
a meter wide snowflakes just sailing down from the sky. It would be a really bizarre
sight. And speaking of bizarre, the largest hailstone ever found measured in at roughly
20 cm, weighing almost 1 kg. It would be like bowling balls falling from the sky.
If you look at a close-up picture of a snowflake it will appear translucent, just like any
other piece of ice. Then why is snow white? Shouldn't it just be a massive pile of translucent
ice? Well when light enters a material some colors are absorbed and others are reflected
away. The reflected color or colors determine what color we perceive that material to have.
In a translucent material like ice light instead passes trough the material which means that
no color is reflected nor absorbed. However it's not completely transparent like a sheet
of glass so the light doesn't always go trough in a straight line. Instead sometimes the
light enters the material from one direction, bounces around a bit, and then exits the material in a new
random direction. Now if you take a bunch of tiny pieces of ice and pack them close
together (just like snow) the light just keeps bouncing around between the snowflakes. Eventually,
the bouncing light will find a way to escape the pile of snow and reflect back into the
eyes of any potential observers. And because this happens to every frequency in the visible
spectrum, the combined color of all colors is white. The same thing happens with ice cubes.
One ice cube is translucent, but if you put a bunch of
them together, they obtain a collective white shade.
Have you ever noticed that when it's snowing outside ambient noise seem to dampen and everything
becomes a lot more quiet and muted. The reason for this is that, like many other porous materials,
fresh snow is very good at absorbing sound. The pockets of trapped air in between the
snowflakes actually weaken the vibrations and converts the sound into small amounts
of heat. That's one of the reasons it can be so difficult to find someone lost in an
avalanche. Just a couple of meters of snow can be enough to make it incredibly difficult
for the sound to reach the surface.
While most people use the words blizzard and snowstorm synonymously there's actually a
few distinctions between the two. A blizzard is typically characterized by strong winds
of at least 56 km/h, has to last at least 3 hours, and visibility must be reduced to
only 400 meters. Anything that does not meat these criteria is classified as a snowstorm.
The deadliest blizzard on record is the 1972 blizzard in Iran. Some places received as
much as 8 meters of snow in the course of a week and around 4000 people lost their lives.
The blizzard was so devastating that entire villages were completely buried in snow.
An interesting survival tactic used by some animals is to change the color of their fur
to white during the winter. One example is the Arctic fox. It's a rather small species
of fox, native to the Arctic regions of the Northern Hemisphere, with thick brown fur
during the summer and white fur during the winter to blend in with the snow-covered landscape.
The fur of the Arctic fox provides the best insulation of any mammal and it can survive
in temperatures as low as -70 °C. Another example is the bird Rock ptarmigan which has
brown feathers during the summer and white during the winter.
When we look at a flat map of the world, it usually looks like this, and it's easy to forget that
Russia and the US are actually right next to each other. In fact, in the middle of the
Bering Straight in between Alaska and Siberia lies two islands. The one on the right belongs
to the United States and is known as Little Diomede and the one on the left belongs to
Russia and is known as Big Diomede. During the winter, an ice-bridge usually spans the
distance between these two islands and thus it's
possible, altough illegal, to walk back and forth between the two nations.
The coldest temperature ever recorded at ground level on Earth was at the Soviet Vostok Station
in Antarctica in 1983. Temperatures dropped to a staggering -89.2 °C.
The Vostok Station is by far the coldest place on Earth with an average summer temperature
of -32 °C. In the winter, the average temperature drops to about -68 °C.
The highest temperature ever recorded at the station was -14 °C.
For many animals and insects, the winter is one of the more challenging of the four seasons.
Not only does food become significantly more scarce but there's a constant struggle to
stay warm in the bitter cold. One of the more fascinating strategies for survival, is not
by fighting the cold, but rather to surrender and give in. It's known as "freeze tolerance"
and it allows some insects and animals to survive the winter by essentially freezing.
The wood frog can be found in many parts of Canada and during summer it just frogs around and does
whatever a normal frog would do. But during winter, something remarkable happens. Ice begins to
penetrate the body of the frog creating large flat ice crystals between layers of skin and
muscle, encasing all the internal organs. The blood stops circulating and there's no
heartbeat, breathing, or any other natural signs of life. By all common sense, the frog
appears to be dead. But it's more accurately in a state of suspended animation because
when the climate warms up again the frog returns to his normal froggy-life.