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.
