1- Hi, it's Doug.
2It's winter time here in California and it just never gets that cold.
3But I have with me here a chunk of dry ice.
4Now this stuff is cold.
5The reason I have to wear gloves is because this is so cold
6if I were to touch it, I would instantly get frostbite.
7Someone named Valeria has a question about the cold.
8Let's give her a call now.
9- Hi, Doug.
10Hi, Valeria.
11I have a question for you.
12What's the coldest place on Earth?
13That's a great question.
14Before I say anything more,
15think about the coldest weather you've ever experienced.
16What was it like?
17Well, for me personally, I grew up near Chicago,
18where I experienced some pretty cold winters.
19But that was nothing compared to what my niece and nephew experience.
20They live in Yellowknife, a town in far northern Canada.
21And in winter, temperatures there get as cold as minus 40 degrees.
22It's hard to even imagine what that's like, so...
23let me give you a sense of it.
24Here's a video of me when I visited there.
25The snow is so deep that if you run and jump into it,
26you sink way down.
27Even more amazing?
28Watch what happens as someone tosses a cup of hot water into the air.
29Do you see that?
30Some of it instantly turns to snow.
31When I visited, I even got to ride on a huge slide made of ice,
32located inside of a snow castle.
33In places this cold, you can even drive a car on a frozen lake.
34Because the ice is that thick.
35Still, northern Canada isn't the coldest place on Earth.
36So, what is then?
37Is it on top of the tallest mountain?
38Maybe the North Pole?
39Don't forget about the South Pole, too.
40Which is it?
41Well, the real question is, how do we know?
42How do scientists figure this out?
43We could visit lots of these places, and...
44just try to decide based on how it feels to us.
45But once things get really cold, it all just kinda seems the same.
46We need some way to measure how cold it is,
47and that's where the thermometer comes in.
48These days, we have digital thermometers that tell us the temperature outside,
49like on this phone.
50Or like ones you see on signs.
51But since you can't see inside one of these digital thermometers,
52it's hard to understand how a thermometer works.
53It's easier if we have a look at an old-school thermometer,
54one like this.
55A few hundred years ago,
56someone noticed that if you take a thin, hollow glass tube
57and you fill it partway with liquid,
58then suck all the air out of the top of the tube,
59the liquid will actually rise up or down in the tube,
60depending on how warm or cold you get the tube.
61If the tube gets warmed up, the liquid rises.
62If the tube gets cooled down, the liquid goes down.
63A few different people had the brilliant idea to add numbers along the side of the tube,
64so that we could have a way to measure how warm or cold something is.
65These numbers are what we mean when we talk about degrees.
66Most of the world uses a temperature scale
67invented by a scientist named Anders Celsius.
68The Celsius scale uses the freezing point of water as its zero degree mark,
69so in other words, when it starts to get cold enough that liquid water becomes solid ice,
70that's zero degrees Celsius.
71Anything colder than that is a minus, or negative temperature like minus 40.
72By comparison, the warmer the weather is, the more the temperature goes above zero.
73On the Celsius scale,
74the 100 degree mark is the temperature at which water starts to boil.
75So basically, the temperature of water on a stove.
76Before the thermometer was invented,
77no one really knew where the coldest place on earth was.
78But once it was invented,
79scientists and explorers could start taking measurements of temperatures all over the world,
80and now, we know exactly where the coldest place in the world is.
81It's probably not going to surprise you
82that both the North and the South Pole are very cold.
83But, what might surprise you is that...
84it's actually colder at the South Pole than it is at the North Pole.
85In wintertime, the North Pole is usually around minus 35 degrees Celsius,
86but the South Pole?
87Temperatures in winter are often around minus 60 degrees,
88and the record all-time coldest temperature ever recorded
89reached almost minus 90 degrees Celsius.
90That's four times colder than it even gets inside your freezer!
91It's not obvious why the South Pole would be colder than the North Pole,
92until you consider that the North Pole is located on basically on a low, frozen section of the Arctic Ocean.
93Whereas the South Pole is on top of a giant ice sheet more than a mile thick.
94So the South Pole is at a higher elevation.
95And high elevation places like mountaintops are always colder than low places.
96So in summary, the South Pole is the coldest place on Earth.
97We know this thanks to thermometers that have been placed there by explorers and scientists.
98That's all for this week's question.
99Thanks, Valeria, for asking it.