00:00Name a desert. Let me guess, Sahara? Well, actually, no matter which one you thought of,
00:06I'm sure of one thing. It's located somewhere below this line.
00:12See, when we hear the word deserts, we usually picture dry, sandy, blazing hot places,
00:18and there's not a single one of those above 45 degrees north latitude or below 45 degrees south.
00:25In the south, that kind of makes sense. I mean, if you leave out Antarctica, there's not a whole
00:31lot of land down there. But things are different above 45 degrees north. There are tons of land.
00:38Countries like Canada, France, the United Kingdom, and a bunch of others. And still, not a single
00:44desert. Actually, not a hot desert. Most of the major hot deserts are up in the northern hemisphere,
00:52near the tropics of Cancer and Capricorn. And the hottest ones are usually found between 15 and 30
00:58degrees north or south of the equator. Just like the Sahara, where average high temperatures can reach
01:04up to 104 degrees Fahrenheit. Together, hot deserts cover almost 15 percent of the Earth's land area.
01:12But if our planet is packed with them, why doesn't a single one appear above this line? I mean,
01:19the 45 degrees north latitude. I'll explain. Let's divide Earth into five main zones. The polar regions
01:27are located at both ends, north and south. Right here, we have the two temperate zones. And finally,
01:34right in the middle of the globe, is the tropical zone. Now, the line we're talking about sits in a
01:40temperate zone, in a colder area. Because the farther you go away from the equator, the colder it gets.
01:46Actually, the average annual temperature drops about 1.2 degrees Fahrenheit for every degree of latitude
01:54you move north in the northern hemisphere. And cold weather is the main reason hot deserts don't
02:00form above this line. It turns out that as you move farther north from the equator, the sun's rays hit the
02:07Earth at a more slanted angle. That spreads the solar energy over a larger area, which means less heat is
02:14concentrated in one spot. Basically, these regions just don't get as hot. And it's one of the reasons
02:20why countries like Finland are much colder than Colombia, for example. Another reason temperatures
02:27are lower in this zone has to do with something called albedo. Basically, this term refers to the
02:33amount of sunlight that a surface reflects back. Let me give you an example. Every surface on Earth
02:40both reflects and absorbs sunlight, but in different amounts. Take snow, for example. It can reflect up
02:46to 90% of the sunlight that hits it and only absorbs about 10%. That's what gives snow a high albedo.
02:54And it's totally different with sand, which has a lower albedo. In places like the Kalahari Desert,
03:00down in southern Africa, it's about a 50-50 situation. Most of the year, roughly 50% of the sunlight gets
03:07reflected around there. And the other 50% is absorbed. The heat absorbed by the sand is part of
03:14why it gets so hot in dry places like deserts. I mean, hot deserts. Simply put, sand soaks up more heat,
03:23while snow reflects most of it. This is one of the reasons why snowy areas stay much cooler. So the
03:30closer you get to the polar zones, the easier it is for sunlight to be reflected instead of absorbed.
03:36And that helps explain why there aren't any hot deserts around those parts.
03:42Then, we also have atmospheric patterns. Let's just say that the air patterns change a lot once you
03:48go above that 45-degree line. That's because this region is controlled by a wind belt known as the
03:54feral cell. Okay, so Earth has three main convection cells. You can think of them as big loops of moving
04:01air. Warm air rises, cools down, sinks, and then the cycle repeats. At the top and bottom of the planet,
04:09we have the polar cells. Right in the middle are the Hadley cells. And sandwiched between those two are
04:16the feral cells. Now, everything that happens between these three cells can get a little complicated.
04:22But for today, the main thing to know is this. In those middle zones, the warm air that comes from
04:28the tropics meets the cold air from the polar regions. So, feral cells play a big role in moving
04:35heat from the equator up toward the poles. This flow of warm air helps smooth out the temperature
04:40differences between the tropics and the polar regions. And that's what shapes the weather in
04:45places like North America, Europe, and parts of Asia. In the mid-latitudes, the feral cell keeps the
04:52air moving around so much that hot, dry air doesn't really get a chance to settle. So, those regions end
04:59up with more moisture, more clouds, and even storms, rather than dry, desert-like weather. That's why you
05:05don't usually see hot deserts around 45 degrees latitude. Basically, what I'm trying to say is that
05:12the feral cell makes the weather less stable. And that's very different from what happens in the Sahara
05:17Desert, for example. Over there, temperatures don't change all that much throughout the year.
05:22Just expect it to be hot. When you put all these factors together, it helps explain why hot deserts
05:28are mostly limited to the middle parts of the Earth, and why you'll never find a Sahara or Kalahari-style
05:34desert in places like Canada or Sweden. But that doesn't mean we don't have any deserts above the 45-degree
05:40line. Turns out, deserts don't always have to be hot and sandy. In fact, they can look like this.
05:49Yep, I'm talking about the Arctic. Because the actual definition of a desert isn't just some hot,
05:55dry, sandy place. It's really any area that gets very little precipitation. In other words,
06:01an area where it doesn't rain much. And it usually gets no more than 10 inches of precipitation per year.
06:08So, no matter if the weather is hot or cold, deserts are really dry places with hardly any
06:13vegetation around. That makes it pretty tough for animals and plants to live in those environments.
06:19If you think about it, that means the biggest desert on Earth is Antarctica. It covers about 5.5
06:26million square miles, just endless ice flats as far as the eye can see. It's also the coldest desert on
06:32the planet, even colder than the Arctic, which is the other polar desert. Temperatures there can drop as
06:39low as minus 128.2 degrees Fahrenheit. If you look at the Earth, about 33% of its land is desert. Now,
06:49pay close attention to the ones located between 15 and 30 degrees latitude. These are called tropical
06:55deserts. But can you notice something curious about their locations? Most of them are found on the
07:02western edges of continents. And there's a reason for that. Actually, four main reasons. First, the winds
07:09that bring rain usually blow from the east. When they move across the land, they drop most of the rain
07:15early on. By the time these winds reach the west side, they're dry, so there's no rain left for those
07:21areas. That already makes things pretty dry. Second, in the western margins of continents, the air
07:28usually goes down instead of up. When air goes down, it gets warmer and can't form clouds. So again, no rain.
07:38Third, if there are mountains nearby, they can block the rain too. One side of the mountain gets all the
07:44moisture, and the other side, called the rain shadow, stays dry. And lastly, the ocean near these
07:51places often has cold water. Cold water cools the air above it and makes it calm, so clouds don't form
07:58easily. No clouds means no rain. So when you put all of that together, it helps explain why those
08:05tropical deserts seem to prefer one side of the continent more than the other.
08:09That's it for today. So hey, if you pacified your curiosity, then give the video a like and share
08:17it with your friends. Or if you want more, just click on these videos and stay on the bright side!
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