Why is Antarctica so Cold?

A very special treat now with a guest post on the climate of Antarctica from the resident Halley MetBabe (atmospheric scientist) Richard “Captain Awesome” Warren.

 

Just why is Antarctica SO cold?!

A common question to ask is “Why is Antarctica so cold, even so much colder than the Arctic? North Pole and South Pole should both be as cold as each other right?” Wrong!

To understand why Antarctica is so much colder, we need to first know why places are warm or cold in the first place.

Hottest and Coldest Places

Look at the image below. It is a map that shows the mean global temperature, with the equator drawn in the middle. Based on the fact that the sun warms the equator most and the poles the least, you would expect the equator to be warmest and the poles to be coldest. As you can see this is generally true, however, note exceptions to the rule. Canada, Greenland and North-eastern Russia are much colder than the ocean between them, and there is a very cold patch to the North-east of India. Antarctica too is exceedingly cold, whereas India itself, North Australia and parts of North Africa are even warmer than the equator.

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Atmospheric Circulation

2So what makes these places warmer or colder than they should be? There are many reasons, but the first thing to understand is how heat moves around the earth. If you stand next to something warm like a radiator, you’ll notice that you can feel the warmth much more by putting your hand above it, rather than below it. This is because warm air rises, and cold air sinks.

 

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If you warm air up it becomes less dense than the air around it, and this causes it to rise, exactly how a hot air balloon rises by trapping a balloon-full of hot, and therefore less dense, air. As air rises and moves away from the source of heat, it cools and becomes denser, and sinks. This circulation from a heat source is called convection. The picture of the radiator in the room is very simple but the principle is exactly the same with the Earth.

The equator is warmed by the sun; the air rises into the atmosphere and moves away from the equator. Here it cools, and descends back to ground level and makes its way back to the equator. The circulation is actually more complicated than this, but it gives you the basic idea of how air moves heat around the Earth.

Based on this, Antarctica should be as cold as the Arctic, as they are the same distance away from the radiator, or the heat source at the equator. As we know however, Antarctica is much colder. Now we’re going to have a quick look at other places that are different too, and once we understand why these differ, we will look at Antarctica again.

 

 

Land vs. Sea

So why are Russia, Canada and Greenland colder than the ocean between them?

Unfortunately, here it gets more complicated! There are other factors at work besides just the circulation in the atmosphere.

Land vs. Sea

  • Have you ever run a bath that was too hot, left it for a while to cool down, yet half an hour later the water is still really warm? Yet if you take a tray out of the oven, 10 minutes later it is cool enough to touch?
  • Water takes much longer to warm up than a solid. If you leave a pebble and a bowl of water in the sun for 1 hour, the pebble will be much warmer than the water.
  • Again, simple principle, but the same with Earth. An ocean will take much longer to warm up than the land, yet it remains warm for longer when the heat source is removed. Warm water in the ocean flows from the equator to the pole, similar to the air in the atmosphere. Whereas the air will be cold by the time it reaches the pole, the water will still be quite warm.
  • Therefore, large land masses such as Russia and Canada get VERY cold in winter when the sun disappears, but can actually be quite warm in summer when the sun warms them up.
  • In comparison, the UK is a small country surrounded by the ocean. Based on how far away from the equator it is, we could expect it to have a mean temperature of between 0 and -10oC, such as central Russia or Canada which are the same distance away. However, the ocean that surrounds the UK warms up slower than the land in summer which has a cooling effect, and cools down slower than the land in winter, having a warming effect. This gives the UK a fairly mild mean temperature of +10oC (thank goodness!). Central Canada and Russia are too far from the sea to feel this moderating effect, and therefore have much more extreme temperatures.

To demonstrate this point, let’s compare the winter temperatures of London and Calgary which are both at 51oN, i.e., the same distance from the equator. London, as I’m sure you know, sits on the Thames estuary where it meets the North Sea, which will be quite mild in winter. On the other hand, Calgary is 600 kilometres from the nearest ocean. The coldest ever recorded temperature in London is a chilly -10oC whereas in Calgary, the coldest temperature is a frost bitingly cold -45oC. Yes this  -45oC was a one off, but still the average low temperature for the winter months in Calgary is -12oC, meaning that on average, every December, January and February in Calgary gets colder than London has ever been.

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The works in reverse too, as shown with countries such as France on the right. The north coast is influenced by the next-door and quite mild North Sea, which also influences London. Compare this to the much warmer southern coast which is in contact with the Mediterranean Sea, a very warm body of water. Notice how quickly the average temperature changes when you move away from either coast.

The temperature difference of Calgary with London is partly due to proximity to the ocean, but there are other factors involved too. London is basically at sea level, whereas Calgary sits at 1100m. Looking at the map of France as well, those cold pockets in the south east stand out from the warm temperatures around them. You may have realised that these are the mountains of the Alps and the Massif Central. Let’s see how altitude can affect temperature.

 

Low vs. High

So, close to a warm ocean and close to the equator are the warmest places? Nope, not exactly. Don’t worry I haven’t lied; we just haven’t seen the whole picture yet. Meet Cotopaxi, not only less than one degree from the equator but also within sight of the warm central Pacific Ocean. It is however, covered in snow. How does this work then?

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A snow covered mountain seems like such an ordinary thing yet when you think about it, why are mountain tops cold enough to be covered in snow when the land around them is so      much warmer? It is due to something called the Lapse Rate.

 

 

Remember our friend convection? Well it happens all over the place; air doesn’t just rise at the equator. Quick recap on atmospheric convection:

12The rate at which the air cools is called the lapse rate. Generally, this is about 0.6oC per 100m. What that means, is that for every 100m higher you go, it will get 0.6oC colder. When you consider that Cotopaxi is almost 6000m high, that means the summit of the volcano is roughly 36oC colder than the air at sea level. Quite a change in temperature!

The most striking examples of this from the mean temperature map are the Himalayas, and the Andes mountain range which runs down the west coast of South America. Look back at the mean temperature map at the Himalayas and note how warm next-door neighbour India is, or look at the Andes, and note the warm ocean to the west, and very hot rainforests of Peru and Bolivia to the East.

Putting all this together, let’s consider what we have learnt so far:

  • It is warmer near the equator.
  • Coastal areas are influenced by the temperature of the nearby ocean.
  • Coastal areas have less extreme temperatures than continental areas.
  • The higher you are, the colder it is.

 

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The two plots above show the mean daily temperatures, and the topography of the United States of America. Compare the two, bearing in mind the lapse rate, and notice how the mid-western states are much colder than those around them. Conveniently, this is where the Rocky Mountain Range is located which has many peaks above 4000m. With a lapse rate of 0.6oC for every 100m, how much colder would it be at 4000m that at sea level?

Look even closer and you’ll notice that apart from the very north, almost all of the coldest mean daily temperatures occur where the large mountain ranges are located. Notice also the warm yellow strip poking north in California (south-western most state) that correlates to a deep valley running between two mountain ranges?

Last thing to spot, is that the south east has a warmer mean temperature than the south west. Yes the south east is generally lower than the south west, but there is something else going on here to. The south east coast is on the shore of the Caribbean; a warm shallow sea, compared to the cooler, deeper Pacific Ocean on the west coast. As we saw before, the sea can have a noticeable effect on the temperatures, especially in winter, when the land cools down quickly yet the sea remains warm. Therefore the winters in California get much colder than the winters in Florida, which has a warmer adjacent sea. There is another way in which the sea affects the land, which is the last thing we are going to have a look at.

That just about wraps up altitude, so the last thing to look at is humidity.

Dry vs. Wet

OK, so one last look at the annual mean temperature graph. Until now, we have gone on the assumption that the further you go from the equator, the colder it gets. The equator gets the most intense sun, and therefore should be warmer. Look at areas like the Sahara desert in North Africa and you’ll see there is actually a belt of the warmest mean annual temperatures in the world at 30oN and 30oS.

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The other figure shows the mean cloud cover, or how much of the sky is on average, covered by clouds. Look at the similarities, because they are striking.

Where are the hottest and coldest temperatures? North Africa, North Australia, Saudi Arabia are the hottest, Greenland and Antarctica are the coldest.

Where are the driest places? North Africa, North Australia, Saudi Arabia, central Greenland and Antarctica.

There is a remarkable correlation between cloud cover and mean annual temperature. To understand why, we need to recall the process of convection, and understand a new one called humidity.

Humidity refers to how much water the air is holding. The warmer the air is, the more water it can hold as invisible water vapour, and the colder the air is, the less water it can hold. When the air is “full” of water and cannot hold anymore, it is at 100% humidity, and known as “saturated”.

You will have noticed this in the past but maybe not known that humidity was to blame. Every time you breathe out, you breathe out water vapour. If you don’t believe me, breathe on a window and you’ll see water droplets form on the glass. Normally, you don’t notice this because the air is not saturated and the moisture in your breath stays as invisible water vapour. However when it is really cold outside, the air cannot hold very much water, i.e., it is nearly saturated or full of water. When you add more water vapour to the air, it condenses, and you can see the tiny water droplets hanging in the air.

The same happens in the atmosphere; as air rises from convection, it cools and suddenly cannot hold as much water vapour. Any excess water vapour then condenses to form clouds. The air above the clouds has therefore lost part of its water content, and is drier than before.

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Remember when I said that atmospheric circulation is actually more complicated? Well we’re going to understand a little more about it now, but don’t worry, we’re almost done!

True to convection, the equator being where the most intense solar heating occurs, this is the area where most air begins to rise into the atmosphere as you can see on the left. As it rises, it cools and cannot hold as much water vapour; hence it condenses and forms clouds. Remember the cloud picture from before? There is a nice strip across the equator of high percentage cloud cover to confirm this. Now this cloud has an interesting effect, as it actually cools the surface below, since it keeps it in shade. It feels hot though; that sort of horrible, sweaty, muggy heat that you associate with rainforests and tropical countries, which is due to the humid, saturated air that is very hot, and therefore is holding lots of water. The high water content is what makes it feel very muggy.

 

 

When the air that rose at the equator descends which is typically at 30oN and 30oS, as it has lost some of its water it is much drier, and therefore clouds are much less common in these areas. This allows the sun, although it is less intense than at the equator, to burn down without any clouds blocking it. Very hot, dry air at the ground without clouds above to provide much rain gives us deserts, and where are the biggest deserts? All 30oN or 30oS of the equator, where the hottest mean annual temperatures are.

If you look at the boundary of the Ferrel and Polar Cells in the picture above, you’ll see that there is another area of rising air which, if you follow the arrows, descends over the North and South Poles. Using what we know from the deserts nearer the equator about what happens when air descends, this is very dry air that is arriving at the poles. However rather than allowing the sun to burn down without any cloud blocking it, since the poles face away from the sun and therefore get very little sunlight anyway, it actually allows heat to escape. View clouds as like a moderator; when it is hot they cool things down by blocking direct sunlight, and when it is cold they keep the heat in like a blanket.

Think of a frosty winter morning. The sky is almost always clear right? If there were any clouds over head during the night, they keep the ground warm and frost doesn’t form. If there are no clouds overnight, heat from the ground can escape so that it is cold enough for frost.

On a global scale then, we have bands of relatively humid and cloudy weather where the air rises, and bands of very dry and cloudless weather where the air, having cooled so that it is denser than the air below, sinks back to surface level. Antarctica is located at the end of one of these “Polar Cells”, where the air is very dry.

Conclusion: Antarctica vs. The Arctic

So let’s review:

  • The further from the equator you go, the colder it gets.
  • The further from the ocean you go, the more extreme the temperatures get.
  • The higher you go, the colder it gets.
  • The drier it gets, the more extreme the temperatures get

Both the Arctic and the Antarctic experience large periods of the year when, due to the tilt of the Earth, they face away from the sun and experience total darkness for months. This is key, as both are as far as each other from the heat source at the equator, and receive the same amount of sunlight each year, which continent gets colder in its winter, and stays cold in summer?

 

Factor The Arctic The Antarctic Which is colder?
How far from the equator? Centred on the geographical North Pole, as far north as you can go Centred on the geographical South Pole, as far south as you can go Draw
Influence from land/sea? The Arctic is not land; it is floating ice on the Arctic Ocean which grows in winter and shrinks in summer. The ocean, although very cold, retains some heat without the sun in winter. Large continent with huge masses of ice on top of it. Has a coast but the centre of the continent is hundreds of miles from any ocean, especially as the sea ice in winter grows up to 400km away and doubles the size of the continent.. No additional warmth from the ocean, loses heat very quickly without the sun. Antarctica
What is the altitude? Virtually sea level, the sea ice doesn’t rise more than a few feet above sea level. Elevation at the North Pole is 2 metres of ice. On average, is the highest continent. Elevation at the South Pole is 3000m of solid vertical ice, sitting on bedrock 100m thick. Antarctica
How humid is it? Air typically descends and therefore is dry, although the surrounding Arctic Ocean does provide some moisture in the air. Air typically descends and therefore is dry. Due to the higher elevation and distance from ocean, this air is very dry indeed. Antarctica

 

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We know that Antarctica is the coldest continent, and hopefully now, we know why it is the coldest. For final confirmation, look at Greenland. Despite being in the Arctic Ocean and very far north, it has many of the characteristics of Antarctica such as it is ice on top of land not sea, and that the ice reaches a very high elevation.

As with everything in meteorology, there are always more factors to consider. For further influences on the temperatures or overall climate of Antarctica, consider looking up “The Albedo Effect”, and “The Polar Vortex”.

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