In Episode 5 of the Shedding Light on the Sun and Earth series, Land and Ocean, we examine how the world’s land masses and oceans affect global climate patterns. We look at why the air gets colder and colder the higher you go, why coastal regions don’t heat up or cool down as much as inland regions, why northern hemisphere winters are so much colder that southern hemisphere winters, and a whole lot more.
A 4-minute excerpt.
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The Transcript (which can be used as a textbook)
Part A: Introduction. Uluru in central Australia often gets to 40°C in summer and often drops below 0°C in winter. Fraser Island, off the coast of Queensland but at the same latitude as Uluru, stays between about 20°C and 30°C all year round. Why the difference?
Part B: The Sun Heats the Earth’s Surface. Why does it get colder and colder the higher up you go?
Part C: Land Gets Warmer Than Water. Why do land masses get so much hotter than oceans, even when they both receive the same amount of sunlight?
Part D: Hot and Cold Winds. Why are winds that blow in from the ocean cooler than winds that are blowing in from land?
Part E: Water’s Moderating Influence. Why are northern-hemisphere winters so much colder than southern-hemisphere winters? Why does Uluru get so much hotter in summer and so much colder in winter than Fraser Island, even though they are at the same latitude?
Part A: Introduction
So, it’s pretty obvious that that the sun warms our planet but not all places are heated equally. Places near the equator are usually warm to hot, while the poles are always cold.
We’ve seen that when the Earth is in this position in December, the southern hemisphere receives more direct sunlight and gets longer days so it gets warmer weather. It’s the southern hemisphere summer. When the Earth is in this position 6 months later, the southern hemisphere receives less direct sunlight and less sunlight since the days are shorter and it gets cooler weather. It’s the southern hemisphere winter. The reverse is true for the northern hemisphere.
However, the temperature of the air around us is influenced not just by the angle that the sun strikes the Earth but by other factors as well.
Two places at the same latitude, like the famous Uluru in the red centre of Australia and the beautiful Fraser Island off the coast of Queensland, are very different in the daily maximum and minimum temperatures that they get.
These two graphs show the daily maximum and minimum temperatures averaged out for each month. Uluru’s average daily maximum temperature in summer is way higher than Fraser Island’s, and its average overnight low in winter is way lower than Fraser Island’s. At night, Uluru occasionally gets down to below zero in winter. The variation between average summer and winter temperatures at Uluru is huge, some 18°C, while the temperatures throughout the year on Fraser Island are much more even.
Also, on Fraser Island, the temperature varies, on average, by about 6 or 7 degrees Celsius between night and day, that is between the overnight low and the daily high. If the overnight low is for example 20°C it might typically only reach a high of about 26 or 27°C. At Uluru though, the temperature varies on a daily basis by an average of about 14 or 15°C. After an overnight low of 20°C, the top temperature typically reaches into the mid-30s. So, at Uluru, there’s a huge variation in temperature on a daily basis and on a yearly basis, whereas the variation on Fraser Island is relatively small. Why is there such a huge difference between Uluru and Fraser Island even though they are both at more or less at the same latitude?
In a similar kind of way, there’s a big difference between southern hemisphere and northern hemisphere locations even if they’re at similar distances from the equator. Melbourne, Sydney, Adelaide, Cape Town and Santiago get hot in summer, but they very rarely get all that cold. Melbourne for example regularly gets days over 30°C in summer. Its average maximum in January is 26°C and its average maximum in July, its coldest month, is about 14°C.
Melbourne occasionally gets a top temperature in winter of only 12°C and it’s considered icy and it’s a big talking point amongst us all who have to suffer through it.
However, though northern hemisphere cities like Shanghai, Tokyo, Rome, and Washington DC, that are at similar distances from the equator as the southern hemisphere cities I’ve mentioned can also get very hot in summer, in winter, top temperatures in single figures (that is below 10°C) are very common and it sometimes doesn’t even get above 0°C during the day.
This NASA animation, which was compiled using data from satellites, shows the changing snow cover over land as the seasons change. It’s pretty obvious that large parts of North America, Europe, and northern Asia get a lot of snow during their winter months and that’s a clear sign that it can get very cold in those places. In contrast, the southern hemisphere generally just doesn’t get all that cold. Of course it can get very cold up in the mountains: the mountains of New Zealand, southern Australia, and South America see a lot of snow but areas near sea level rarely get below 10°C and it even more rarely gets below 0°C. So, again, why the difference?
Well, it comes down to water’s amazing ability to absorb loads of energy without actually increasing in temperature all that much, certainly compared to rock and sand and land in general. The fact that land and ocean increase in temperature by different amounts during the day, even if the same amount of sunlight hits both of them affects local and global climate patterns. It’s these climate patterns that we’ll be looking at in this episode of the Shedding Light on the Sun and Earth series.
To begin though, we need to look into the more basic question of why it gets hotter on average near sea level than it does at the top of a mountain.
Part B: The Sun Heats the Earth’s Surface
Behind me in the city of Melbourne, which is more or less at sea level, the air temperature is 25°C. However, up here, 600 metres above sea level, the air temperature is only 22°C.
This graph shows the temperature for a six-hour period of a place near where I live in the Melbourne suburbs (at sea level) and up in a place called Mt Dandenong (630 m above sea level). Though the two locations are only about 20 km apart, it’s usually (not always, but usually) a few degrees cooler at the top of Mt Dandenong than it is in the Melbourne suburbs. The higher you go, the colder it gets on average.
This may seem counter-intuitive. It’s easy to think that if you’re higher up, you’re closer to the sun, which means it should get warmer, but it doesn’t. It gets colder.
This is Africa’s tallest mountain, the nearly 6000-m tall Mt Kilimanjaro. Though it’s located only 3° south of the equator, it’s permanently covered in ice and snow. The surrounding plains are always hot though. In fact all tall mountains around the world—even those near the otherwise hot equator—are always cold. What’s going on?
Whenever you hear that the temperature is 30°C, or 20°C, or whatever, that figure is telling you the air temperature, not the temperature of the ground or the ocean or anything like that. To measure air temperature, you need a thermometer of some sort of course and you have to place the thermometer in the shade.
In this simple experiment, I held one thermometer in the sun and the other in the shade for about 3 minutes. The thermometer in the sun was not only surrounded by warm air, it was being hit directly by the sun’s rays. As a result it got hotter than the one in the shade.
This one’s at 29°C and this one’s at only 24°C. A thermometer in the shade gives a much better indication of air temperature.
So why is the air temperature typically much colder the higher you go? The reason, is that the sun doesn’t really warm the air directly. Most sunlight passes straight through the air without getting absorbed.
When light hits an object it can reflect off it, leaving the object unchanged, or pass through it, which also leaves the object unchanged, or be absorbed by it. When the light energy is absorbed, the object gets hotter. Quite often a combination of all these things happens at the same time.
Only a small percentage of the sunlight that hits the Earth is absorbed by the air since most sunlight passes straight through the air, but a large percentage is absorbed by the ground. So, the ground heats up when the sunlight strikes it and then the ground heats the air that is in direct contact with it. It’s not just the ground of course, it could be water, plants, buildings or whatever.
So air in contact with the warm ground heats up much more than air that is far away from the ground.
Since most of the world’s land isn’t very high above sea level, and the ocean is at sea level, the air is warmer near the surface and it’s cooler higher up. Air near a mountain may warm up a little but it’s generally surrounded by cooler air far from the ground so the higher you go, the colder it gets. The always-freezing-cold peak of Mt Everest is about 9 kilometres above sea level. Commercial jets typically fly at an altitude of about 11 kilometres. The air temperature up there, outside the plane, is typically about -50°C. Sunlight heats it a little, but since the air is so far from the ground, it doesn’t get heated by the ground, so it’s really cold.
Now since the air is not really heated much directly by the sun but mostly by contact with the surface of the Earth, then the air will get hotter if the ground gets hotter. The air over land, solid ground, will usually warm up more during the day than air over the ocean; it turns out that land heats up more than the sea during the day even if an equal amount of sunlight strikes them. Why is that? Let’s take a look.
Part C: Land Gets Warmer Than Water.
Sometimes when you go to the beach, the sand gets really hot, to the point where your feet are burning. You then step into the water and you cool down immediately. Why does land get so much hotter than water, even when they both receive the same amount of sunlight? Well, there are three main reasons.
Firstly, when the sun’s rays hit water, some are reflected, but they mostly pass into the water. The energy is absorbed by the water, but because the energy is spread out over a huge mass of water, the water’s temperature doesn’t increase all that much. Shallow water can get relatively warm because there is literally less water, but deeper water hardly heats up at all.
When the sun’s rays hit the non-see-through surface of the land though, a lot is reflected, but the energy that is absorbed is absorbed almost entirely on the surface. The heat energy penetrates into the ground a little, but not much, so it’s concentrated on the surface. Land surfaces therefore usually warm up more during the day than what ocean water does.
So, because the land gets warmer than the ocean when the sun shines on it, the air in contact with the land gets warmer as well. The ocean water stays cooler, and so the air above it is cooler as well.
Another reason that ocean stays cooler than land is that when ocean water warms up, some of the water evaporates. Evaporation has a cooling effect.
Humans sweat because as the sweat evaporates, it cools the body down. For more on this, you can watch Shedding Light on Heat Episode 6: Heat and the Human Body.
The third reason that ocean doesn’t heat up as much as land during the day is that water has a huge what’s called “specific heat capacity”. What is specific heat capacity? Let me explain.
In this simple experiment, we heated, separately, exactly 1 kg of water and 1 kg of olive oil for 4 minutes and measured the temperature of the two liquids every minute. We used the same saucepan, the same burner, and the same mass to ensure a fair test.
When we look at the graph, it’s pretty obvious that the oil had a much quicker rise in temperature than the water did. Even though they both absorbed more or less the same amount of energy in the 4 minutes that they were heated, the temperature change was different. Water takes a lot more energy to raise its temperature by a given amount than most other substances. The specific heat capacity of a substance is the amount of energy needed to raise the temperature of 1 kilogram of the substance by 1°C.
We covered this concept in the Shedding Light on Energy series and in the Shedding Light on Heat series, but even if you’ve seen those series, we’re going to go over it again and extend it.
Temperature is typically measured in degrees Celsius. Energy is measured in Joules. When water absorbs energy, its temperature rises. If you have 1 kilogram of water and you heat it up so that its temperature rises by 1°C, (let’s say from 25°C to 26°C), then it will have absorbed 4,200 Joules of energy. One Joule of energy is obviously a very small amount of energy. If you continue heating and the temperature rises by another degree Celsius to 27°C (so that the temperature change is 2°C) it needs another 4,200 J of energy bringing the total to 8,400 Joules of energy.
However, if you raise the temperature of 1 kilogram of olive oil by 1°C (for example from 25°C to 26°C), it will have absorbed only 1,800 Joules of energy. If you take it up by 2°C, it will have absorbed 3,600 Joules of energy. If you have equal amounts of water and olive oil, the water needs more energy to change its temperature by 1°C than olive oil does.
The unit for specific heat capacity of a substance is Joules per kilogram per degree Celsius.
The specific heat capacity of water is 4,200 Joules per kilogram per degree Celsius. This is often written like this: 4,200 J/kg/°C. For every kilogram you have and for every degree Celsius the water’s temperature changes, you need 4,200 Joules of energy. Let me put the information into a table. The specific heat capacity of olive oil is 1,800 Joules per kilogram per degree Celsius.
Earthen materials have specific heat capacities of about 800 J/kg/°C. So, if for example sand absorbs the same amount of energy as water, then it will get much hotter than water since it takes less energy to heat sand up per °C. One kilogram of sand only needs to absorb about 800 Joules of energy before its temperature rises by 1°C. If 1 kilogram water absorbs the same amount of energy, it’s temperature will rise by about 1/5 of a degree Celsius.
A simple mathematical formula can put all these things together. Some people say they don’t like maths, but maths can often make things much easier to understand.
This is the formula:
Energy absorbed = specific heat capacity × the mass of substance × the temperature change
(in Joules) (in J/kg/°C) (in kg) (in °C)
The temperature change is the final temperature minus the initial temperature.
So let’s do an example. How much energy is absorbed by 2 kilograms of water that is heated from 15°C to 90°C? Well, the specific heat capacity of water is 4,200 J/kg/°C, the mass is 2 kg and the temperature change is 75°C (since 90 – 15 is 75). Multiplying them gives us the answer of 630,000 Joules of energy. However, example 2, how much energy is absorbed by 2 kg of olive oil if it is heated from 15°C to 90°C? Using the same formula but using olive oil’s specific heat capacity of 1,800 J/kg/°C, we get 270,000 Joules of energy.
So, since 270,000 Joules can be delivered in less time than 630,000 Joules, olive oil heats up faster than water does.
Rocks and sand and concrete also heat up much faster. Water has a huge specific heat capacity compared to most common things around us, so it takes longer to heat up.
Now the reverse is also true. Not only does water heat up more slowly than olive oil and pretty much everything else, it also cools down much more slowly than everything else.
In this experiment, we heated two equal masses of water and olive oil to 32°C and we then allowed them to cool down in the freezer. The olive oil cooled down more quickly because of its lower specific heat capacity. All other factors being equal, water takes a longer amount of time to heat up and to cool down than oil and in fact rocks, dirt, sand and land in general.
So, the fact that the sunlight that hits water spreads out in the water, and the fact that water evaporates, and the fact that water has a huge specific heat capacity means that the world’s oceans don’t heat up as quickly or as much during the day as the world’s land masses do.
Water’s huge specific capacity also means that oceans don’t cool down as quickly or as much at night as land does. In fact on a daily basis, ocean water barely changes temperature at all during the day.
The oceans also don’t show as much variation in temperature throughout the year.
For example, the average water temperature of Port Phillip Bay doesn’t change all that much on average throughout the year, but the average air temperature of Melbourne, which reflects how hot the land gets, varies quite a lot. Land temperatures rise and fall much more than ocean temperatures do.
These simple facts are responsible for a huge part of global weather patterns. Let’s take a closer look.
Part D: Hot and Cold Winds
So the sun doesn’t really heat the atmosphere directly all that much since sunlight passes straight through the atmosphere without being absorbed all that much. The air is heated mostly by the ground.
Since the air temperature is influenced by how hot the Earth’s surface is, and land gets hotter than ocean during the day, air blowing over land generally warms up more than air blowing over ocean.
In my home city of Melbourne, when the wind blows from the north in summer, it has travelled over the hot land and so Melbourne gets hot weather.
By the way, a wind that blows from the north is called a north wind or a northerly wind. A southerly wind blows from the south. If the wind changes from being a northerly to a southerly, this air is cooler, since the ocean doesn’t heat up as much and so Melbourne gets cooler weather. The same is generally true for all coastal cities. Easterly winds blowing over land towards Perth bring hot air and therefore hot weather, while westerly winds bring cooler air and cooler weather.
So as I said, a north or northerly wind blows from the north, towards the south. It may seem strange because it would make just as much sense to call a wind blowing towards the south a southerly wind. But, the temperature of the air is influenced by where it’s coming from not where it’s going. When it comes to weather, we don’t really care where the wind’s going we only care about where it’s coming from.
So northerly winds in Melbourne, coming from the north, are usually warmer winds, as are easterly winds, coming from the east, for Perth. In fact Melbourne, Sydney, and Adelaide have all on certain days each been the hottest places in the world during the southern summer. On average, they’re nowhere near the hottest places on earth, but if the wind is blowing in the right direction over hot land, they can all get very very hot. (Adelaide’s highest ever temperature was 46.6°C, Melbourne’s, 46.4°C, and Sydney’s, 45.8°C.)
In summertime, Uluru is usually hot regardless of the wind direction because the wind has blown over land, which, as I’ve said, gets hotter than ocean does. Now complicating things a little is the fact that wind blowing from the direction of the equator starts off a little warmer already, since the land and ocean near the equator is always warm to hot, but everything I’ve just spoken about is still entirely true.
Winds blowing from the direction of the land towards the ocean are often called offshore winds and winds blowing onto land from the ocean are called onshore winds. Onshore winds aren’t just cooler, they bring water which has evaporated from the ocean. A lot of the water ends up condensing and becoming rain.
This is especially true if the winds hit a mountain range. The moist air is deflected upwards, it cools down, and, because cooler air can’t hold as much moisture, the water condenses and then often falls as rain. This is the main reason that mountains tend to get more cloud cover and rain than flatter regions. On the other side of the mountain range there’s often less rain because most of the moisture has already fallen as rain. This is called a rain shadow. Most of the rain that falls on the east coast of Australia comes when the wind is blowing onshore towards the 3,500-km-long Great Dividing Range. On the satellite image you can see that coastal regions are greener than inland areas.
At a local level, since the temperature of ocean water doesn’t change all that much but land temperature does, winds call sea breezes and land breezes form.
During the day, when the land gets warmer and the ocean water stays cooler, the air over the land heats up more than the air over the water, so the warm air rises and cooler air flows in from the ocean to replace it. This is called a sea breeze. Sea breezes often bring much-needed relief to people in hot summer weather.
The opposite happens at night.
Since land cools down more than ocean at night but the ocean stays more or less the same temperature, the air above the land also cools down more than the air above the ocean. The warmer air (over the water) rises and the cooler air from the land blows over to replace it. This is called a land breeze. Remember, it’s not the actual air temperature that matters; it’s the difference in air temperature that causes a breeze.
Part E: Water’s Moderating Influence
Now earlier on, I said that the daily temperature fluctuations at Uluru are much bigger than they are on Fraser Island. Why is that? Well, it’s because of the vast amounts of water that are all around Fraser Island. If the sun shines down on both Uluru and Fraser Island with exactly the same intensity, the ground in the middle of Australia will rise to a higher temperature than the water in the Pacific Ocean that is near Fraser Island. As a result, the air near Uluru, will also get hotter. At night, the land around Uluru will cool back down faster than the water around Fraser Island, and so the air temperature will also drop down a lot more.
Basically, coastal regions generally don’t show as much variation between the lowest temperature and the highest temperature they reach each day because water just doesn’t heat up or cool down all that quickly.
In winter, when the nights are longer than the days, Uluru cools down much more than Fraser Island does and while it gets pleasant weather during the day, it often gets very cold at night. It’s very common for Uluru to get to an overnight low of 2 or 3°C in winter before climbing to a maximum of 20°C. Occasionally it actually drops to below 0°C.
However, Fraser Island stays relatively warm in winter during the day and during the night because it’s surrounded by water that still hasn’t cooled down all that much from summertime (since water takes ages to cool down compared to land).
Which leads us to the reason that cities in the northern hemisphere gets so much colder in winter than cities in the southern hemisphere, like Melbourne, do.
It’s fairly obvious if you look at a map of the Earth that most of the Earth’s surface is covered by oceans. In fact, globally, 71% of the surface is covered by water which means only 29% is land. However there’s a huge difference between the two hemispheres. In the northern hemisphere, 61% percent of the surface is ocean and 39% is land. In the southern hemisphere, however, 81% of the surface is ocean and only 19% is land. More than 2/3 (68% in fact) of all land is in the northern hemisphere. Now there are a lot of numbers here, and I’m not expecting you to remember them of course, but the important thing is that the southern hemisphere has a far higher amount of water compared to land (it’s about a 4:1 ratio) and the northern hemisphere has a more even amount of ocean and land. The ratio of ocean to land is about 1½:1.
Since, as we’ve seen, water takes a long time to either heat up or to cool down compared to most other things, the southern hemisphere—on average—just doesn’t get as hot during its summer as the northern hemisphere does during its summer (although individual places can get very hot). Southern hemisphere winters, on average, don’t get anywhere near as cold as northern hemisphere winters do.
As we saw earlier, large parts of the northern hemisphere get snow in winter, whereas the southern hemisphere hardly gets any snow. The higher abundance of water in the southern hemisphere makes the temperature change from summer to winter smaller there.
During winter, the oceans of the southern hemisphere cool down, but thanks to the huge amount of water, they don’t cool down all that much before spring returns and they start heating up again.
And so we come to the end of the Shedding Light on the Sun and Earth series. We’ve seen how the sun heats our precious planet but different parts of the planet are heated by different amounts depending on their location and on the time of the year.
Understanding the Earth’s climate is extremely important, because it helps us to, for example, forecast the weather more accurately and to determine where tropical cyclones will make landfall. However, it’s a pretty complicated topic and there’s still a lot that we don’t know so the world still needs good scientists who can continue to add to our knowledge of it. You might want to consider making a career of it
Anyway, I’m off to enjoy some good summer weather. See you next time.
Written and directed by Spiro Liacos
Time Lapse de Ayers Rock by Tómate algo – Tu cuaderno de bitácora. Creative Common Licence.
https://en.wikipedia.org/wiki/File:Reliefmap_of_Australia.png by Hans Braxmeier via http://www.maps-for-free.com. This file is licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.
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Snow cover animation by NASA https://earthobservatory.nasa.gov/global-maps/MOD10C1_M_SNOW
Specific Heat Capacity Tables