Why are temperatures different at the equator than at high latitudes?

Written by: YUNG Chung-hoi

The sun is so far away that it practically shines equally at the equator and the poles. But why is the equator very hot and the poles very cold?

There are several factors that influence the amount of sunshine received at various places on the earth.

The major factor is that while the sun is overhead at the equator, it is at a rather slant angle at the poles. This effect can be demonstrated using a torch, which can be taken as the sun. See Figure 1. What really counts is the amount of sunshine that falls on a given area. At the equator, the area A receives the same amount of sunshine as another area B at a higher latitude. Since, from Figure 1, B is clearly greater than A, the amount of sunshine that falls on a unit area (for instance a square metre) at area B is less than that at area A. In other words, for the same area of 1 square metre, the amount of sunshine received is greater at A than at B.

The other factors, which are less important, include:-

a) Absorption and scattering of sunshine when passing through the atmosphere - the amount of absorption and scattering depends on the nature and concentration of air molecules and small particles in the atmosphere. All things being equal, at higher latitudes the sun's path is longer. Naturally, there are more air molecules and particles along the way, resulting in greater absorption and scattering. Hence less solar energy reaches these places.

b) Reflection by the earth's surface. As the sun's ray reaches the earth, some of the energy is reflected back and does not warm up the place. The amount of reflection depends on the nature of the surface. Snow is an extreme in terms of reflection, since as much as 75 to 95% of the incoming sunshine is bounced back - not much help for the already cold weather in snow-covered areas. However, with an overall decreasing trend in snow cover in recent years due to climate change, more and more solar energy gets absorbed. A very likely consequence is still warmer conditions and less snow cover, further aggravating the equilibrium.

[Reference: The Wonder of the Weather, Bureau of Meteorology, An AGPS Press publication.]

Essentially, there arc two main reasons that climate varies from place to place; first, the amount of energy arriving from the sun, and second the circulation of the atmosphere and oceans which carry heat and moisture from one placc to another.

One of the major factors determining the relative warmth of a climate is the angle of the sun in the sky. The sun shines almost straight at the earth's equator, because the equator sits in the direct plane of the sun within the solar system. So, if you stand on the equator during the middle part of the day, the sun passes straight overhead. At higher latitudes, such as in Europe or North America, you would be standing a little way around the curve of the earth and so the sun always stays lower in the sky. The farther away from the equator you go. the lower the sun stays until at the poles it is really only barely above the horizon during the day.

Having the sun directly overhead gives a lot more energy to the surface than if the sun is at an angle. It is rather like shining a flashlight down onto a table. Hold the flashlight pointing straight down at the table and you have an intense beam on the surface. But hold it at an angle and the light is spread out across the table top and much weaker. If the sun is high in the sky, a lot of light energy hits each square kilometer of the earth's surface and warms the air above. If the sun is low in the sky, the energy is splurged out across the land; so there is less energy falling on the same unit area (Figure 1.1a). This tends to make the poles colder than the tropics, because they are getting less heat from sunlight.

A second factor relating to sun angle, which helps make the high latitudes cooler, is the depth of atmosphere that the sun's rays must pass through on the way to the earth's surface. Because at high latitudes the sun is lower in the sky, it shines through the atmosphere on a slanting path. At this angle, the light must pass a longer distance through more gases, dust and haze. This keeps more of the sun's energy away from the surface, and what is absorbed high in the atmosphere is quickly lost again up into space. Think how weak the sun is around sunset just before it sinks below the horizon—so weak that you can stare straight into it. The dimness of the setting sun is an example of the effect of it having to shine through a longer path of atmosphere, which absorbs and scatters the sun's light before it can reach the surface. So, the lower in the sky the sun is, the longer is its path through the atmosphere, and the less energy reaches the ground.

Sun's beam from above

Light spread across large area Light concentrated onto small area

Sun's beam spread across surface

Top of atmosphere

Only in the tropics is the sun right overhead throughout the year, giving the maximum amount of energy. This then is the key to why the poles are cooler than the tropics.

The seasons of the year arc also basically the result of the same sun angle effects (Figure 1.2). The earth is rotating on its axis at a slight angle to the sun, and at one part of its yearly orbit the northern hemisphere is tilted so the sun is higher in the sky; it gets more energy. This time of year will be the northern summer. At the same time, the southern hemisphere is getting less energy due to the sun being lower. During the other half of the year, the southern hemisphere gets favored and this is the southern summer. Adding to these effects of sun angle is day length; the "winter" hemisphere is in night more of the time because the lower sun spends more time below the horizon. This adds to the coldness—the warming effect of the sun during the day lasts less time, bccause the days are shorter.

Why mountains are colder

If you climb up a mountain, the air usually gets colder. The temperature tends to decline by about 0.5 C for every hundred meters ascended, although this does vary. The rate of decrease of temperature w ith altitude is callcd the "lapse rate '. Lapse rate tends to be less if the air is moist, and more if the air is dry. Generally, every 10 meters higher up a mountain is the climatic equivalent of traveling about 15 km towards the poles. Unlike the decline in temperature with latitude, sun angle docs not explain why higher altitudes are generally colder. The relative coldness of mountains is a byproduct of the way that the atmosphere acts as a blanket, letting the sun's light in but preventing heat from being lost into space (sec Box Section 1.1 on the greenhouse

Why the upper parts of mountains are colder. A thinner layer of greenhouse gases causes them to lose heat rapidly.

Because they protrude up into the atmosphere, mountain tops have less of this blanket above them, so they are colder.

There are however some exceptions to this pattern of temperature decline with altitude: places where the mid-altitudes of a mountain arc warmer on average than the lowest altitudes. This occurs where there are enclosed valleys between mountains, where there is not much wind. At night, cold air from the upper mountain slopes tends to drain as a fluid into the valley below, and accumulate. Just above the level that this cold draining air tops up to, there is a warm mid-altitude belt that can have warmer-climate plants than the valley below (Figure 1.4). Mid-altitude warm belts like this often occur in the Austrian Alps, for example.

How mid-altitude warm belts form. Cold air drains down as "rivers" from the upper slopes of the mountain, and fills up the valley below. Just above the top of the accumulated cold air, temperatures are warmer.

The general pattern of cooler temperatures at higher altitudes occurs not only on mountains, but through the atmosphere in general, essentially because of the same factor—a thinner blanket of greenhouse gases higher up. If air is rising up from the surface due to the sun's heating, it will tend to cool as it rises due to this same factor. Another thing that will tend to make it cool is that it expands as it rises into the thinner upper atmosphere—an expanding gas always takes up heat. If the rising air is moist, the cooling may cause it to condense out water droplets as cloud, and then perhaps rain drops which will fall back down to earth.

WINDS AND CURRENTS: THE ATMOSPHERE AND OCEANS

Differences in the amount of the sun's energy received by the surface drive a powerful global circulation pattern of winds and water currents. The most basic feature of this circulation, and a major driving force for almost everything else, is a broad belt of rising air along the equator. This is known as the intertropical convergence zone, or ITCZ for short. The air within the ITCZ is rising by a process known as convection; intense tropical sunlight heats the land and ocean surface and the air above it warms and expands. Along most of this long belt, the expanding air rises up into the atmosphere as a plume, sucking in air sideways from near ground level to replace the air that has already risen up. Essentially the same process of convection occurs within a saucepan full of soup heated on a hot plate, or air warmed by a heater within a room; any fluid whether air or water can show convection if it is healed from below. The difference with the ITCZ, though, is that it is convection occurring on an enormous scale. Because air is being sucked away upwards, this means that the air pressure at ground level is reduced so the ITCZ is a zone of low air pressure in the sense that it would be measured by a barometer at ground level.

What goes up has to come down, and the air that rises along the equator ends up cooling and sinking several hundred kilometers to the north or south of the equator. These two belts of sinking air press down on the ground from above, imposing higher pressure at the surface as they push downwards.

The air that sinks down in these outer tropical high-pressure belts gets sucked back at ground level towards the equator, to replace the air that is rising up from being heated by the sun. It would be easiest for these winds blowing back to the equator to take a simple north-south path; this after all is the shortest distance. But the earth is rotating, and in every 24 hour rotation the equator has a lot farther to travel round than the poles. So, the closer you are to the equator, the faster you are traveling as the earth turns. When wind comes from a sliuhtlv hiaher latitude, it comes from a part of the earth that is rotating more slowly. As it nears the equator, it gets "left behind"—and the closer to the equator it gets, the more it lags behind. So. because it is getting left behind the wind follows a curving path sideways. This lagging effect of differences in the earth's rotation speed with latitude is known as the "Coriolis effect", and any wind or ocean current that moves between different latitudes will be affected by it. It also explains, for example, why hurricanes rotate.

The intertropical convergence zone, a belt of rising air heated by the equatorial sun.

Air descends Air rises at zone Air descends further away of maximum further away heating from sun being directly overhead

Although it has been moving towards the equator, much of this wind does not get there bccausc the Coriolis cflcct turns it sideways. It ends up blowing westwards as two parallel belts of winds, one belt either side of the equator (Figure 1.6a). These are the trade winds, so-called because in the days of sail, merchant vessels could rely on these winds to carrv them straight across an ocean.

There is another related effect—the "Ekman spiral"—when a wind bent by the Coriolis effect blows over the rough surface of the earth, the friction of the earth's surface—which remember is rotating underneath it at a different speed— will drag the wind along with the rotating earth, canccling out the Coriolis cflcct (Figure 1.6b). This causes the wind direction to change near the earth's surface, and is part of the reason why winds by the ground can be blowing in one direction, while the clouds up above arc being blown in a different direction. Between the air nearest the ground and the air way above, the wind will be blowing at an intermediate angle; it is "bent" around slightly. The closer it gets to the surface the more bent off course it gets.

There arc many other aspects to the circulation pattern of the world's atmosphere. too many to properly describe here in a book that is mainly about vegetation. For instance, there is another convection ccll of rising and sinking air just to the north of the outer tropical belt, and driven like a cog w heel by pushing against the cooling air that sinks back down there. A third convection cell sits over each of the poles.

Outside the tropics, air lends to move mostly in the form of huge "blobs" hundreds of miles across. These are known as "air masses". An air mass is formed when air stays still for days or weeks over a particular region, cooling off or heating up, and only later starts to drift away from where it formed. You might regard an air mass as resembling a big drop of treacle poured into a pan of water. It tends to spread out sideways, and also mix sideways with what is around it. The collision zone between an air mass and the air that it is moving into is known as a "front". When a front passes over, you get a change in the weather, and often rain.

Higher altitude wind direction is (b) dominated by the Coriolis effect

Higher altitude wind direction is (b) dominated by the Coriolis effect

But dragging of wind near surface changes its direction to follow the rotation of the part of the Earth it N. is blowing over

In a sense, the detailed patterns of moving individual air masses are controlled by thin belts of higher altitude winds (at between 3 and 12 km altitude) in the atmosphere at the edge of the polar regions, and also at lower latitudes where the air from the ITCZ starts descending.

These eastward-trending winds arc the jet streams. They "push around" the lower-level air masses like chcss picccs. There is the subtropical jet stream and the polar jetstream in each hemisphere. That makes four jet streams in all. The jet streams are fed by air rising up into them moving in a polewards direction, and they are propelled east by the Coriolis force because the air comes from the faster-rotating lower latitudes.

THE OCEAN CIRCULATION

Just as the winds move through the atmosphere, there are currents in the oceans. These too transport an immense amount of heat from the equator towards the higher latitudes. For the most part, ocean currents only exist because winds blow them along, pushing the water by friction. But part of the reason winds blow is that there are temperature differences at the surface, and ocean currents sometimes bring about such contrasts in temperature (especially if there is upwelling of cool water from below). So the water moves because the wind blows across it, yet the wind may blow because of the very same temperature contrasts that are brought about by the water moving!

Wind skimming across the surface will drive the top layer of water as a current in a particular direction, and if it moves towards or away from the equator the current will eventually gel bent round by the Coriolis effect. So, for example, in each of the world's main ocean basins there are eastward-curving currents that travel out from the equator because of this mechanism (see below). But below the surface of a current being bent by the Coriolis effect, the deeper part of the current is being dragged by contact with the still waters below it. That dragging tends to move it along in the direction that the earth is rotating locally. So because of this dragging, this deeper water in the ocean ends up traveling in a slightly different direction. The deeper you go, the more the angle of the current is diverted by dragging against water below, and different layers in the ocean can be traveling in quite different directions. This is the same Ekman spiral effect as occurs in the atmosphere.

Winds blow fast but per volume of air they don't carry very much heat. The heat-carrying capacity of ocean water is much greater, but the ocean currents move much more slowly than the winds. In fact, both ocean currents and winds are important in transporting heat around the earth's surface.

Ocean gyres and the "Roaring Forties" (or Furious Fifties)

The most prominent feature of the world's ocean circulation are currents that run in big loops, known as gyres. They start off in the tropics moving west, and curve round eastwards in the higher latitude parts of each ocean basin, eventually coming back down to the tropics and completing a circle.

These gyres originate from the powerful trade winds that blow towards the west in the outer tropics. The winds push against the surface of the ocean producing these currents. But why does an ocean gyre eventually turn around and flow eastwards? It happens because the ocean currents are slammed against the shores on the west sides of ocean basins by the trade winds that blow west along the equator. Both the winds and the currents bounce oil'the western side of the basin, and start to head away from the equator. Because they arc traveling with the same rotation speed as the equatorial zone, the Coriolis effect bends them off towards the east, diagonally across the ocean towards higher latitudes.

The winds that follow the outer parts of these ocean gyres, and help drive them, are powered by the big contrast in temperature created as the ocean currents move polewards and cool oil'. In the southern hemisphere these winds are known as the Roaring Forties, blowing west-to-east just south of South Africa and Tasmania, and hitting the southern tip of South America with a glancing blow. The nickname that generations of sailors have given these winds comes from their unrelenting power and their tendency to carry storms, and the fact that they stay within the 40s latitudes. In the northern hemisphere, the equivalent belt of winds is located more in the fifties and low sixties, hitting Iceland, the British Isles and the southwest Norwegian coast. These winds, even stormier, are known as the "Furious Fifties".

Winds and ocean currents push against one another

As I've implied above, surfacc ocean currents arc driven by winds, but to some extent the winds arc responding to pressure and temperature diflercnccs creatcd by ocean currents beneath them. So it is a rather complex circular chicken-and-egg situation.

Actually, there is something peculiar about the North Atlantic circulation, beyond just the push of equatorial trade winds, which partly explains why it is strong enough to produce the Furious Fifties. As well as being pushed, it is also pulled along by another mechanism, the thcrmohaline circulation.

The Thermohaline Circulation

Ocean currents do not just move around on the surface. In some places, the upper ocean waters sink down into the deep ocean. This happens for example in the North Atlantic off Greenland, Iceland and Norway. Where the surface water sinks, this sends a "river" of surface water down into deep ocean. A similar sinking process happens off Antarctica, and in a small patch of the Mediterranean Sea (just south of Marseilles, France) in winter.

The reason these waters sink is that they are denser than the surrounding ocean. But why are they denser? It is mostly due to their higher salt content. Pour a dense brine solution into a bowl of fresh water and it will sink straight down to the bottom, and the same principle applies here. These denser, saltier ocean waters are derived from areas that undergo a lot of evaporation, because the climate is hot. Evaporation of water leaves a more concentrated salt solution behind, and this is the key to the whole mechanism. So. for example, the waters in the north Atlantic gyre are derived from the Gulf Stream that comes up from the Caribbean. Heated by the tropical sun, it has lost a fair amount of water by evaporation. After water vapor is transported away, the remaining seawater is left saltier and denser as it leaves on its path northwards across the surfacc of the Atlantic (Figure 1.7a). But the water is not yet dense enough to sink because the Gulf Stream is still warm as it is transported northwards. Warm water tends to be less dense than cold water. Even though it is saltier, its extra warmth is keeping its density quite low and it can still float over the less salty but cold water below.

Only when it reaches northern latitudes does the Gulf Stream water cool off drastically, giving up its heat to the winds that blow east over Hurope. Because it has cooled, the Gulf Stream water is now left heavier than the surrounding waters and it finally sinks, as "pipes" of descending water about a kilometer across that lead down to the ocean floor. These pipes tend to form in the spaccs between sea ice floes when a cold wind skips across the surface. On reaching the bottom, the sunken waters pan out to form a discrete layer that spreads through all the world's ocean basins.

The thermohaline circulation in the Atlantic. Relatively salty warm water (a) comes north from the tropics, then (b) cools ofTand sinks down into the deep ocean, pulling more water in behind it.

There arc several different sinking regions that feed water down into the deep (the North Atlantic being just one of them), and they each produce their own mass of water. These different waters sit above one another in a sort of "layer cake" arrangement, that shows up in a cross section down through the ocean. Each layer has its own density, temperature/salinity balance, chemistry and is travelling in its own particular direction!

Just about all the world's deep occan waters—those below about 300 metres—arc cold (about 2 to 4 C), even though most of the ocean surface area is warmer. Even in the tropics, where surface water temperatures may be 32°C, the water below 300 m depth is about as cold as it would be in a domestic refrigerator. Why then are these deeper waters so cold? Because they originate as water that sinks in winter in the high latitudes, when the sea surface is cold. If other warmer waters at other temperatures had instead been filling the deep occan. the mass of ocean water would reflect their particular temperature instead.

In fact, at other times in past (e.g., the early Eocene period, around 55 million years ago) the whole deep ocean was pleasantly warm—18-20 C instead of about 3°C at present. Why? Because the ''feeding1' of sinking water must have been occurring not in chilly sub-polar seas but down in tropical latitudes, from places similar to the Arabian Gulf at present where warm but salty water (concentrated by evaporation) spills out into the Indian Ocean. What did this opposite circulation system do to climate? The climate scientists have no idea, really. But it could perhaps help explain the warmer world at such times, a world that, for example, had palm trees and crocodiles living near the poles.

The greenhouse effect

The atmosphere tends to trap heat, through a process known as the "greenhouse effect". The gases in the atmosphere are mostly transparent to visible light, which is the main form in which the sun's energy arrives on earth. But many of these same gases tend to strongly absorb the invisible infra-red light that the earth's surface radiates to loose heat back to space. Some of the infra-red captured by the gas molecules in the atmosphere is sent back down to earth (as infra-red again) where it is absorbed by the surface once more and helps keep it warm. This is known as the "greenhouse effect".

If it were not for the combined greenhouse effect of naturally occurring gases in the atmosphere, the earth's temperature would naturally be somewhere around -20"C to -30 C on average. Thus this extra warming is very important in keeping the earth at a moderate temperature for life.

At present there is a lot of concern about an ongoing increase in the atmospheric levels of certain greenhouse gases due to human activities. For instance, carbon dioxide is building up at around 1% a year due to it being released by fossil fuel burning and forest clearance around the world (Chapter 7). It is set to reach double the concentration it was at 250 years ago some time during the mid-21 st century. The worry is that the increase over the background level of this and other greenhouse gases will lead to major climate changes around the world over the coming centuries. Already, detectable warming does seem to be occurring and the likelihood is that this will intensify. Since plants are strongly affected by temperature, it is likely that global warming will change the distribution of biomes. Shifts in rainfall that result from the changing heat balance and circulation of the atmosphere may also turn out to be important.

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