1. Global Energy Balance

#TestThenCheck

Key concepts for your schema. Use these questions to test your knowledge before scrolling down!

The atmosphere likes balance.
If there is too much heat energy somewhere and not enough elsewhere the atmosphere will move heat from the areas of what to areas of what?
These are known as the heat transfers and there are two types:
- Vertical (from ground to air) - what are the three vertical transfers?
- Horizontal (from low latitudes to high latitudes)
  - What are the two broad categories of horizontal heat transfer?
  - What are the three ways heat is redistributed by winds?
There is a pattern to global temperatures -
- This can be explained with reference to four factors - what are they and how do they operate?
- What is the global pattern of temperature and how & why is it modified?

The Earth's Heat Budget

The global energy balance refers to the balance between:

inputs of heat energy (insolation) from the Sun
outputs of heat from the Earth

In addition, there are stores of heat energy within the system (e.g. the Earth, the atmosphere itself) and transfers (vertical and horizontal transfers of heat - see below).

Although there is an overall balance between inputs and outputs, there are imbalances in the distribution of heat within the atmosphere. It is the atmospheric processes that act to redress this imbalance which are responsible for producing our weather.

There are two imbalances that need to be addressed:

The imbalance of heat between the Earth and the atmosphere
The imbalance of heat at different latitudes (It's warmer at the Equator and colder at the Poles).

1.1 Vertical Heat Transfers

The heat energy input into the atmosphere system comes from the Sun.

You do NOT need to learn the following in great detail (no need to learn the percentages, for instance). Instead, it is to give you an overall sense of the heat budget balance the atmosphere reaches.

Earth's Heat Budget

But not all of the Sun's energy that reaches the edge of our atmosphere arrives at the surface.

In fact, for every 100 units of insolation that reach the edge of the atmosphere, on average:

30% is reflected or scattered back into space
20% is absorbed by gases in the atmosphere such as ozone
50% is absorbed by the earth’s surface

KEY POINT

The atmosphere likes balance, and so the amount of energy inputted to the atmosphere overall is equal to the energy outputted (otherwise the Earth would keep getting warmer and warmer).

The process by which this happens is known as the VERTICAL HEAT TRANSFERS. There are 4 of them to look at.

Conduction

This is the transfer of heat energy by contact. The air directly above the ground is heated due to contact with the ground.

Convection

A gas that is heated becomes less dense and so will rise by convection. The air has vertical rising currents of warmer air (thermals) which transfer the heat upwards into the atmosphere.

Radiation

Warm surfaces emit heat via radiation (transfer by electromagnetic waves).

The Sun emits short wave radiation and the gases in the atmosphere cannot absorb that very effectively.
The Earth, however, emits long wave radiation. The gases in the atmosphere can absorb this much more readily, and so this transfers heat from the Earth upwards through the atmosphere directly into space.

This diagram summarises how each method of heat transfer works:

Latent heat

When water from the Earth's surface evaporates, the heat energy used to change the state from liquid to gas (a change of phase state) does not produce an increase in temperature. Instead, that heat energy is stored with the water molecules as they rise up. Stored, that is, until condensation occurs. At that point, the latent heat is released.

This is a sigificant factor in vertical heat transfers from the Earth to the Atmosphere, moving close to a quarter of the heat as part of that transfer.

This20% radiated into Space directly from the Eart

The following Slides shows how latent hea transfer happens. You do not need to learn this in detail.

Latent heat

A bit more physics behind latent heat

Heat can be categorised into two types:

Sensible heat - When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat. Heat that causes a change in temperature in an object is called sensible heat.
Latent heat - All pure substances in nature are able to change their state. Solids can become liquids (ice to water) and liquids can become gases (water to vapour) but changes such as these require the addition or removal of heat. The heat that causes these changes is called latent heat. Latent heat however, does not affect the temperature of a substance - for example, water remains at 100°C while boiling. The heat added to keep the water boiling is latent heat. Heat that causes a change of state with no change in temperature is called latent heat.

The amount of energy needed to change water from liquid to gas is actually quite staggering.

To increase 1 g of water by 1C needs 1 calorie of heat.
But the amount of latent heat needed when water evaoprates is around 540 calories!
This heat is then stored as latent heat until, when condensation occurs as the air rises and cools, all 540 of those calories of latent heat are released!

So, if we were to take 1 km2 of open water at 26C, in one hour, around 2,500,000 litres of water would evaporate, which is around 2,500,000,000 grams of water.

The amount of latent heat energy that would be released by the water vapour when it condenses would be a staggering 1,347,500,000,000 calories of latent heat!!!

That's equivalent to

The energy released is approximately equivalent to the amount of energy used by a typical household in the United States in about 50 years.
The energy released is about 135 times greater than the energy needed to launch a Space Shuttle into orbit.
The energy released is roughly equivalent to the energy that could be produced by burning 400,000 kg of coal.

Once conduction, convection and latent heat have transferred heat energy into the atmosphere, this is then transferred back out into space by long wave radiation.

1.2 Horizontal Heat Transfers

Okay, so the Earth's heat budget is in balance. The Earth is not getting naturally warmer and warmer over time, I get it.

But what about this fact: if this is true, why is the equator warmer than the Poles? Where is your precious balance there...?!

Fair enough. Let's examine that and see if we can find some dynamic equilibrium at work.

This map shows the average annual temperature of the Earth. And it reveals the startling fact: it's warmer at the Equator and colder at the Poles!

Clearly this is an imbalance of heat energy. If the atmosphere did not act to redress that, then the Equator would get warmer and warmer over time, and the Poles would get colder and colder.

This is not happening, so there must be something at work to redress this imbalance.

Welcome to the horizontal heat transfers!

Before we look at how, let's look at a way of presenting the information in this map on a graph.

At first, this graph may look a bit complicated. But let's take a look bit by bit at what it shows.

The red line is the input - the amount of heat received by the Earth from the Sun.
The blue line is the output - the amount of heat given off by the Earth into the atmosphere.
The scale at the bottom is the latitude: 0 is the equator and 90 is the Poles.

What is the pattern shown in this graph?

Between 0 and 40 N/S, the red line is above the blue line - what does this indicate in terms of the balance of inputs and outputs?
From 40 degrees to the Poles, the blue line is above the red line - what does this indicate in terms of the balance of inputs and outputs?

This imbalance is redressed by the horizontal heat transfers.

There are two broad categories into which horizontal heat transfers can be placed: Winds and Ocean Currents.

A. Winds

About 70% of this heat imbalance is redressed by global winds. Winds carry heat energy from areas of surplus to areas of deficit.

i. Air masses

Air masses play a role in this. In the mid latitudes , the General Circulation of the Air means that there is a lot of mixing of cold air from the Poles and warm air from the Tropics.

For example, in this weather chart, the Low Pressure is to the NW of the British Isles. The UK is in the warm sector of the depression which as Tm air. But note the wind direction here: the wind is coming all the way from the tropics, bringing warm air from areas of surplus to areas of deficit.

In contrast, in this weather chart, only 4 days later, the UK is dominated by the High Pressure over Scandanavia. Note the wind direction now - it's coming from the east.

In fact, follow the isobars back up and you'll see it originates from the far north east. This is bringing cold air from areas of deficit to areas of surplus.

Below is a video for the air masses from the above synoptic charts. Notice two things:

how the wind direction affecting the UK changes from southerly to easterly, and how the temperatures get much colder as a result.
the parcel of warm air that is detached and moved north - bringing heat that originated over the Sahara desert all the way up to the Arctic Circle.

Here's what the air temperature was for the days shown in the above maps (map 1 is 'Today', map 2 is Sunday). Notice how the temperatures fall as the air mass changes. It shows how much heat energy was being transported up from the south in the first map!

ii. Hurricanes

The map shows the tracks of Atlantic hurricanes over a 150 year period. As you can see:

They originate off the west coast of Africa in the area of heat surplus.
They track west towards the Caribbean and USA.
Most of them then track north and north west towards the area of heat deficit, and so transfer heat from the areas of surplus to deficit.

Find out about one example of a hurricane that followed this route here.

B. Ocean currents

There are some consistent ocean currents that flow around our planet, north to south and south to north, and they play an important role in redistributing the heat energy around our planet.

Let's look at two examples. [You do not need to learn all the ocean currents for your exam. You need to know (1) the principles of how they redistribute heat and (2) the two examples below to illustrate the principles.]

North Atlantic Drift

This is a current that originates in the warm oceans west of Africa, travels west towards the Gulf of Mexico and then on north east towards Western Europe. It transfers heat from the areas of surplus to deficit, keeping NW Europe warmer during the winter than other areas at the same latitude.

Labrador Current

This is a current that originates from the North Pole. It transfers cold from the areas of deficit southwards, keeping NE USA cooler during the winter than other areas at the same latitude.

Here's is a map of all the ocean currents to give you context, in case you get a resource based question about one or two of them.

1.3 Global pattern of temperature

We have already seen the overall pattern of world temperature: it's warmer at the Equator and colder at the Poles.

Or, more formally, as latitude increases, temperature decreases.

That said, if latitude was the only factor affecting temperature, we would expect the temperature bands to correspond precisely with the lines of latitude. However, it's clear that they don't. Although in the general sense it's true to say temperature decreases as latitude increases, there are modifications to this. Let's explore this more with these two maps for winter and summer.

December

The green line shows a line of latitude. Look carefully at how the blue line rises and falls as we move around this line of latitude.

In the winter, the land is colder than the sea at any given line of latitude.

July

Look carefully now at how the red line rises and falls as we move around this line of latitude.

In the summer, the land is warmer than the sea at any given line of latitude.

So, the overall pattern of the maps is this:

as latitude increases, temperature decreases

But some modifications are apparent to this pattern:

in the winter, the land is colder than the sea at any give latitude
in the summer, the land is warmer than the sea at any given latitude.

This must mean that factors other than latitude alone influence temperature. Let's have a look at these now.

1.4 Factors influencing temperature

Retrieval Practice video on the factors affecting temperature

A. Latitude

As latitude increases, temperature decreases.

Why is this the case? Three reasons:

i. Angle of incidence

As the Sun's rays approach the atmosphere at a more gentle angle at the Poles, the insolation is more likely to be reflected off at Polar regions.

Conversely, at the Equator, the insolation approaches from more directly overhead and so it less likely to be reflected off.

So, the insolation is more likely to be reflected off at the Poles.

The differing angles of approach of insolation between the equator and the poles has two other impliactions:

ii. Concentration of insolation

The Sun's rays approach from more of an overhead direction close to the equator.

This means that the insolation is concentrated more there and so it is warmer.

Conversely, at the Poles, the Sun's rays approach at an gentle angle.

This means that the insolation is more spread out - the same amount of heat has to cover a larger area - and so it is colder there.

So, the insolation is more spread out at the Poles.

iii. Thickness of atmosphere

As the Sun's rays approach the atmosphere more head on at the Equator, they travel through less thickness of the atmosphere than is the case at the Poles, where the insolation approaches at a more gentle angle (note the different lengths of the two blue lines in the diagram).

As we have seen, the atmosphere reflects, scatters and absorbs some insolation, so less insolation reaches the Earth’s surface at the Poles, hence it is colder there.

So, less insolation reaches the surface at the Poles.

To summarise: due to the low angle of the Sun at the Poles, (i) less insolation reaches the surface; (ii) that which does is spread out more; (iii) and it's more likely to be reflected off than at the Equator.

B. Continentality

Let's consider that map for global temperatures in December. Look again at the green line (line of latitude) and the blue line (line of zero degrees).

If we were to travel around the globe, following that line of latitude, what we would discover is:

In the winter, the land is colder than the sea at any given line of latitude.

And this is for the following reason:

The land cools down more quickly than the sea due to continentality.

Why do land and sea heat up and cool down differently?

Land heats up and cools down quickly

Water heats up and cools down slowly

What are the implications of this?

Continentality can be seen a the distance from the moderating influence of the ocean. As the oceans heat and cool more slowly, they tend to buffer the temperatures of coastal areas, making them cooler than inland areas in the summer and milder than inland areas in the winter. In contrast, areas far inland have much greater extremes - warmer summers and colder winters - than coastal areas.

A bit more of the physics behind this

The specific heat of a substance is the amount of heat energy required to raise the temperature of 1 gram of the substance by 1 degree Celsius. Water has a very high specific heat compared to most other substances, including soil, rock, and trees. The specific heat of water is about 4.18 joules per gram per degree Celsius (J/g°C). In comparison, the specific heat of soil is typically around 0.84 J/g°C, while the specific heat of rock is around 0.22 J/g°C, and the specific heat of wood is around 1.76 J/g°C.

The high specific heat of water is due to the unique hydrogen bonding between water molecules. When heat is added to water, the molecules start to move faster and their kinetic energy increases. However, this increased kinetic energy also causes the hydrogen bonds to stretch and become more rigid. As a result, water molecules require a significant amount of heat energy to increase their temperature, and this causes water to have a high specific heat.

To understand this concept, you can think of the analogy of human sweat. When we sweat, our bodies release water onto the skin surface. As the sweat evaporates, it absorbs heat energy from our bodies and cools us down. This is because water has a high latent heat of vaporization, which means that it requires a lot of energy to convert liquid water into water vapor. Similarly, water requires a lot of energy to raise its temperature, which is why it takes longer for water to heat up or cool down compared to other substances with lower specific heats.

The high specific heat of water has important implications for climate and weather. Oceans, lakes, and rivers can absorb large amounts of heat energy from the sun and store it as thermal energy, which helps to regulate temperature and stabilize the climate. This also means that bodies of water can take longer to warm up or cool down compared to land surfaces, which can affect local weather patterns.

Still struggling to get this idea? Maybe this will help!

Think of hot sand on a beach on a hot summer’s day. It can be painful to walk on, but if you dig your toes down even a few centimeters into the soil, it is much cooler. That evening, the sand will quickly cool down again as the heat quickly escapes from the surface layer.

Now think of the water. If you go in for a swim on that same hot summer's day, the water temperature is more or less the same all the way from the surface down at least deep enough to cover you. That night, the sea temperature wouldn’t be too different to what it was during the day.

Let's look at similar information presented in a different style of map. This time, the map displays annual temperature ranges.

The map clearly shows differences in patterns between land and sea in terms of annual temperature ranges:

Annual temperature ranges are lower over the sea and they are greater over the land masses.

The larger the land masses, the greater the temperature ranges.

Use these principles to help you describe the map in more detail in your Docs. You should refer to places (continents) and quote figures.

C. Altitude

As altitude increases, temperature decreases.

This is Kilimanjaro. In Kenya. That's in Africa - and at the Equator. And yet, there's still snow on top of it? How come there is snow in Africa?!

Look at this Slides presentation for the explanation.

Why temperature decreases with altitude

The Sun emits short wave radiation which is not easily absorbed by the atmosphere.
So the atmosphere is heated indirectly by the long wave radiation given off by the Earth as part of the vertical heat transfers.
This heat is absorbed by the molecules in the air (as part of the greenhouse effect).
But, as height increases, air pressure decreases - and so does the number of molecules per unit volume of air.
This means that more long wave radiation can be absorbed lower down where there are more air molecules than higher up - thus temperature decreases as height increases. The rate at which this happens is around 5° C per km.

D. Seasonality

Seasonality & continentality

The main seasonal differences in summer/winter temperatures are related to continentality. For example, in Siberia at the eastern part of the largest continental landmass in the world, temperature ranges can go from as much as +35 C to -60 C, an annual range of 95 C. In contrast, the Faroe Islands (a group of small islands in the Atlantic Ocean between Scotland and Iceland) has winter temperatures of around 4 C and summer temperatures of arround 11 C, an annual range of only 7 C!

Seasonality & latitude

Due to the tilt of the Earth on its axis of rotation, there are seasonal variations in temperature across different latitudes. The higher the latitudes, the more pronounced these seasonal variations are.

June

On 21 June, the overhead Sun is directly above the Tropic of Cancer. In the higher latitudes in the northern hemisphere, this means that, compared to higher latitudes in the southern hemisphere:

the insolation passes through less thickness of atmosphere (so less is reflected & absorbed on the way through)
it is more concentrated
the higher angle of incidence means that heat is less likely to be reflected off (and, with less sea ice in the Arctic Ocean, the albedo is reduced)

December

On 21 December, the overhead Sun is directly above the Tropic of Capricorn. In the higher latitudes in the northern hemisphere, this means that, compared to higher latitudes in the southern hemisphere:

the insolation passes through more thickness of atmosphere (so more is reflected & absorbed on the way through)
it is less concentrated
the lower angle of incidence means that heat is less likely to be reflected off (and, with more sea ice in the Arctic Ocean, the albedo is increased)

A consequence of this is that the extremes of temperature ranges are greater at higher latitudes than lower latitudes

Seasonal variations in temperature

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