Energy Balances

Chapter 4

Praise the LORD!
Praise the LORD from the heavens!
Praise him from the skies!
Praise him, all his angels!
Praise him, all the armies of heaven!
Praise him, sun and moon!
Praise him, all you twinkling stars!
Praise him, skies above!
Praise him, vapors high above the clouds!
Let every created thing give praise to the LORD,
for he issued his command, and they came into being.
He established them forever and forever.
His orders will never be revoked.
Praise the LORD from the earth, you creatures of the ocean depths,
fire and hail, snow and storm, wind and weather that obey him,
mountains and all hills, fruit trees and all cedars,
wild animals and all livestock, reptiles and birds,
kings of the earth and all people, rulers and judges of the earth,
young men and maidens, old men and children.
Let them all praise the name of the LORD.
For his name is very great; his glory towers over the earth and heaven!
He has made his people strong, honoring his godly ones – the people of Israel who are close to him.
Praise the LORD!

 Psalm 148 (NLT)

There is an introductory video here:

Insolation, discussed in Chapters 2 and 3, represents solar energy coming into the earth-atmosphere system (energy input).  Remember INSOLATION is INcoming SOLar radiATION (not the pink fiberglass stuff in your attic – insUlation!).  This insolation may be absorbed, refracted, or reflected once it enters the atmosphere or reaches the surface of the earth.  This chapter looks at the movement of energy within the earth-atmosphere system.

I. Energy Principles

In Chapter 2 we noted that at the top of the Earth’s atmosphere, insolation varies with latitude (it decreases toward the poles).  At the Earth’s surface insolation also varies according to dominant weather patterns.  Regions that are typically always clear receive more insolation than regions that are frequently cloud-covered.

Consider Figure 4.3, “Insolation at the Earth’s surface, “  4CE, p. 94. (4.2; 3CE, p.87).

  •  Note the highest insolation at the Earth’s surface does NOT occur at the Equator.  Equatorial weather is characterized by afternoon cloudiness and heavy rainstorms – reducing insolation at the surface; this coincides with rainforest/jungle regions.
  •  The highest insolation is received along the Tropics (23½° N and S).  In this region the weather is almost always clear, allowing for higher insolation.  Note this coincides with major deserts.
  •  North and south of the Tropics (from 23½° N and S towards the poles), surface insolation does decrease uniformly with latitude. Weather (cloudiness) in these regions is fairly uniform everywhere.
A.   Transmission

Energy passes through air and water; this is referred to as transmission.  As energy is transmitted through the atmosphere, things can happen to it …

1. Refraction
4 Brocket AB rainbow

Rainbows are a result of light refraction. Brocket, AB

When insolation passes from one medium to another (from space → atmosphere, from atmosphere → water, from atmosphere → glass), or from one part of a medium to another part of the medium with different characteristics (from cold air → warm air) its speed and direction change.  This referred to as refraction.  When you experiment by bending a beam of light by shining it through a prism, you are refracting it.  The light is going from one medium (air) to another medium (glass), and back into the first medium (air).

Different wavelengths may refract (or bend) at different angles.  Thus, when insolation passes through a raindrop, it refracts into a rainbow.  The different wavelengths of visible light refract at different angles.  See Figure 4.4, “A rainbow,” 4CE, p. 95 (4.3; 3CE, p. 88).

Rainbows are caused by refraction of light within raindrops (Everything you want to know about rainbows but were afraid to ask). Sometimes you get double rainbows.  The primary bow, which is most visible, is caused by one refraction. The secondary bow is caused when some light reflects within the raindrops before exiting. This “extra” reflection/refraction diminishes the light’s intensity causing the fainter appearance of the second rainbow.In theory you can get a third rainbow as well by a triple reflection.  If you look carefully, in a double rainbow, the colours in the second are reversed.

Air with layers of different temperatures (and thus different densities) also refracts light.  In air with different temperatures, light bends and distorts insolation to produce a mirage.  On very hot days, air near the surface is super-heated to temperatures much warmer than temperatures higher up.  As visible light passes through these layers of differentially heated air, the different temperature layers within the air refract the light at different angles – creating multiple images.

Refraction is also responsible for the apparent “swelling” of the sun at sunset … and for our ability to see the sun, even when technically it’s below the horizon.  See Figure 4.5, “Sun refraction,” 4CE, p. 95 (4.4; 3CE, p. 88).  A similar effect happens with the moon when it is near the horizon. When the moon is just above the horizon (or even if it’s actually just over the horizon), it may appear larger than normal.

2. Reflection

Some energy is reflected back into space.  Much of this occurs as insolation is reflected by clouds and particles in the atmosphere.  Some is reflected by the surface of the earth.

Overall the Earth and its atmosphere reflect an annual average of 31% of all insolation back into space:

  • 21% is reflected by clouds
  • 3% reflected by the Earth’s surface (land and water)
  • 7% scattered in the atmosphere

Albedo is a term used to describe how reflective a surface is.  See Figure 4.6, “Various albedo values,” 4CE, p. 96 (4.5;3CE, p. 89).

  • Lighter colors have a high albedo; they are highly reflective (they reflect a high percentage of solar energy); therefore lighter surfaces absorb less solar energy and heat up less than darker surfaces.. Snow reflects 80-95% of insolation. Water bodies well north or south of the Equator (like North America) tend to have a high albedo because they sun strikes them at a sharp angle and rays reflect off the water like a mirror.  People who wear light clothes in the summertime put themselves at a greater risk of heatstroke than those who wear white clothes.
  • Darker colors have a low albedo; they are not very reflective (they reflect a low percentage of solar energy); therefore darker surfaces absorb more solar energy and heat up!.  Asphalt reflects only 5-10% of insolation. Water bodies nearer the Equator (like Central America) tend to have a low albedo because the sun’s rays strike them at a higher angle.  The sun warms these waters more because less insolation is reflected.  People who wear dark clothes in the summertime put themselves at a greater risk of heatstroke than those who wear white clothes.

Consider the changes a snowfall can bring.   Before a snowfall, the albedo on  campus is quite low – lots of grass, trees, asphalt (albedo levels of 5-30%).

Once snow falls, the albedo rises sharply (to albedo levels of 80-95%).    Much more (50-90% more!) insolation is reflected after a snowfall than before.

What does this mean?  Before the snowfall, 95-70% of the insolation was used for heating the ground (see absorption, below).   After the snowfall, only 20-5% of the insolation is available for heating the ground – the rest is all reflected!

Everything else being equal, daytime temperatures will fall significantly after a snowfall, simply because more insolation is reflected and cannot be absorbed and used for heating.

Clouds have a high albedo.  This reduces insolation and thus would be expected to cool the Earth’s surface.   However clouds also act as insulation, trapping longwave radiation from the Earth.  This tends to raise temperatures at the Earth’s surface.  See Figure 4.7, “The effects of clouds …” 4CE, p. 97 (4.6; 3CE, p. 90).

Most human activities tend to increase albedo.  Pollution, for instance, tends to make the atmosphere more reflective.  Urban areas, with many light roofs and light-colored buildings, tend to have a higher albedo than rural areas.

The moon appears to be very bright; it appears to have a high albedo.  However the moon actually has a very low albedo of about 6-8% (it’s mostly rock, not dissimilar in albedo to worn asphalt or a coniferous forest).  The Earth has a much higher albedo (average 31%) than the moon (6-8%), making the earth much “brighter” than the moon.  If we could see the Earth in the night sky (rather than the moon), the Earth would appear to be 5-6 times brighter than the moon!  Astronauts are often incredibly astonished at the brightness of the Earth from space!

Venus, which is shrouded in clouds, has a very high albedo.  Not surprisingly it appears very bright!  It is normally the first celestial object we see at night (the “evening star”) and the last object we see in the morning (the “morning star”).  Of course Venus is not a “star” at all, but a planet!

4 digby

The sky is blue because: the sun produces much radiation in the blue spectrum and it is more easily scattered by the atmosphere (Digby, NS)

3.  Scattering

Some insolation is “scattered” by particles in the atmosphere.  In general, the shorter the wavelength, the greater the scattering.  The longer the wavelength, the less the scattering.

3 Sunset

Sunrises/sunsets are more colourful with more pollution, which tends to scatter longer wavelengths (Howe Sound, BC)

Shortwave lengths of visible light (blues and violets) are scattered the most by air.  Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light.  The result is that the human eye perceives blue when looking toward parts of the sky other than the sun.  Note that the sun produces more blue wavelengths than violet.  Thus blue light is most scattered by gas molecules in the atmosphere – and our sky appears blue!
At sunrise and sunset, when the sun’s rays have to pass through more of the lower atmosphere (where most of the pollution is), they often appear red or orange because solid particles more effectively scatter these longer wavelengths – creating colorful sunsets and sunrises.

4. Absorption

Insolation that is not reflected is absorbed.  When it is absorbed it is converted into either longwave (infrared) radiation or chemical energy (by photosynthesis in plants).  Absorption raises the temperature of the absorbing body.  When solar energy is absorbed by asphalt, for instance, it warms up!

Note that air, however, is primarily heated by the longwave radiation reradiating from the Earth, not directly from incoming solar radiation!  The sun does not directly heat the atmosphere.  Rather, the sun heats the Earth’s surface, which reradiates longwave radiation, which heats the air.  In the lower few meters of the troposphere (where we live!) air tends to heat upward from the Earth’s surface.

  • This is why snowfalls drop temperatures!   More insolation is reflected.  Less insolation is absorbed.   Less heating occurs!
  •  Try this experiment next time it snows: clear a portion of the parking lot down to the asphalt.   Leave another part snow-covered.  See what happens.
  •  Where you exposed the asphalt, the surrounding snow will melt much quicker than snow at the unexposed spot.
  • Why?  Because the exposed asphalt absorbs more insolation, heats up, and begins to melt the surrounding snow. The snow-covered site does not absorb much insolation and does not melt much.

  The moral of the story: to effectively melt snow, expose surfaces with low albedos (like asphalt)!

B.  Conduction, Convection, and Advection

Heat is transferred by:

  1. Conduction:  molecule-to-molecule heat transfer.  As particles warm up, they vibrate, colliding with others, physically transferring heat to cooler materials.  Generally denser materials (rock, soil) conduct better than less dense materials (water, air).  When you touch a hot burner on a stove, the heat you feel is transferred by conduction!
  1. Convection: vertical physical mixing.  For instance, air, heated by conduction from warmer ground, rises in convection currents.  The hot air from your forced air furnace, coming out of the vent in your room, is heating your room by convection.
  1. Advection or Radiation: horizontal physical mixing.  For instance, winds can blow warmer air masses or ocean currents can move warmer water into a region.  On a blistering hot day, when you open the door of your air conditioned home and the hot air rushes in, you are experiencing advection.
C. Review and a bit more:

1. Shortwave Radiation (Insolation) in the Atmosphere

Shortwave radiation passes through the upper layers of the atmosphere relatively easily.  In the troposphere shorter waves are often scattered (creating the blue sky).  Or they are reflected by clouds (which have a high albedo).  On cloudy days some shortwave radiation is diffused by water droplets and still reaches the earth (so we still have light, but it isn’t as bright as on sunny days).

Thus clear days tend to be warmer than cloudy days!

At the surface the albedo will determine how much of this energy is absorbed (and can become thermal energy or heat) and how much is reflected by the surface.  In general human activity results in surfaces with higher albedo which has possible global environmental consequences.

2.  Longwave (Infrared) Radiation in the Atmosphere

The earth emits mostly longwave radiation (because it’s relatively cool).  The atmosphere (especially carbon dioxide and water vapor (i.e. clouds)) are very good “absorbers” of longwave radiation.  They, in turn, reradiate this energy toward the earth.  They act like a blanket over the earth’s surface.

Thus, at night when skies are clear the longwave radiation emitted by the earth is largely lost and temperatures fall rapidly.  But at night when skies are cloudy, much of the longwave radiation emitted by the earth is absorbed by the clouds and reradiated to the surface.  Consequently the earth-atmosphere system cools more slowly on cloudy nights.  Cloudy nights tend to have less dramatic temperature drops than clear nights!

II.   Energy Balance in the Troposphere

A.   The Greenhouse Effect
  •  See Figure 4.8, 4CE, p. 99 (4.9; 3CE, p. 93).

A greenhouse (or a car) heats up when short-wave radiant energy passes through the glass, and is absorbed by the surfaces inside.  Albedo is important!  A car with a light color interior has a higher albedo and tends to reflect more short-wave isolation directly back into the atmosphere.  A car with a dark interior tends to absorb more insolation – it will heat up more quickly!  Think about this when you consider buying the car with black vinyl seats!

The energy that is absorbed in the car (or in the greenhouse) is reradiated as long-wave heat.  The glass of the car windows/greenhouse does not allow long-wave energy to pass through as easily as the incoming short-wave energy.  Thus the heat energy is trapped and the car/greenhouse warms up!  Energy comes in through the glass but cannot escape.  Unless you open a vent or your car window it may get hotter and hotter!

4 ge

How the greenhouse effect works …

The atmosphere does not work as efficiently as a greenhouse, but the same effects can occur.  Short-wave radiation comes in, passing through most of the atmospheric gases.  When it is reradiated as long-wave heat by the earth, some is reflected back, particularly by water vapour (clouds), carbon dioxide and methane (which are often called the “greenhouse gases”).

The proportion of these gases in the atmosphere is increasing, suggesting the net result will be a rise in global temperatures.  This is particularly true of carbon dioxide.

Naturally occurring greenhouse gases (water vapour, carbon dioxide, methane, nitrous oxide) keep the Earth warm enough to support life. Scientific studies show that a variety of human activities release greenhouse gases, that enhance this greenhouse effect, leading to global warming and climate change. These activities include the burning of fossil fuels for producing electrical energy, heating and transportation.  By increasing their concentrations and by adding new greenhouse gases like CFCs, humankind is capable of raising the average global temperature.

Before the Industrial Revolution, atmospheric carbon dioxide is estimated to have been about 0.0294%.  The burning of fossil fuels (coals, petroleum and natural gas) releases water vapor and carbon dioxide. Clear cutting of forests also releases more carbon dioxide, plus it reduces organic carbon dioxide consumption.  From 1860 to 1990 atmospheric carbon dioxide has increased about 22% to over 0.035%; by the year 2000 to about 0.0380% (up 35%).  We will double our current levels of carbon dioxide by 2030.  See the data from Mauna Loa.

These increases in carbon dioxide (in particular) appear to be contributors to global warming and climate change.  Temperature records shows the Earth has warmed an average of 0.6°C over the past 100 years. There appears to have been a warming until the early 1940’s then a moderate cooling until the mid 1970’s, followed by a renewed and pronounced warming continuing through the present. The 1990’s was the warmest decade on record. (In fact, ice core and other proxy data indicates that the 1980’s and 1990’s were the warmest decades of the past millennium).

The U.N. World Meteorological Organization’s International Panel on Climate Change (IPCC) is the world’s premier peer-reviewed scientific authority on climate change, with several thousand atmospheric scientists involved.  Recent Assessment Reports highlight that:

  • Warming of the atmosphere and ocean system is unequivocal. Many of the associated impacts such as sea level change (among other metrics) have occurred since 1950 at rates unprecedented in the historical record.
  • There is a clear human influence on the climate
  • It is extremely likely that human influence has been the dominant cause of observed warming since 1950, with the level of confidence having increased every year.
  • The longer we wait to reduce our emissions, the more expensive and/or uncontrollable it will become.

Recent reports also noted:

  • Recent years/decades have been the warmest in recent centuries.
  • The upper ocean has warmed .
  • The Greenland and Antarctic ice sheets have been losing mass in the last two decades and that Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent.
  • Sea level rise since the middle of the 19th century has been larger than the mean sea level rise of the prior two millennia.
  • Concentration of greenhouse gases in the atmosphere has increased to levels unprecedented on earth in 800,000 years.

Global warming gases have hit record levels in the world’s atmosphere, with concentrations of carbon dioxide up 39 percent since the start of the industrial era in 1750.  CO2 is the greenhouse gas of greatest concern to policy makers looking to stem human-induced climate change:  fossil fuel-burning, loss of forests that absorb CO2 and use of fertilizer are the main culprits.

Levels of methane – considered the second most important greenhouse gas – have risen after a period of relative stabilisation from 1999 to 2006.  This could be due to the thawing of the Northern permafrost and increased emissions from tropical wetlands.  Nitrous oxide, emitted into the atmosphere from natural and man-made sources, including biomass burning and fertiliser use, was 323.2 parts per billion in 2010 – 20% higher than in the pre-industrial era.  Read the World Meteorological Organization (WMO)  Greenhouse Gas Bulletin GHGbulletin.html.

HFCs (Hydrofluorocarbons), a popular choice by refrigeration manufacturers of because they are are deemed to be a “like-for-like” replacement substance for ozone-depleting Chlorofluorocarbons (CFCs) and hydrofluorochlorocarbons (HCFCs) are also dramatically increasing in the atmosphere — and are potent greenhouse gases .(BBC News – Climate concerns as ‘ozone-friendly’ HFCs use grows).  HFCs are much more potent global warming agents than carbon dioxide:  Global Warming Potentials

While many politicians and media types (for various reasons) may question the reality of global warming, it is a bit like those who argued the earth was flat back in Columbus’ day.  The scientific evidence is overwhelming.  (More in Chapter 10-11 on Climate and Climate Change).

The political debate is far from over!

For current information on the greenhouse effect, climate changes, and political responses to the issues, see:

B.  Energy Balance

The Earth-atmosphere energy balance is described on pp. 100-101 (4CE) or (Figure 4.10, “Detail of the Earth-atmosphere energy balance“ 3CE, p.94).  Don’t worry about the numbers!  Simply note the various parts – reflection, scattering, absorption – and the various places this occurs: in the atmosphere, (e.g. clouds) and at the surface.

Consider Figure 4.10, “Energy budget by latitude” 4CE, p. 103 (Figure 4.12, 3CE, p. 95).  Highlight the comments in your text that read, “Figure 4.10/4.12 summarizes the Earth-atmosphere energy/radiation balance (for all shortwave and longwave energy) by latitude …”

In the middle latitudes (between 36°S and 36°N latitude), a surplus of energy occurs (more energy comes in that is lost).  Toward the poles a deficit occurs (more energy is lost than received).  Energy transfers by warm air masses, precipitation, and warm ocean currents moving from tropical areas toward the poles balance this.

For any point on the earth the amount of incoming and outgoing solar radiation can be determined.  This referred to as the energy balance.

For a given spot this can be expressed as:

(Insolation – reflection) + (incoming longwave – outgoing longwave) = net radiation

Net radiation is available to be used as

  • sensible heat (that is, heat you feel).  This is transferred from land/water to air as heat (especially important in the subtropics).
  • latent heat (heat that is used to evaporate water into water vapour).  This heat is stored (you don’t feel it as sensible heat), but it is released when water vapour condenses to form droplets in the atmosphere.
  • ground heating (the ground (land/water) warms up to a certain depth).

III.  Atmospheric Temperature Variation

A.  Daily Variations

As the earth rotates (in about 24 hours), the amount of solar radiation any spot receives varies.  See the orange line on Figure 4.11, “Daily radiation curves” 4CE, p. 103 (Figure 4.13, 3CE, p. 96).  This will be helpful on your lab!

  • At night, no radiation is received.
  • At “high noon” maximum solar radiation is received.
  • At dawn and dusk, when the sun is low in the sky, rays strike the earth obliquely and the radiation is spread over a wide area; also much radiation is intercepted by dust, gases, and vapour in the atmosphere. Relatively little radiation is received.

The effect on temperature follows logically.  See Figure 4.11/4.13, the purple line

  • As the sun rises it slowly begins to heat the earth and the air above it.
  • At noon, solar radiation is maximum; but
  • Maximum temperature occurs about 2-3 p.m. This is because there is a lag between when the earth heats up (because of insolation) and when this heat is conducted to the air above it).
  • As the sun drops, radiation drops and so does the temperature.
  • Cooling continues through the night as the earth reradiates stored energy
  • Maximum cooling occurs just before dawn, as the Earth has radiated energy all night long.

EXAM/LAB NOTE:  Make sure you understand Figure 4.11/4.13, “Daily radiation curves.”  Why is the orange “insolation” curve the shape it is (why is insolation 0 at night?  Why is it maximum at noon?)?  Why is the purple “temperature” curve the way it is?  (why is the warmest time of day later than noon?  Why is it coolest just before dawn?)

This pattern is affected by the presence of water:

  1. Water heats more slowly (and cools more slowly) than soil
  1. Shortwave solar radiation can heat up to 50 m of water, but only the upper few centimeters of soil
  1. Water is mobile; warm water can move great distances
  1. Water transfers heat relatively easily through mixing
  1. Water bodies provide ready sources of water for evaporation and cloud formation; reducing insolation and heating

The combination of these factors tend to moderate temperatures (cooler during the day, warmer at night) at locations near bodies of water.

This effect is called continentality; areas far from oceans tend to have higher temperature extremes than coastal areas.

5-1 Victoria

February in Victoria, BC (coastal, moderated by ocean) – roughly same latitude as Lethbridge

5-2 Lethbridge

February in Lethbridge, AB (continental inland, not moderated by ocean)

Do not worry about the “surface energy balance/budget,” or “daily radiation budget” discussions 4CE pp. 104-107 (3CE, pp. 97-101).

B.  Seasonal Variation

The changing position of the earth relative to sun also affects atmospheric temperature because both day length and the angle of incoming solar radiation varies.

In winter, our area experiences a net radiation deficit (more is lost than comes in).  In summer we experience a surplus.  Some of this surplus is stored in the earth and then released through the winter.  Also, warmer air from other regions (the coast, the south) helps to balance out our radiation loss.

Again, the presence of water has a moderating influence (cooler summers, warmer winters); continentality is again a factor.

Seasonal variation also tends to be more pronounced further north or south of the Equator (at the equator, seasons aren’t a real factor); temperatures vary little from month to month.

Toward the poles the length of daylight varies more and more (to 0-24 hours); and the angle of insolation varies more than at the equator, too.

At the poles, during summer, the sun never sets so no nighttime cooling occurs.

In contrast, during winter no insolation is received at all so no warming occurs during daytime (heat is only received by air and water moving up from the south).

In a city like Edmonton (54°N), for example,

  • In winter the days are quite short ( 7 hours, 27 minutes) and the sun never gets above about 15° above the horizon.  The amount of insolation, therefore, is relatively low.
  • As spring approaches, days lengthen and the sun rises higher; more insolation and more heat.  On June 21, insolation reaches its maximum (daylight for 17 hours, 2 minutes – 9.5 hours more than in December!).
  • Temperature reaches it s maximum 3-6 weeks after (in July-August), delayed  as the earth absorbs energy, reradiates it and heats the atmosphere.  As winter comes on, days shorten, the sun’s angle drops, and radiation decreases; so do temperatures.
  • Again there is a lag (about 4 weeks) between maximum insolation (June) and maximum temperatures (July), and between minimum insolation (December) and minimum temperatures (January).

Further south the daylength differences are less extreme.  The delay between the longest day of the year and the hottest time is still consistent at southern latitudes.  As is the lag between the shortest day and coldest weather.

Find all the details for sunrise/sunset and daylength on any day of any year in your town/city at

IV. Boundary Layer Climates

We live in the lowest few metres of the atmosphere.  This is sometimes called the “Boundary Layer” (the boundary between air and earth).  Within this boundary layer, dramatic local variations in climate can exist. Often these are due to local variations in albedo (surface colour, wetness, vegetation), topography, aspect and winds.

A. Heating & Cooling

During daylight, solar radiation is absorbed by the ground, which is heated and which, in turn, heats the air above it (by conduction).  This heated air rises (by convection) and cools.  Wind (advection) speeds up this mixing process.

Consequently, air is warmest right at the ground and cools with distance from the ground.  On windy days, because of greater mixing, it will cool less quickly with height.

At night the ground cools more quickly than the air above it.  Consequently temperatures are least at the ground, and increase with height (to a point), and then decrease again.  Again winds will moderate this effect.

B. Surface Type & Local (Micro-) Climates
1. Bare Surfaces

The boundary layer effects of bare surfaces (soil, sand, asphalt, concrete, etc) are influenced by:

a. Colour – darker surfaces (lower albedo), result in more absorption and heating

  • Soil absorbs more insolation (more heating) than sand,
  • Asphalt absorbs more insolation (more heating) than concrete,
  • dark soil absorbs more insolation (more heating) than light soil.

b. Wetness – wet surfaces are often darker, causing more heating; BUT water heats more slowly and cools more slowly than dry surfaces.  Water also transfers heat deep into the soil.

The net effect is that water tends to result in less dramatic heating during day; but also less dramatic cooling at night because the water cools more slowly and heats the surface to deeper depths.

c. Texture – loose soils and sand only heat at the surface but not at depth (air does not transfer heat well).  Thus loose materials tend to be very hot at surface, but are cooler just below the surface.

e.g. at the beach on a hot day, the top of the sand is very hot, but the sand just below the surface.  (if the bottom of your feet are burning, bury them!)

d. Snow is a bit different.  Snow has a high albedo, reflecting much insolation.  But it also does transfer some heat downward.  Snow also acts as an insulator, preventing the ground from cooling too quickly so that frost is limited to the uppermost layers.

2. Vegetated Surfaces

Vegetation provides a variety of surfaces for solar energy absorption — leaves, stems and soil.  The amount of leaves and vegetation height affect the amount of local climate changes.

The main effects of vegetation on local climates include:

a. Vegetation blocks insolation, reduces wind speed, traps long-wave radiation and maintains high humidity.  Thus vegetated surfaces heat slower, but also cool slower.

b. Much insolation is used to evaporate moisture from soil and leaves (transpiration); reducing energy for heating.

c. The size and density of the vegetation determine the amount of modification  (more vegetation, less energy available for heating)

C. Urban Micro-Climates

Buildings modify local climates, too – as the density of buildings increases (i.e. in cities), so does climatic modification.

  • Study   Table 4.1 “Urban Heat Islands: Driving Factors and Climatic Response” (4CE, p. 110).    Table 4.1, “Urban Physical Characteristics and Conditions” (3CE, p. 104).   Figure 4.22, “The urban environment” (3CE, p. 105).
  • Read Table 4.2, “Average Differences in Climatic Elements …” 3CE, p. 105 (Combined into 4.1, in 4CE).
  1. Urban Heat Island
  • See  Figure 4.17, “Typical urban heat island profile” 4CE, p. 110 (4.23; 3CE, p. 106).

During the day cities may actually receive a little less insolation than rural areas because of pollution.  However the physical structure of cities absorb more insolation than is absorbed in rural areas.  Thus most cities are, on average, warmer than surrounding rural areas.

Trees and crops in rural areas tend to result in cooler climates.  In cities, asphalt, concrete, brick, stone, etc. all absorb and store heat well. Cities provide many surfaces that absorb insolation well, and these surfaces are often vertical, getting more direct insolation.

In cities water is frequently removed rapidly by sewers and drains, so little insolation is used for evaporation; most is used for heating.  In rural areas, moisture is left on leaves and soil – much energy is used to evaporate moisture and is not available for heating.

Also, almost every human activity generates heat (cars, industry, heating buildings, etc.).  In Manhattan, in January, this human heat energy production alone amounts to 2.5 times the amount of energy received from the sun.

At night buildings radiate long-wave radiation; emit heat from furnaces (in winter) and air conditioners (in summer), reducing the rate of cooling in cities.

In winter this is great!  In summer it means cities are often hotter and more humid than rural areas!

For a great resource on urban heat islands, their effects, and engineering options, check out

  1. Urban Pollution

When winds are light, pollutants tend to form a “pollution dome” over a city.  If a temperature inversion develops (warmer air aloft), this can become trapped over the city for extended periods of time. “Smog” (smoke + fog) is a real problem in some coastal areas where inversions are common (e.g. Los Angeles, Vancouver).

Stronger winds produce a “pollution plume” carrying materials downwind, effecting the air quality of large regions beyond the city.

  1. Urban Winds

When winds are light, the urban heat island tends to create highest temperatures downtown (highest building density), and decreasing temperatures through the suburbs.

Winds do tend to reduce this effect by mixing the warmer and cooler air.  However, cities also tend to reduce wind strength!  In general, buildings reduce wind speeds; downtown winds tend to be less than in rural areas because increased friction.

Urban structures (especially high-rises) do create local wind gusts and eddies, however, which can be very strong; this has been a big challenge for modern architects!

  1. Clouds and precipitation

Cities appear to create clouds and precipitation by causing heating (rising air), pollution (condensation nuclei), producing moisture (combustion); cities may have 5-10% more precipitation than surrounding areas.

V. Human Use of Solar Energy

Solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP). Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. Photovoltaics convert light into an electric current using the photovoltaic effect.

By 2050, solar photovoltaics and concentrated solar power together are anticipated to contribute close to 30% of the worldwide electricity consumption.  Most solar installations would be in China and India.

Photovoltaics were initially solely used as a source of electricity for small and medium-sized applications, from the calculators powered by a single solar cell to remote homes powered by an off-grid rooftop PV system.  As the cost of solar electricity has fallen, the number of grid-connected solar PV systems has grown and utility-scale solar power stations with hundreds of megawatts are being built. Solar PV is rapidly becoming an inexpensive, low-carbon technology to harness renewable energy from the Sun.

Commercial concentrated solar power plants were first developed in the 1980s. The 392 MW Ivanpah installation is the largest concentrating solar power plant in the world, located in the Mojave Desert, California.

Canada has been relatively slow at expanding solar power projects.  Photovoltaic (PV) cells are often used as standalone units, mostly as off-grid distributed electricity generation to power remote homes, telecommunications equipment, oil and pipeline monitoring stations and navigational devices.

Ontario has been the most proactive province.  Since  2009, the Feed-In-Tariff (FIT) program has invested $2.3 billion in solar technology.  For a brief time in 2010, Ontario was actually home to the largest solar farm in the world,  the 97 megawatt Sarnia Photovoltaic Power Plant (which can power more than 12,000 homes), until larger farms came online in China and India.

Worth reflecting on …
  • John Houghton, former member of the IPCC and the UK Government Panal on Sustainable Development – and a Christian – talks about the many different ways in which we can generate power without the use of fossil fuels.  Sustainable Living – YouTube
  • “We live in exciting times. All over the world the church is recovering the truth that the gospel is concerned with restoring three areas of relationship. Principally it brings peace between people and their God, through Jesus Christ the Redeemer. Secondly it can restore relationships between people, through Christ who breaks down all the walls of hostility. Finally it makes possible a renewed relationship between people and God’s Creation itself.

“We begin (discussing this third relationship) by taking seriously the perspective of scripture that makes it plain that we live in the creation, not the environment. That is to say that when we have anything to do with the material world, it is with God’s handiwork that we are involved, and thus all we do in our lives, all our actions, have to do with what the Creator has made, and so of course with the Creator himself. If we think of the world merely as “the environment”, or “nature”, or even more selfishly as “natural resources” we tend to think of it as what is around us, and there for us. We take our place, idolatrously, in the centre of things. But biblically the world and all we see, at least that which we have not ourselves transformed, is both from God, and for him. That brings much of our daily living into the conscious context of our personal and communal relationship with God; just as every Christian is inevitably a witness, every Christian and Christian community must inevitably care for Creation if it means anything at all to say that we believe in a Creator God. So whatever our work, our relationship to creation is part of it, and reflects our relationship to God. If we wish to care for Creation better, we need to worship God more fully! If we wish to worship as the Bible intends, we must recognise that we join the worship which creation itself offers to God. And if we wish to worship in offering our souls and bodies as God encourages us to do, we must make our care for his creation part of that.

“We go on to discover that the creation is everywhere, and not merely in the countryside. We often think that environmental issues are for the foresters, or the biologists, or maybe the agriculturalists and rural development workers. But we all turn on taps or fetch water, we all switch on or off lights or light fires, we all breathe and eat. All of these things involve us in choices, and responses to God’s good gift of creation. They may well also involve us in the suffering that has been brought into the world through the Fall. The water may be polluted, the light comes at a high cost to creation, the very fabric of our food is increasingly an artefact of profit-driven choices that have major implications for many other species of plants and animals that share the planet with us. So whether we live in a mega-city, or a tiny hut in the rainforest, we live in Creation, and its care is our concern.

“Most of all, we express our relationship to God, and therefore witness to it and make it evident, by all that we are, and not merely by all that we say. So an abusive or indifferent relationship to the Creation sends confused signals if we wish to proclaim Christ the Creator. Often it is our concern for the church that encourages us to cross cultures and undertake the challenges of work in all kinds of places, regardless of the difficulties that we may encounter. We know that Christ died for his church, and so our concern is not a trivial one. But what is the church for which we are concerned, and what is the gospel that we wish to see spread through all the earth? The church is the community of people redeemed from the three broken areas of relationship that we have talked about above. But if we ourselves continue to live that broken-ness, what do we bring with us to the different societies that we enter, and the churches elsewhere that we go to serve?”

Adapted from “Down-to-earth Christianity” edited by R W Dayton and P E Pretiz, 2000. ISBN 0-9678717-0-0 (

Do you agree with Dayton and Pretiz?  Why?  Why not?  Feel free to discuss this quote on the course discussion site …

To review …

Check out the resources at

This page is the intellectual property of the author, Bruce Martin, and is copyrighted © 2020 by Bruce Martin.  This page may be copied or printed only for educational purposes by students registered in courses taught by Dr. Bruce Martin.  Any other use constitutes a criminal offence.

Scripture quotations marked (NLT) are taken from the Holy Bible, New Living Translation, copyright © 1996. Used by permission of Tyndale House Publishers, Inc., Wheaton, Illinois 60189. All rights reserved