Chapter 4 Atmosphere and Surface Energy Balances Robert W. Christopherson Charlie Thomsen © 2012 Pearson Education, Inc.

Atmosphere and Surface Energy Balances Energy pathways and principles Scattering Refraction Reflection and albedo Absorbtion Energy balance in the troposphere The greenhouse effect and atmospheric warming Clouds and the earth’s natural ‘greenhouse’ Earth-atmosphere energy balance Daily radiation patterns The urban environment

Energy Essentials The amount of insolation received at the surface and the affect of this insolation on temperature will be influenced by the atmospheric gases through which it passes, dust, clouds and the surface type eg. land or water. The energy received will effect the weather patterns, ocean currents and vegetation type. Energy patterns will differ for deserts, mountains, oceans, forests etc. Solar energy heats the earth’s atmosphere and surface unevenly and is affected by latitude and seasonal fluctuation.

Energy Pathways Figure 4.1 © 2012 Pearson Education, Inc.

Energy Pathways Insolation input All radiation received at Earth’s surface – direct and indirect (diffuse). Patterns of insolation Insolation decreases poleward from about 25˚ latitude on either side of the equator. Equatorial and tropical latitudes receive high insolation But highest insolation is received in the cloudless sub- tropical deserts due to low cloud cover. Radiation: Energy travelling though air or space

Insolation at Earth’s Surface Figure 4.2 © 2012 Pearson Education, Inc.

Energy Principles Refraction: light entering the earth’s atmosphere is refracted due to a change in the density of the medium through which it is moving. This causes the light to refract or bend to a different angle. This can have the effect of bending different wavelengths to different angles and separating the colours. It is the effect of refraction that we see when we see a rainbow, water droplets in the atmosphere refract and reflect light at precise angles to the observer and thus separate the colours.

Refraction Figure 4.4 © 2012 Pearson Education, Inc.

Energy Principles Amount of insolation reaching the surface is affected by: Scattering Reflection Absorbtion

Scattering Scattering: this process involves radiation being scattered by gases or particles in different directions, but it does not change the wavelength of the radiation. Dust, ice, pollutants, cloud droplets and water vapour can cause scattering. Much of the scattered radiation still reaches the earth’s surface and the rest exits the earth’s atmosphere back to space.

Reflection Reflection: some insolation that enters the atmosphere is reflected out again directly without altering its wavelength and without it having any effect on the temperature of the earth’s atmosphere. Factors / characteristics that increase albedo include: Cloud cover Lots of particulate matter in the atmosphere White/ light coloured surfaces such as ice or snow Water surfaces if the angle of the sun’s rays is more oblique. Low vegetation cover

Albedo Albedo is a concept used to identify precisely the amount of insolation that is being reflected by a particular surface or a molecule in the atmosphere. An albedo measurement is given as a percentage. High reflection: 80-90% albedo Low reflection: 10-15% albedo

Albedo Figure 4.5 © 2012 Pearson Education, Inc.

Clouds and Albedo Figure 4.6 © 2012 Pearson Education, Inc.

Absorption Absorption: is the opposite of reflection. This refers to the capacity of a gas, particle or surface type to absorb the radiation which it receives. Absorption is the assimilation of radiation into a surface and during this process the radiation is converted from shorter wave incoming radiation to longer wave outgoing radiation. Some gases causing absorption include: CO 2, water vapour, methane, nitrous oxide, CFCs. Absorption of the atmosphere is responsible for the earth’s energy balance and the temperature at the surface.

Absorption (contd.) Absorption causes heating. Factors / characteristics which increase absorption include: Darker surfaces Vegetated surfaces Lower cloud cover Water surfaces if the angle of the sun’s rays is direct (close to 90˚) Higher proportions of absorbing gases in the atmosphere eg. CO 2

July and January Albedos Figure 4.6

Heat Transfer Conduction Direct transfer or heat / energy. Molecule-to- molecule transfer. Convection Energy transferred by movement / circulation Can be movement in air or water bodies Advection Also energy transfer through movement but this is a term for horizontally dominant movement.

Heat Transfer Figure 4.8

The Greenhouse Effect and Atmospheric Warming Atmosphere absorbs short wave radiation from the sun and converts it to long wave radiation, in the process giving off heat. Atmospheric gases responsible for this include: CO 2, water vapour, methane, nitrous oxides and CFCs. This process is responsible for warming the earth’s atmosphere. It is roughly similar to the effect of a greenhouse on temperature and thus was given the name greenhouse effect.

The Greenhouse Effect and Atmospheric Warming The passage of long wave radiation to space is delayed by atmospheric greenhouse effect. This is the natural greenhouse effect and it is a vital effect in keeping the earth’s surface temperatures at livable levels. Current concern is over the enhancement of this process through anthropogenically induced changes in the amounts of some of the gases mentioned. Specifically industrial activity producing increases in CO 2, agricultural activity and waste increasing methane and the production of CFCs.

Clouds and Atmospheric Temperatures Clouds have differing effects on temperatures at the surface depending on percentage cloud cover, type, height and thickness. High-altitude, ice-crystal, cirrus clouds reflect insolation with albedos of about 50%, whereas thick, lower cloud cover of cumulus clouds with a higher density of water droplets can have an albedo of up to 90%.

Clouds and Forcing Figure 4.10

Figure 4.11 Shortwave and Longwave Energy © 2012 Pearson Education, Inc.

Energy Balance at Earth’s Surface If the earth and the atmosphere are looked at separately: The earth has an energy surplus (energy absorbed from the sun) The atmosphere has an energy deficit (energy lost to space) However considered together there is a balance between the surplus at the surface and the loss of energy to space. That is the incoming and outgoing radiation from the earth- atmosphere system as a whole is in perfect balance.

Energy Balance at Earth’s Surface Radiation is not even across the surface of the earth, with more being absorbed at the tropics and less at the poles and seasonal and regional differences. It is these differences in temperature which are responsible for the atmospheric circulation patterns which cause our winds and our weather. About 45% of the insolation which is received at the outermost part of the earth’s atmosphere finally reaches the surface. This is then given off by the surface as longwave radiation.

Energy Balance at Earth’s Surface Latent heat. This is temperature which is lost or gained through the changes in the state of water. This heat is taken up during evaporation, and released to the atmosphere during condensation. Sensible heat: This is the actual heat stored in an object and released through conduction or convection. Ground heat: the energy flowing into and out of the ground surface by conduction. Through the year this value is 0˚C as heat absorbed is equal to heat released.

Earth–Atmosphere Radiation Balance Figure 4.10 © 2012 Pearson Education, Inc.

Energy Budget by Latitude Figure 4.12 © 2012 Pearson Education, Inc.

Daily Radiation Curves Figure 4.13 © 2012 Pearson Education, Inc.

Global NET R Figure 4.17

Urban Environments and Temperature Urban microclimates cause an increase in surface temperatures, both maximun and minimum temperatures. Almost 50% of the world’s population live in cities. The increased temperatures in cities is termed the urban heat island effect, urban areas are found to be warmer than the rural areas which immediately surround them. These differences are caused by different temperature and moisture effects in cities.

Urban Heat Islands: Causes Pollution dust domes over cities caused by transportation, energy usage and industrial activities, which increase absorption. Urban surfaces tend to absorb more radiation than rural surfaces Wind flow is decreased by high density of building, reducing cooling by wind. Urban generated sensible heat increases winter temperatures Water gets channeled fast so the heat reduction effect caused by evaporation is reduced in cities.

The Urban Environment Figure 4.21

Urban Heat Island Figure 4.22

Robert W. Christopherson Charlie Thomsen Geosystems 8e An Introduction to Physical Geography End of Chapter 4