GEO 200: Physical Geography Insolation and Temperature.

GEO 200: Physical Geography

Insolation and Temperature

Rev. 17 February 2006 Insolation and Temperature 2

Insolation and temperature

• Temperature is the result of the interaction among a number of factors – particularly those that affect the energetics of the atmosphere:– Insolation: Incoming solar radiation

– The extent of heating and cooling

– The transfer of heat from place to place


Measuring temperature, part 1

• There are a number of instruments for measuring temperature. All work on the principle that most substances expand when heated, calibrating this change in volume to measure temperature.

• There are three temperature scales used in the United States: the Fahrenheit Scale, the Celsius Scale, and the Kelvin Scale.



• Three temperature scales (continued):– The Fahrenheit scale is used by public weather reports

from the National Weather Service and the news media; few other countries than United States use it.

– The Celsius scale is used either exclusively or predominately in most countries other than United States, which uses it for scientific work. It is slowly being established to supersede the Fahrenheit scale.

– The Kelvin scale is used in scientific research, but not by climatologists and meteorologists.



• Temperature data is recorded throughout the world at thousands of locations, following specific rules for providing accurate and important raw material for weather reports and long-run climatic analyses.– Official temperatures must be taken in shade so

measure air temperature, not solar radiation.

– Official thermometers are usually mounted in an instrument shelter that shields them from sunshine and precipitation while providing air circulation.



• Recording temperature data (continued):– Thermographs are often used to continuously record

temperature.• The highest and lowest temperatures are recorded for each 24-

hour period.

– Temperature statistics:• Daily mean temperature is the average of the maximum and

minimum temperatures of a 24-hour period.

• Daily temperature range is the difference between the highest and lowest recorded values of a 24-hour period.

• Monthly mean temperature is determined by averaging the daily means for each day in the calendar month.



• Recording temperature data (continued):– Temperature statistics (continued):

• Annual mean temperature represents the average of the 12 monthly means.

• Annual temperature range is the difference between the means of the warmest and coldest months.


Temperature and landscape

• Long-run temperature conditions affect the organic and inorganic components of the landscape.– Animals and plants often evolve in response to hot or

cold climates.

– Soil development is affected by temperature, with repeated fluctuations in temperature being the primary cause of breakdown of exposed bedrock.

– Human-built landscape is created in response to temperature considerations.


A measure of heat, part 1

• Energy and heat are different.– Energy is the capacity to do work; It can take on

various forms.– Heat is a measure of energy; It refers to speed of

molecular vibration.

• Temperature expresses the degree of hotness or coldness of a substance.

• Sensible temperature is a concept of relative temperature that is sensed by a person’s body because of factors such as moisture or air movement.


Solar energy

• The Sun is the only important source of energy for Earth’s atmosphere.– Solar energy consists of electromagnetic waves, which

do not diminish in energy despite traveling 150 million kilometers to Earth.

– Because waves diverge from a spherical body, they do diminish in intensity with distance from the Sun.

– Energy travels at speed of light, so takes 8 minutes to reach Earth.

• Radioactive decay within the Earth and tidal energy are of minor importance.


Insolation, part 1

• Wavelength is measured as the distance from the crest of one wave to the crest of the next.

• The electromagnetic spectrum consists of waves of various lengths; Only three areas of the spectrum are important to study of physical geography:– Ultraviolet waves – 0.01 to 0.4 micrometers; too short

to be seen by human eye; could cause considerable damage to living organisms if the shortest ones reached Earth’s surface, but atmosphere filters out.


Insolation, part 2

• The electromagnetic spectrum (continued):– Visible light – 0.4 to 0.7 micrometers; makes up only 3

percent of all electromagnetic spectrum, but large portion of solar energy.

– Infrared waves – 0.7 to 1,000 micrometers; too long to be seen by human eye; emitted by hot objects and sometimes called heat rays; Earth radiation is entirely infrared (sometimes called thermal infrared), but only small fraction of solar radiation.


Insolation, part 3

• Not all insolation stays in the atmosphere; some is reflected back to space.– Solar radiation arrives primarily in short wavelengths.

– Terrestrial radiation leaves primarily via long wavelengths.


The solar constant

• The solar constant is the fairly constant amount of solar insolation received at the top of the atmosphere, slightly less than 2 langleys per minute.– A Langley is a unit of measure of radiation intensity

that is 1 calorie per square centimeter (a calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C).


Heating and cooling

• To understand how energy travels from the Sun to Earth, it’s best to examine how heat energy moves. – Heat energy moves from one place to another in three

ways:• Radiation

• Conduction

• Convection


Radiation

• Radiation is the process by which electromagnetic energy emits from an object; radiant energy flows out of all bodies, with temperature and nature of the surface of the objects playing a key role in radiation effectiveness.– Hot bodies are more potent than cool bodies (and the hotter the

object, the more intense the radiation and the shorter the wavelength).

– A blackbody is a body that emits the maximum amount of radiation possible, at every wavelength, for its temperature.


Absorption and reflection

• Absorption is the ability of an object to assimilate energy from the electromagnetic waves that strike it. – Different objects vary in their capabilities to absorb

radiant energy (and thus increase in temperature).• Color plays a key role in an object’s absorption ability; dark-

colored surfaces more efficiently absorb the visible portion of the electromagnetic spectrum than light-colored surfaces.

• Reflection is the ability of an object to repel waves without altering either the object or the waves.


Scattering

• Scattering is the process by which light waves change in direction, but not in wavelength. Occurs in the atmosphere when particulate matter and gas molecules deflect wavelength and redirect them. – Sometimes when insolation is scattered, the waves are

diverted into space; but most continue through atmosphere in altered, random directions.

– Amount of scattering depends on wavelength of wave and the size, shape, and composition of the molecule or particulate.


Why is the sky blue?

• Violets and blues in the visible part of the spectrum are shorter in wavelength than the oranges and reds. Shorter waves like violets and blues are more readily scattered by the gases in the atmosphere, so they are more likely to be redirected.

• The sun appears reddish at sunrise and sunset because the path of light through atmosphere is longer, so most of the blue light is scattered out before the light waves reach Earth’s surface.


Transmission

• Transmission is the process by which electromagnetic waves pass completely through a medium; ability of objects to transmit these waves varies greatly according to their makeup; also, transmission depends on the wavelengths themselves.– Shortwave radiation has wavelengths less than 4

micrometers; almost all solar radiation is shortwave.

– Longwave radiation has wavelengths more than 4 micrometers; all terrestrial radiation is longwave.


Greenhouse effect, part 1

• The greenhouse effect is directly related to how these different wavelengths are transmitted through objects.

• It is more appropriately called the atmospheric effect, because the warming of the atmosphere is not the same as what happens in actual greenhouses, as originally thought. Greenhouses stay warm because warm air is trapped inside and does not mix with the cooler air outside, so it does not dissipate.



• The warming up of the atmosphere is more similar to what occurs in a closed automobile parked in the sunlight. The window glass transmits shortwave radiation, which is then absorbed by the upholstery. The car emits longwave radiation, which is not readily transmitted through the glass.



• In the atmosphere, atmospheric gases, known as greenhouse gases, transmit the incoming solar shortwave radiation, which are absorbed by Earth’s surface. They do not transmit the outgoing longwave terrestrial radiation, but instead absorb it, then reradiate the terrestrial radiation back toward the surface. Heat is then trapped in the lower troposphere.



• The most important greenhouse gas is water vapor, followed closely by carbon dioxide, then to a lesser degree by methane and some kinds of clouds.


Global warming, part 1

• Air temperature increases when certain atmosphere gases (such as carbon dioxide, methane, and nitrous oxide) inhibit the escape of longwave terrestrial radiation. It is a naturally occurring process; without it, Earth would be a frozen mass. Now, however, there are strong indications that this effect has been intensified by human actions.



• According to climate data, the average global temperature has increased about 0.6 degree C during the 20th century, with the warmest records occurring since 1990s.

• Measurements of this temperature increase, both direct and proxy, have pointed toward a clear warming trend on the Earth in recent decades.



• Although the climate changes do occur naturally, the evidence is increasingly pointing to these changes being caused by anthropogenic sources.– The International Panes on Climate Change (IPCC)

released a report in 2001 discussing climatic changes on both global and local scales and the strong evidence pointing to this change being a result of human activities.



• Carbon dioxide and other “greenhouse gasses” appear to be the principal offenders.– Carbon dioxide is believed to be responsible for about

75 percent of the human-enhanced greenhouse effect.

– Since 1750 carbon dioxide levels have increased by more than 30 percent.

– This increase in carbon dioxide is attributed to the increased burning of fossil fuels in recent decades.

– The warming is further exacerbated by the burning of the tropical rainforests which serve to absorb carbon from the atmosphere.



• Principal offenders (continued):– A 50 percent decrease in fossil fuel consumption would

be required to eliminate the warming trend.

– The increased use of other gasses (methane, chloroflurocarbons, and nitrous oxides) have also contributed to the increase in global temperatures.



• The warming has not been globally uniform, but rather widespread.– Warming has been greatest over North American and

Eurasia.

– On regional scales, there is clear evidence of changes in variability or extremes, with a general trend in the reduction of diurnal temperature range over more than 40 percent of the global landmass.

– There has been an increase in the number of frost free days, and an increase in the number of days in the growing season.



• Widespread warming (continued):– Arctic sea ice thickness had decreased between 10

percent and 15 percent, and nonpolar glaciers have shown widespread retreat.

– Average sea level rise has been increasing at a rate of 1 to 2 mm per year.



• According to the IPCC Synthesis report, it is estimated that:– There will be an average global temperature increase of

1.4 to 5.8 degrees C between 1990 and 2100;

– There will be an increase in climate variability;

– There will be increasing socioeconomic costs related to weather damage.



• Computer modeling shows that if the trend continues, heat and drought would become more prevalent in much of the midlatitudes, and milder temperatures would prevail in the higher latitudes. Arid lands might receive more rainfall. Ice caps would melt and global sea levels would rise. Current living patterns would have to change over much of the world.



• The Kyoto Protocol tried to address this situation by having the top six producers cut their global warming gas emissions by an average of 5% below 1990 levels by the year 2012. Although the United States is the leading producer of carbon dioxide, it rejected this standard, however, on the basis that developing nations did not have to meet as high a benchmark.



• (The Kyoto Protocol gave developing nations some leeway to allow them to address their economic needs while they attempted to raise their industrial capabilities; developing countries argued that they did not create the problem, so shouldn’t have to pay consequences.)

• Even if the Kyoto Protocol were adopted, we would still have problems because those gases already released still persist for decades.


Global Warming


Conduction, part 1

• Conduction is the movement of energy from one molecule to another without changes in the relative positions of the molecules. It enables the transfer of heat between different parts of a stationary body, or from one object to a second object when the two are in contact.


Conduction, part 2

• Conduction does require molecular movement, however. Although the molecules do not move from their relative positions, they do become increasingly agitated as heat is added. – An agitated molecule will move and collide against a

cooler, calmer molecule, and through this collision transfer the heat energy. Thus, heat energy is passed from one place to another, without the molecules actually moving from one place to another, just vibrating back and forth from agitation. (Thus, it’s the opposite of convection.)


Conduction, part 3

• Conduction ability varies with the makeup of the objects; metals are excellent conductors in comparison to earthy materials like ceramics.


Conduction question No. 1

• Why does Earth’s land surface warm up during day?– Earth’s land surface is a good absorber, but it is not a

good conductor. Thus, although some of the warmth that the land surface absorbs is transferred deeper underground most stays on the surface and is transferred back to the atmosphere. Air, however, is a poor conductor too, so only the air layer touching the ground is heated very much (unless wind circulates the heat).


Conduction question No. 2

• Why do you stay warmer on a dry day? – Moist air is a slightly more efficient conductor than dry

air. The moist air will conduct heat away from you, while dry air will let it stay in closer contact.


Convection

• Convection is the transfer of heat by a moving substance (opposite of conduction).

• Molecules actually move from one place to another, rather than just vibrating from agitation. – The principal action in convection is vertical, though

there is some horizontal movement.

• Advection is when a convecting liquid or gas moves horizontally as opposed to vertically as in convection.


Convection question

• Why can you warm your hands by holding them above a campfire?– Heat caused the air to expand, thus become less dense,

so the warm air can rise. This creates a convective circulation pattern:

• Heated air expands and moves upward in the direction of lowest pressure. The cooler surrounding air then moves in to fill the empty space, and the air from above moves in to replace that cooler air. One ends up with an updraft of warm air (that will warm your hands), and a downdraft of cool air.

• A similar convective system occurs in each hemisphere during its summer and throughout the year in the tropics.


Adiabatic processes, part 1

• When air rises or descends, its pressure changes, which in turn changes its temperature, without needing an external source.– The temperature depends on the extent of molecular

collisions.



• Adiabatic cooling is cooling by expansion in rising air.– Rising air expands because there is less air above it, so

less pressure exerted on it.

– The molecules spread over a greater volume of space, which requires more energy.

– The molecules slow down and don’t collide as much.



• Adiabatic warming is warming by contraction in descending air.– Descending air contracts because there is more pressure

being exerted on it, thus compressing the molecules in the air and making them collide more frequently.


Latent heat, part 1

• Latent heat is the energy stored or released when a substance changes state; can result in temperature changes in atmosphere.

• Changes of state:– Evaporation is when liquid water converts to gaseous

water vapor; it is a cooling process because latent heat is stored.

– Condensation is when gaseous water vapor condenses to liquid water; it is a warming process because latent heat is released.


Latent heat, part 2

• Changes of state (continued):– Freezing is when liquid water converts to solid water

(ice); it is a warming process because latent heat is released.

– Sublimation is when ice converts to gaseous water vapor; it is a cooling process because latent heat is stored.


Atmospheric heating, part 1

• Why doesn’t Earth get progressively warmer or cooler?– Because in the long run there is an apparent balance

between the total amount of insolation received by Earth and the total amount of terrestrial radiation returned to space.

• However, a closer look shows that the atmosphere experiences a net gain of 14 units every year in terms of its annual balance, which is the result of longwave radiation being trapped in the atmosphere by greenhouse gases. Without it, Earth would not store the heat necessary for life.



• Outgoing energy from Earth also depends on transport of latent heat from process of evaporation. There is more water than land, so more than three-fourths of sunshine hits water, which evaporates moisture from bodies of water.

• Ultimately, atmospheric heating is a complicated sequence that has many ramifications:– Atmosphere is heated more from below than above;

– There is an environment of almost constant convective activity and vertical mixing.



• Albedo is ability of an object to reflect radiation; in case of Earth, it relates to the amount of solar radiation or insolation that Earth scatters, or reflects back, into space.


Atmospheric Energy Balance


Variations in heating

• Earth does not evenly distribute heat through time and space; instead, there are variations in its radiation budget that relate to latitudinal and seasonal variations in how much energy is received by Earth. – These imbalances are among the fundamental causes of

weather and climate variations, as they cause unequal heating of Earth and its atmosphere.


Latitudinal differences, part 1

• There is unequal heating of different latitudinal zones for four basic reasons, angle of incidence, day length, atmospheric obstruction, and latitudinal radiation balance:

• The angle of incidence is the angle at which rays from the Sun strike Earth’s surface; always changes because Earth is a sphere and Earth rotates on own axis and revolves around the Sun.– Angle of incidence is the primary determinant of the

intensity of solar radiation received on Earth.



• The angle of incidence (continued).– Heating is more effective the closer to 90°, because the

more perpendicular the ray, the smaller the surface area being heated by a given amount of insolation.

• Angle is 90° if Sun is directly overhead.

• Angle is less than 90° if ray is striking surface at a glance.

• Angle is 0° for a ray striking Earth at either pole.



• Day length is important because the longer the day, the more insolation can be received and the more heat can be absorbed. – Middle and high latitudes have pronounced seasonal

variations in day length, while tropical areas have little variation.



• Atmospheric obstructions – such as clouds, particulate matter, and gas molecules – absorb, reflect, or scatter insolation.– How much effect they have depends on path length, the

distance a ray must travel. • Because angle of incidence determines path length,

atmospheric obstruction reinforces the pattern established by the varying angle of incidence.

• Because they must pass through more atmosphere than high-angle rays, low-angle rays are subject to more depletion through reflection, scattering, and absorption.



• The latitudinal radiation balance occurs because the belt of maximum solar energy swings back and forth through tropics as the direct rays of sun shift northward and southward in course of a year. – Low latitudes (about between 28° N and 33° S) receive

an energy surplus, with more incoming than outgoing radiation.

– There is an energy deficit in latitudes north and south of these low latitudes.

• This simple latitudinal pattern is interrupted principally by atmospheric obstruction.


Land versus water, part 1

• Different materials react differently to solar energy, which plays a major role in how the Earth surface affects the heating of the air above it.– There are almost limitless kinds of surfaces on Earth,

both natural and human-made. Each varies in its receptivity to insolation, which in turn affects the temperature of overlying air.

• The most significant contrasts occur between land and water surfaces.



• Heating: Generally, in comparison to water, land heats and cools faster and to a greater degree.– There are four main reasons why water and land are

different:• Specific heat is the amount of energy it takes to raise the

temperature of 1 gram of a substance by 1 degree C. Water’s specific heat is about five times as great as that of land, so it takes about five more times the energy to raise its temperature.

• Transmission: Water, which is transparent, is a better transmitter of heat than land, which is opaque. Heat diffuses over a much greater volume (and deeper) in water and reaches considerably lower maximum temperatures than on land.



• Heating (continued):– Four main reasons why water and land differ

(continued):• Mobility: Water’s mobility disperses heat both broadly and

deeply; on land, heat can be dispersed only by conduction, and land is a very poor conductor.

• Moisture and evaporation: Water has more moisture, so more potential for evaporation and losing heat; cooling effect of evaporation slows down any heat buildup on water surface.



• Cooling: Water surface cools more slowly and to a higher temperature as compared to land for one main reason:– Heat in water is stored deeply and brought only slowly

to surface.• Circular pattern is created so that entire body of water must be

cooled before the surface temperature decreases significantly.



• Implications: Oceans create more moderate climates for maritime areas, so that interiors of continents hold the hottest and coldest places on Earth.– Distinction between continental and maritime climates

is the most important geographic relationship in study of atmosphere.



• Implications (continued).– Oceans provide a sort of global thermostatically

controlled heat source, moderating temperature extremes.

• Northern Hemisphere has greater extremes in average annual temperature range because it is the land hemisphere – 39 percent of its area is land surface.

• Southern Hemisphere is the water hemisphere – only 19 percent of its area is land.


Heat transfer, part 1

• The tropics would become progressively warmer (and less habitable) until the amount of heat energy absorbed equaled the amount radiated from Earth’s surface if not for two specific mechanisms moving heat poleward in both hemispheres:– Atmospheric circulation is the most important

mechanism, accomplishing 75 to 80 percent of all horizontal heat transfer.



• Heat transfer mechanisms (continued):– Oceanic circulation (ocean currents) reflect average

wind conditions over a period of several years.• Current refers to various kinds of oceanic water movements.

• The atmosphere and oceans serve as thermal engines; their currents are driven by the latitudinal imbalance of heat.

• There is a direct relationship between these two mechanisms:– Air blowing over ocean is the principal driving force of major

surface ocean currents;

– Heat energy stored by ocean affects atmospheric circulation.



• Heat transfer mechanisms (continued):– All Earth’s ocean basins are interconnected:

• North Pacific

• South Pacific

• North Atlantic

• South Atlantic

• Indian

• All the basins have a single simple pattern of surface currents: – Basically, warm tropical water flows poleward along the western

edge of each ocean basin, and cool high-latitude water flows equatorward along the eastern margin of each basin.

– This pattern is impelled by the wind and caused by the Coriolis effect, the deflective force of Earth’s rotation.



• Heat transfer mechanisms (continued):– Northern and Southern variations

• In the Northern Hemisphere, the bulk of the current flow from North Pacific and North Atlantic is prevented from entering the Arctic Ocean because continents are close together.

– Flow is more limited in the North Pacific because Asia and North America are very close together.

• In the Southern Hemisphere, distance between continents permits continuous flow around the world.

– West wind drift is a circumpolar flow in the Southern Ocean around latitude 60° S.


Ocean Circulation Patterns


Vertical patterns, part 1

• Rate at which temperature drops as altitude increases can vary according to season, time of day, amount of cloud cover, and other factors.

• The lapse rate is the normal vertical temperature gradient, with temperature dropping 6.5°C per kilometer.



• Temperature inversions are prominent exception to average lapse rate, in which temperature increases with increasing altitude. – Common but usually brief and only to a restricted

depth.

– Affect weather by cutting possibility of precipitation and creating stagnant air conditions.



• Temperature inversions (continued)– There are three kinds of surface inversions:

• Radiational inversions are surface inversion that results from rapid radiational cooling of lower air, typically on cold winter nights (and thus in high latitudes);

• Advectional inversions are surface inversion caused by a horizontal inflow of colder air into an area (as in cool maritime air blowing onto a coast); usually short-lived and shallow and can occur any time of year, but are more common in winter than in summer;

• Cold-air-drainage inversions are surface inversion caused by cooler air sliding down a slope into a valley; these are fairly common during winter in some midlatitude regions.



• Temperature inversions (continued)– Upper-Air inversions, or subsidence inversions are

temperature inversions that occur well above Earth’s surface as a result of air sinking from above.


Global patterns, part 1

• An individual map can only reveal so much.– Maps of global temperature patterns display seasonal

extremes, not annual averages.• January and July are chosen because, for most places on Earth,

they are the months with the lowest and highest temperatures.

• Temperature maps are based on monthly averages, which use daily averages (not maximum daytime heating or maximum nighttime cooling).

– Viewed correctly, they permit a broad understanding of Earth’s temperature patterns.

– Isotherms are lines joining points of equal temperature.



• Prominent controls of temperature– Four factors control gross patterns of temperature:

altitude, latitude, land-water contrasts, and ocean currents:

– Altitude: Most maps displaying world temperature patterns adjust for altitude by reducing temperature to what it would be if station giving temperature were at sea level.

• Use average lapse rate to convert to sea-level temperature.

• Must realize that while these maps are useful for showing world patterns, they do not indicate actual temperatures for locations not at sea level.



• Prominent controls of temperature (continued):– Latitude: If the Earth had a uniform surface and did not

rotate, the isotherms would probably coincide with parallels (with temperature progressively decreasing poleward from equator).

• Latitude is the primary governor of insolation received, the fundamental cause of temperature variation over world.

– Land-water contrasts: Continents have higher summer temperatures than do oceans.

• Likewise, continents have lower winter temperatures than do oceans.



• Prominent controls of temperature (continued):– Ocean currents: Because of land–water heating

contrasts, cool currents deflect isotherms equatorward, whereas warm currents deflect them poleward.

– The maps that follow show how isotherms have a general east-west trend, in conjunction with the influence of latitude, which shows that temperatures tend to correspond with latitude, with warmer temperatures toward the equator and cooler temperatures toward the poles.



• Seasonal patterns (continued)– Between summer and winter, there is a latitudinal shift

of isotherms, with them moving northward from January to July and returning southward from July to January.

• This latitudinal shift is much more pronounced in high latitudes than in low, and much more pronounced over continents than over oceans.

– The temperature gradient, or the rate of temperature with horizontal distance, is steeper in winter than in summer, and steeper over continents than over oceans.



• Seasonal patterns (continued)– Coldest places on Earth: landmasses in higher latitudes.

• In July, in Antarctica;

• In January, in subarctic portions of Siberia, Canada, and Greenland.

– Hottest places on Earth: subtropical latitudes, where clear skies do not give the protection that clouds give in the tropics.

• In July, in northern Africa and southwestern portions of Asia and North America;

• In January, subtropical parts of Australia, southern Africa, and South America.



• Seasonal patterns (continued)– Highest average annual temperatures: in equatorial

regions, because they do not have winter cooling.

• Annual temperature range– Maps showing average annual temperature range,

which is the difference between the average temperatures of the warmest and coldest months.

– Interiors of high-latitude continents and continental areas in general have much greater ranges than do equivalent oceanic latitudes.



• Annual temperature range (continued)– Tropics have only slight average temperature

fluctuations.

GEO 200: Physical Geography Insolation and Temperature.

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Transcript of GEO 200: Physical Geography Insolation and Temperature.