seasons: temperature and time - lakeheadu.ca · temperature of earth is determined by energy stored...
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Seasons: Temperature and Time
GEOG/ENST 2331 – Lecture 5 Ahrens: Chapters 2 and 3
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Graph from Exxon documents in 1981
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Energy Balance and Temperature Global energy budget
Incoming solar radiation must be balanced by outgoing terrestrial radiation Temperature of Earth is determined by energy stored in the system
Earth-atmosphere exchange
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Shortwave Radiation (imagine 100 total units)
Ahrens: Figure 2.15
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Earth-atmosphere energy balance
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Convection Conduction: direct heat exchange
Warm air becomes less dense
Convection: Rising air carries
heat away from the surface
Ahrens: Fig. 2.4
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Free Convection
A&B: Figure 3-12
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Forced Convection
A&B: Figure 3-13
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Latent heat Energy required to change the state of a substance Liquid to gas: heat of evaporation Solid to liquid: heat of fusion
Heat is ‘hidden’ No change in temperature
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Thermal Storage and phase change
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Latent heat Liquid to gas
Absorbs heat (at the surface)
Gas to liquid Releases heat (in the atmosphere)
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Radiative, convective and latent transfers
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Global coordinate system
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Beam spreading A beam of sunlight spread over a large area is less intense Higher latitudes receive less solar energy per unit area Also passes through more air
Ahrens: Fig. 3.7
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Axial tilt Axis of rotation is offset 23.5o from being perpendicular to the orbital plane Hemispheric orientation changes as the Earth orbits the Sun
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Ahrens: Fig. 3.5
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Solstices One hemisphere axis of rotation is pointed toward the Sun; the other is pointed away The hemisphere pointed toward the Sun receives its maximum insolation on this date Astronomically, these dates designate the first day of winter or summer
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Equinoxes March (Vernal) Equinox
On or about March 20
September (Autumnal) Equinox On or about September 22 September 23 this year
The sun is visible for 12 hours everywhere
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Intensity of Radiation Beam spreading
Incident radiation is directly proportional to solar angle Higher solar angles incorporate reduced beam spreading
Lower angles induce less intense illumination and heating per unit area
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Beam depletion Solar radiation is diminished relative to the amount of atmosphere the radiation passes through (distance through the air) Significant beam reduction occurs at low solar angles
Period of Daylight Axial tilt influences day length Days are longer in summer and shorter in winter Effect is more pronounced at high latitudes
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Ahrens: Table 3.1
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A&B: Table 2-2
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Diurnal heat budget
A&B: Figure 3-23
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Cloudy days
A&B: Figure 3-23
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Diurnal surface air temperature
Ahrens: Fig. 3.12 and 3.14
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Extemes of Temperature in Thunder Bay
Heat 40.3° C
August 7, 1983
Cold -41.1° C
January 30, 1951
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Next lecture Temperature and Geography Ahrens: Chapters 2 and 3