NATS 101 05 Lecture 6 Seasons and Temperature

NATS 101 -05 Lecture 6 Seasons and Temperature Variations

Supplemental References for Today’s Lecture on Seasons Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2 nd Ed. 505 pp. Prentice Hall. (ISBN 0 -13 -027394 -5) Danielson, E. W. , J. Levin and E. Abrams, 1998: Meteorology. 462 pp. Mc. Graw -Hill. (ISBN 0 -697 -21711 -6) Gedzelman, S. D. , 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0 -471 -02972 -6) Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Intro-duction to the Atmosphere, 8 th Ed. 484 pp. Prentice Hall. (ISBN 0 -13 -087957 -6) Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science, An Introductory Survey. 467 pp. Academic Press. (ISBN 0 -12 -732950 -1)

Reasons for Seasons • Tilt of Earth’s Axis - Obliquity Angle between the Equatorial Plane and the Orbital Plane • Eccentricity of Earth’s Orbit Elongation of Orbital Axis

Eccentricity of Orbit Perihelion Aphelion Ahrens (2 nd Ed. ), akin to Fig. 2. 15 Earth is 5 million km closer to sun in January than in July. Solar radiation is 7% more intense in January than in July. Why is July warmer than January in Northern Hemisphere?

147 million km Ahrens, Fig. 2. 17 152 million km

Solar Zenith Angle Equal Energy e rg La ea Ar Long Path o 23. 5 Short Path Small Area Ahrens, Fig. 2. 19 Depends on latitude, time of day & season Has two effects on an incoming solar beam Surface area covered or Spreading of beam Path length through atmosphere or Attenuation of beam

Beam Spreading Ahrens, Fig. 2. 16 Large Zenith Angle Zero Zenith Angle Small Zenith Angle Large Zenith Angle Low Zenith - Large Area, Much Spreading High Zenith - Small Area, Little Spreading

Beam Spreading Schematic Ignores Earth’s Curvature

Atmospheric Path Length Schematic Ignores Earth’s Curvature Cloud

Length of Day Lutgens & Tarbuck, p 33

Day Hours at Solstices - US Sites Summer-Winter Arctic Circle Gedzelman, p 67 Tucson (32 o 13’ N) 14: 15 - 10: 03 Seattle (47 o 38’ N) 16: 00 - 8: 25 Anchorage (61 o 13’ N) 19: 22 - 5: 28 Fairbanks (64 o 49’ N) 21: 47 - 3: 42 Hilo (19 o 43’ N) 13: 19 - 10: 46

Path of Sun Hours of daylight increase from winter to summer pole Equator always has 12 hours of daylight Summer pole has 24 hours of daylight Winter pole has 24 hours of darkness Note different Zeniths Danielson et al. , p 75

Noon Zenith Angle at Solstices Summer-Winter Aguado & Burt, p 46 Tucson AZ (32 o 13’ N) 08 o 43’ - 55 o 43’ Seattle WA (47 o 38’ N) 24 o 08’ - 71 o 08’ Anchorage AK (61 o 13’ N) 37 o 43’ - 84 o 43’ Fairbanks AK (64 o 49’ N) 41 o 19’ - 88 o 19’ Hilo HI (19 o 43’ N) 3 o 47’ (north) - 43 o 13’

Is Longest Day the Hottest Day? Consider Average Daily Temperature for Chicago IL: USA Today WWW Site

Annual Energy Balance Radiative Cooling NH Radiative Warming Ahrens, Fig. 2. 21 Radiative Cooling SH Heat transfer done by winds and ocean currents Differential heating drives winds and currents We will examine later in course

Summary • Tilt (23. 5 o) is primary reason for seasons Tilt changes two important factors 1. Angle at which solar rays strike the earth 2. Number of hours of daylight each day • Warmest and Coldest Days of Year Occur after solstices, typically around a month • Requirement for equator to pole Heat Transport Done by Atmosphere-Ocean System

NATS 101 -05 Now on to Temperature Variations

Supplemental Reference for Today’s Lecture on Temperature Variations Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science, An Introductory Survey. 467 pp. Academic Press. (ISBN 0 -12732950 -1)

Temperature Questions • What causes diurnal temperature variations? • What physical processes can influence daily temperature variations? • Why is MAX temperature after solar noon? • Why is MIN temperature just after sunrise? • What is Wind Chill Factor? (if time allows)

MAX Temperature near Surface Ahrens, Fig 3. 1 Solar SW Convection Conduction

MIN Temperature near Surface Outgoing Infrared Absorbed & Re-emitted Infrared Conduction Ahrens, Fig 3. 3

12 and 00 UTC TUS Sounding iso th er m s isobars Inversion Diurnal Range MAX-MIN Range o 12 C at 925 mb 6 o. C at 910 mb 2 o. C at 800 mb 0 o. C by 700 mb Range decreases with height

Growth and Decay of Inversion Height Morning Height Evening t 3 Temperature t 2 t 1 t 0 t 2 t 3 t 0 Temperature

What Affects Inversion Strength? Cloud Cover Clear skies-strong inversion Cloudy skies-weak inversion Land Characteristics Snow cover-strong inversion Bare ground-weaker inversion Wind Speed Calm winds-strong inversion Strong winds-weak inversion Absorption Re-Emission Weak IR Strong IR Warm Cold Mixing with Fast Winds

When Does MAX-MIN Occur? Ahrens, Fig 3. 2 When incoming SW exceeds outgoing IR Temperature rises When outgoing IR exceeds incoming SW Temperature falls MAX occurs Late afternoon MIN occurs Just after sunrise

Winter-Summer Temperature Variations at Sea Level DJF 100 o. F 10 o. F Ahrens, Figs. 3. 8, 3. 9 JJA Continents undergo larger changes than oceans High latitudes undergo larger changes than low latitudes

Controls of Temperature • Latitude Average temperatures in middle latitudes decrease by 5 -10 o. C every 10 o latitude • Elevation Lapse rate in troposphere is 6. 5 o. C/km Tucson (2, 500 ft) July Max - 100 o. F Mt. Lemmon (8, 500 ft) July Max - 76 o. F

Controls of Temperature • Ocean Currents and Prevailing Winds Warm-Gulf Stream Cold-California Current • Land versus Water Heat capacity of water is 5 X that of land Absorbed solar energy is distributed a greater depth in water than in land

Specific Heat Capacity Heat required to raise temperature of 1 gm of mass 1 o. C. Rock has lower heat capacity than water

Water-Soil Heating Depth Incoming Solar Energy Deep Penetration Convective Conduction Mixing No mixing Large Heat Capacity Small warming over great depth Shallow Penetration Small Heat Capacity Large warming in shallow layer

Soil Temperature Wallace and Hobbs, p 347

Ocean Temperature Wallace and Hobbs, p 348

Wind Chill Still air is poor conductor; lack of wind allows insulating layer of still air to form near skin Wind blows insulating layer of air from skin Forced convection or heat transport by advection

Summary • Balance between incoming and outgoing energy controls temperature rises and falls MAX late afternoon, MIN just after sunrise • Diurnal temp. changes are largest at ground Affected by wind, cloud cover, land type • Winter-Summer changes Largest over land, high latitudes • Temperature Controls Latitude, Altitude, Land-Sea, Ocean Currents

Assignment • Ahrens Atmospheric Moisture Pages 77 -89, B: 430, D: 433 -436 Problems 4. 1, 4. 2, 4. 5, 4. 6, 4. 9, 4. 10