NATS 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 ) Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN ) Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN ) Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Intro-duction to the Atmosphere, 8 th Ed. 484 pp. Prentice Hall. (ISBN ) Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science, An Introductory Survey. 467 pp. Academic Press. (ISBN )

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

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? Eccentricity of Orbit Aphelion Perihelion Ahrens (2nd Ed.), akin to Fig. 2.15

147 million km152 million km Ahrens, Fig. 2.17

Solar Zenith Angle 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 Ahrens, Fig Large Area Small Area Short Path Long Path Equal Energy 23.5 o

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

Beam Spreading Schematic Ignores Earth’s Curvature

Atmospheric Path Length Schematic Ignores Earth’s Curvature Cloud

Length of Day Lutgens & Tarbuck, p33

Day Hours at Solstices - US Sites Summer-Winter Tucson (32 o 13’ N) 14: :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: :46 Gedzelman, p67 Arctic Circle

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., p75

Solar Declination Aguado & Burt, p46 Solstice Equinox

Noon Zenith Angle at Solstices Summer-Winter 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’ Aguado & Burt, p46

Incoming Solar Radiation (Insolation) at the Top of the Atmosphere Wallace and Hobbs, p346 C C W W

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

Radiation Budget Summer hemisphere shows a surplus, warms Winter hemisphere shows a deficit, cools Equator/S. Pole always shows a surplus/deficit Why doesn’t the equator warm and S. Pole cool? Lutgens & Tarbuck, p51 NH SH

Annual Energy Balance Heat transfer done by winds and ocean currents Differential heating drives winds and currents We will examine later in course NHSH Radiative Warming Radiative Cooling Ahrens, Fig. 2.21

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 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 )

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 Convection Conduction Solar SW Ahrens, Fig 3.1

MIN Temperature near Surface Ahrens, Fig 3.3 Outgoing Infrared Absorbed & Re-emitted Infrared Conduction

Daily Range of Temperatures Ahrens, Fig 3.10 MAX-MIN difference decreases with height above ground level

12 and 00 UTC TUS Sounding MAX-MIN Range 12 o 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 isotherms isobars Diurnal Range Inversion

Temperature Height t1t1 t2t2 t3t3 t0t0 Temperature Height t1t1 t2t2 t3t3 t0t0 Growth and Decay of Inversion Evening Morning

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 Weak IRStrong IR Absorption Re-Emission Warm Cold Mixing with Fast Winds

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

When Does MAX-MIN Occur? 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 Ahrens, Fig 3.2

Winter-Summer Temperature Variations at Sea Level Continents undergo larger changes than oceans High latitudes undergo larger changes than low latitudes Ahrens, Figs. 3.8, o F 10 o F DJF JJA

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 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 5X 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 Deep Penetration Shallow Penetration Incoming Solar Energy Convective Mixing Small warming over great depthLarge warming in shallow layer Conduction No mixing Large Heat Capacity Small Heat Capacity

Wallace and Hobbs, p347 Soil Temperature

Wallace and Hobbs, p348 Ocean Temperature

Wallace and Hobbs, p348 Ice Formation

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: Problems 4.1, 4.2, 4.5, 4.6, 4.9, 4.10