NATS 101-06 Lecture 6 Seasons and Temperature Variations

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NATS 101-06Lecture 6Seasonsand Temperature
Variations

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Supplemental References for Todays Lecture on
Seasons

• Aguado, E. and J. E. Burt, 2001 Understanding
  Weather Climate, 2nd Ed. 505 pp. Prentice Hall.
  (ISBN 0-13-027394-5)
• Danielson, E. W., J. Levin and E. Abrams, 1998
  Meteorology. 462 pp. McGraw-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,
  8th 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)

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Reasons for Seasons

• Tilt of Earths Axis - Obliquity
• Angle between the Equatorial Plane and
  the Orbital Plane
• Eccentricity of Earths Orbit
• Elongation of Orbital Axis

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Eccentricity of Orbit
Perihelion
Aphelion
Ahrens (2nd 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?
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147 million km
152 million km
Ahrens, Fig. 2.17
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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

Long Path
Large Area
Equal Energy
23.5o
Small Area
Short Path
Ahrens, Fig. 2.19
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Beam Spreading

• Low Zenith - Large Area, Much Spreading
• High Zenith - Small Area, Little Spreading

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Beam Spreading
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Atmospheric Path Length
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Length of Day
Lutgens Tarbuck, p33
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Day Hours at Solstices - US Sites

• Summer-Winter
• Tucson (32o 13 N) 1415 - 1003
• Seattle (47o 38 N) 1600 - 825
• Anchorage (61o 13 N) 1922 - 528
• Fairbanks (64o 49 N) 2147 - 342
• Hilo (19o 43 N) 1319 - 1046
  

Arctic Circle
Gedzelman, p67
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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
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Solar Declination
Solstice
Equinox
Solstice
Aguado Burt, p46
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Noon Zenith Angle at Solstices

• Summer-Winter
• Tucson AZ (32o 13 N) 08o 43 - 55o 43
• Seattle WA (47o 38 N) 24o 08 - 71o 08
• Anchorage AK (61o 13 N) 37o 43 - 84o 43
• Fairbanks AK (64o 49 N) 41o 19 - 88o 19
• Hilo HI (19o 43 N) 3o 47 (north) - 43o
  13

Aguado Burt, p46
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Incoming Solar Radiation (Insolation) at the Top
of the Atmosphere
W
C
C
W
Wallace and Hobbs, p346
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Is Longest Day the Hottest Day?
Consider Average Daily Temperature for Chicago IL
USA Today WWW Site
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Radiation Budget

• Summer hemisphere shows a surplus, warms
• Winter hemisphere shows a deficit, cools
• Equator/S. Pole always shows a
  surplus/deficit
• Why doesnt the equator warm and S. Pole cool?

NH
SH
NH
SH
Lutgens Tarbuck, p51
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Annual Energy Balance
Radiative Warming
Radiative Cooling
Radiative Cooling
NH
SH
Ahrens, Fig. 2.21

• Heat transfer done by winds and ocean currents
• Differential heating drives winds and currents
• We will examine later in course

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Summary

• Tilt (23.5o) is primary reason for seasons
• Tilt changes two important factors
• Angle at which solar rays strike the earth
• 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

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NATS 101-06Now on to Temperature Variations

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Supplemental Reference for Todays Lecture on
Temperature Variations

• Wallace, J. M. and P. V. Hobbs, 1977 Atmospheric
  Science, An Introductory Survey. 467 pp. Academic
  Press. (ISBN 0-12-732950-1)

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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)

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MAX Temperature near Surface
Ahrens, Fig 3.1
Solar SW
Convection
Conduction
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MIN Temperature near Surface
Outgoing Infrared
Ahrens, Fig 3.3
Absorbed Re-emitted Infrared
Conduction
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Daily Range of Temperatures
MAX-MIN difference decreases with height above
ground level
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12 and 00 UTC TUS Sounding

• MAX-MIN Range
• 12oC at 925 mb 6oC at 910 mb 2oC at 800 mb
  0oC by 700 mb
• Range decreases with height

isobars
isotherms
Diurnal Range
Inversion
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Growth and Decay of Inversion Evening
Morning
Height
Height
t3
t3
t2
t2
t1
t1
t0
t0
Temperature
Temperature
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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

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Review Is Longest Day the
Hottest Day?
Average Daily Temperature for Chicago IL
USA Today WWW Site
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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
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Winter-Summer Temperature Variations at Sea Level
DJF
Ahrens, Figs. 3.8, 3.9
100oF
10oF

• Continents undergo larger changes than oceans
• High latitudes undergo larger changes than low
  latitudes

JJA
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Controls of Temperature

• Latitude
• Average temperatures in middle latitudes
  decrease by 5-10oC every 10o latitude
• Elevation
• Lapse rate in troposphere is 6.5oC/km
• Tucson (2,500 ft) July Max - 100oF
• Mt. Lemmon (8,500 ft) July Max - 76oF

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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

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Specific Heat Capacity

• Heat required to raise temperature of 1 gm of
  mass 1oC.
• Rock has lower heat capacity than water

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Water-Soil Heating Depth
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Soil Temperature
Wallace and Hobbs, p347
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Ocean Temperature
Wallace and Hobbs, p348
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Ice Formation
Wallace and Hobbs, p348
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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

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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

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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