Transcript PowerPoint Presentation - Length of Day at Solstice

NATS 101
Intro to Weather and Climate
Lecture 7
Seasonality
Supplemental References for
Today’s Lecture
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 Introduction 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)
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 (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?
147 million km
Ahrens, Fig. 2.17
152 million km
Solar Zenith Angle
Long
Path
Equal
Energy
Short
Path
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
Zenith Angle
Equivalent Area
0o
10o
30o
50o
70o
80o
Horizon
1.00
1.02
1.15
1.56
2.92
5.76
Infinite
Atmospheric Path Length
Schematic Ignores
Earth’s Curvature
Cloud
Zenith Angle
Equivalent Atmospheres
0o
10o
30o
50o
70o
80o
Horizon
1.00
1.02
1.15
1.56
2.92
5.70
45.0
Length of Day
Lutgens & Tarbuck, p33
Day Hours at Solstices - US Sites
Summer-Winter
Arctic Circle
Gedzelman, p67
Tucson (32o 13’ N)
14:15 - 10:03
Seattle (47o 38’ N)
16:00 - 8:25
Anchorage (61o 13’ N)
19:22 - 5:28
Fairbanks (64o 49’ N)
21:47 - 3:42
Hilo (19o 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., p75
Noon Zenith at Solstices
Summer-Winter
Aguado & Burt, p46
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’
C
W
W
C
Wallace and Hobbs, p346
Incoming
Solar
Radiation
(Insolation)
at the Top
of the
Atmosphere
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 (23.5o) 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 Heat Transport
Done by Atmosphere-Ocean System
Assignment for Lecture
• Ahrens
Pages 55-64
Problems 3.1, 3.2, 3.5, 3.6, 3.14