NATS 101Intro to Weather and Climate

Lecture 7Seasonality

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 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)

Reasons for Seasons

• Eccentricity of Earth’s Orbit

Elongation of Orbital Axis

• Tilt of Earth’s Axis - Obliquity

Angle between the Equatorial Plane and the Orbital Plane

Earth is 5 million km closer to sun in January than in July.

Eccentricity of Orbit

AphelionPerihelion

Ahrens (2nd Ed.), akin to Fig. 2.15

Solar radiation is 7% more intense in January than in July.

Why is July warmer than January in Northern Hemisphere?

147 million km 152 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 beamAhrens, Fig. 2.19

Large

Large

Area

Area

Small Small AreaArea

Short Path

Long Path

Equal Energy 23.523.5

oo

Ahrens, Fig. 2.16

Large Zenith Angle Zero

Zenith Angle Large

Zenith Angle

Small Zenith Angle

Beam Spreading

High Sun – Power Spread over Smaller Area

Low Sun – Power Spread over Larger Area

Quantifying Beam Spreading

Zenith Angle Equivalent Area 0o 1.00

10o 1.02 30o 1.15 50o 1.56 70o 2.92 80o 5.76

Horizon Infinite

Schematic Ignores Earth’s Curvature

Atmospheric Path Length

Zenith Angle Equivalent Atmospheres 0o 1.00

10o 1.02 30o 1.15 50o 1.56 70o 2.92 80o 5.70

Horizon 45.0

Schematic Ignores Earth’s Curvature

Cloud

Length of Day

Lutgens & Tarbuck, p33

Day Hours at Solstices - US Sites

Summer-WinterTucson (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

Gedzelman, p67

Arctic Circle

Path of SunHours 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 ZenithsDanielson et al., p75

Noon Zenith at Solstices

Summer-WinterTucson 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

Incoming Solar

Radiation (Insolation) at the Top

of the Atmosphere

http://web.geog.arizona.edu/~comrie/nats101/wa/wa1insol.jpg

C

C

W

W

Is Longest Day the Hottest Day?

USA Today WWW Site

Consider Average Daily Temperature for Chicago IL:

equilibruimwarmingwarming cooling

Annual Energy Balance

Heat transfer done by winds and ocean currents

NH SH

Radiative WarmingRadiative

CoolingRadiative Cooling

Ahrens, Fig. 2.21

Differential heating drives winds and currentsWe 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 a month later

• 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