NATS 101-06

Lecture 6Seasons

and Temperature Variations

Supplemental References for Supplemental References for Today’s Lecture on SeasonsToday’s 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)

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

AphelionPerihelion

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

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 beam

Ahrens, Fig. 2.19L

arge

Large

Area

Area

Small Small AreaArea

Short Path

Long Path

Equal Energy 23.523.5

oo

Beam Spreading

Low Zenith - Large Area, Much Spreading

High Zenith - Small Area, Little Spreading

Ahrens, Fig. 2.16

Large Zenith Angle Zero

Zenith Angle Large

Zenith Angle

Small Zenith Angle

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 Zeniths

Danielson et al., p75

Noon Zenith Angle 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

Is Longest Day the Hottest Day?

USA Today WWW Site

Consider Average Daily Temperature for Chicago IL:

Annual Energy Balance

Heat transfer done by winds and ocean currentsDifferential heating drives winds and currents

We will examine later in course

NH SH

Radiative WarmingRadiative

CoolingRadiative Cooling

Ahrens, Fig. 2.21

Summary

• Tilt (23.5o) 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 101-06

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 0-12-732950-1)

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

ConvectionConvection

ConductionConduction

Solar SWSolar SW

Ahrens, Fig 3.1

MIN Temperature near SurfaceAhrens, Fig 3.3Outgoing Outgoing

InfraredInfrared

Absorbed & Absorbed & Re-emitted Re-emitted

InfraredInfrared

ConductionConduction

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 Range decreases with heightwith height

isoth

erm

sisobars

Diurnal Range

Inversion

Temperature

Hei

ght

t1

t2

t3

t0

Temperature

Hei

ght

t1

t2

t3

t0

Growth and Decay of Inversion Evening Morning

What Affects Inversion Strength?

Cloud CoverCloud CoverClear skies-strong inversionCloudy skies-weak inversionLand CharacteristicsLand CharacteristicsSnow cover-strong inversionBare ground-weaker inversionWind SpeedWind SpeedCalm winds-strong inversionStrong winds-weak inversion

Weak IR Strong IR

Absorption Re-Emission

Warm

Cold

Mixing with Fast Winds

When Does MAX-MIN Occur?

When incoming SW exceeds outgoing IRTemperature rises

When outgoing IR exceeds incoming SWTemperature falls

MAX occurs Late afternoon

MIN occurs Just after sunriseAhrens, 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, 3.9Ahrens, Figs. 3.8, 3.9100100ooFF 1010ooFF

DJFDJF

JJAJJA

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

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 1oC.

Rock has lower heat capacity than water

Material Specific Heat Capacity (Cal gm -1 oC-1)

Still Water 1.00

Dry Air 0.24

Granite (Rock) 0.19

Water-Soil Heating Depth

Deep Penetration

Shallow Penetration

Incoming Solar Energy

Convective Mixing

Small warming over great depth Large 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

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 fallsMAX late afternoon, MIN just after sunrise

• Diurnal temp. changes are largest at groundAffected by wind, cloud cover, land type

• Winter-Summer changesLargest over land, high latitudes

• Temperature ControlsLatitude, 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