NATS 101

Lecture 8Temperature Variations

Supplemental References for Today’s Lecture

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

Review

• Seasons Due to Tilt of Earth’s Axis

Affects Two Things:

Solar Zenith Angle - Length of Day

• Differential Solar Heating and IR Cooling

Cause of Atmospheric-Ocean Motions

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

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

Air at ground level is heated the most

MIN Temperature near SurfaceAhrens, Fig 3.3Outgoing Outgoing

InfraredInfrared

Absorbed & Absorbed & Re-emitted Re-emitted

InfraredInfrared

ConductionConduction

INVERSION – Temp decreases w/height

Air at ground level is cooled the most

Daily Range of TemperaturesAhrens, Fig 3.10

MAX-MIN difference decreases with height above ground

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

Temperature

Hei

ght

t2

t3

t1

t4

Growth and Decay of Inversion Evening Morning

t4

Cooled the most at the ground level Warmed the most at the ground level

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

When outgoing IR equals incoming SWTemp. is constant

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, 3.9Ahrens, Figs. 3.8, 3.9100100ooFF 1010ooFF

DJFDJF

JJAJJA

Ahrens, Figs. 3.8, 3.9Ahrens, Figs. 3.8, 3.9100100ooFF 1010ooFF

DJFDJF

JJAJJA

Winter-Summer Temperature Variations at

Sea Level

Continents undergo larger changes than oceans

High latitudes undergo larger changes than low latitudes

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

Still water warms or cools very slowly compared to air and land

Water-Soil Heating Depth

Deep Penetration

Shallow Penetration

Consider Incoming Solar Energy

Penetrating Water-Soil

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