Chapter 3

Why the Earth has seasons Earth revolves in elliptical path around

sun every 365 days. Earth rotates counterclockwise or

eastward every 24 hours. Earth closest to Sun (147 million km) in

January, farthest from Sun (152 million lm) in July.

Distance not the only factor impacting seasons.

Why the Earth has seasons The amount of energy that reaches the

Earths surface is influence by the distance from the Sun, the solar angle, and the length of daylight.

When the Earth tilts toward the sun in summer, higher solar angles and longer days equate to high temperatures.

Why the Earth has seasons Seasons in the Northern Hemisphere

Summer solstice: June 21, Sun directly above Tropic of Cancer, Northern Hemisphere days greater than 12 hours

Winter solstice: December 21, Sun directly above Tropic of Capricorn, Northern Hemisphere days less than 12 hours

Autumnal and Spring Equinox: September 22, Marc 20, Sun directly above Equator, all locations have a 12 hour day

Stepped Art

Fig. 3-8, p. 63

Why the Earth has seasons Special Topic: First day of winter

December 21 is the astronomical first day of winter, sun passes over the Tropic of Capricorn; not based on temperature.

Why the Earth has seasons Seasons in the Southern Hemisphere

Opposite timing of Northern HemisphereCloser to sun in summer but not significant

difference from north due to:○ Greater amount of water absorbing heat○ Shorter season

Simplified Surface Energy Balance

NET R = + SW (insolation) – SW (reflection) + LW (infrared) – LW (infrared)

Figure 3.13

Local temperature variations Southern exposure: warmer, drier

locations facing south. Implications forVegetationVinicultureSki slopesLandscapingArchitecture

Local temperature variations Environmental Issues: Solar Heating

In order to collect enough energy from solar power to heat a house, the roof should be perpendicular to the winter sun.

For the mid-latitudes the roof slant should be 45°- 50°

Daily temperature variations Each day like a tiny season with a cycle

of heating and cooling Daytime heating

Air poor conductor so initial heating only effects air next to ground

As energy builds convection begins and heats higher portions of the atmosphere

After atmosphere heats from convection high temperature 3-5PM; lag in temperature

Daily temperature variations Properties of soil affect the rate of

conduction from Earth to atmosphere Wind mixes energy into air column and

can force convection.

Daily temperature variations Nighttime cooling

As sun lowers, the lower solar angle causes insolation to be spread across a larger area

Radiational cooling as infrared energy is emitted by the Earth’s surface

Radiation inversion: air near ground much cooler than air above

Thermal belt

Stepped Art

Fig. 3-14, p. 69

Daily temperature variations Protecting crops from cold

CoverSmudge potsFansSprinklers

The controls of temperature Latitude: solar angle and day length Land & water: specific heat Ocean currents: warm and cold currents Elevation: cooling and increase range

Land–Water Heating Differences  Evaporation

Transparency Specific heat Movement Ocean currents and sea surface

temperatures Marine vs. continental effects

Land–Water Heating Differences  

Figure 3.20

Global Temperature Ranges

Figure 3.28

Air temperature data

Daily, monthly, yearly temperatureRange: maximum minus minimumMean: average of temperature observationsMaximum: highest temperature of time

periodMinimum: lowest temperature of time period

Air temperature data

Special topic: What’s normal?Climate normal is the 30 year average for a

given temperature variable.

Air temperature data

The use of temperature dataHeating degree-day: people heat when

temperature below 65°FCooling degree-day: people cool when

temperature above 65°FGrowing degree-day: temperature above of

below base temperature for specific crop

Air temperature and human comfort Body heats through metabolism

wind-chill indexHypothermia

Body cools through emitting infrared energy and evaporation of perspiration

Wind Chill Table

Figure 1

Heat Index Table

Figure 2

Air temperature and human comfort Observation: 1000 degrees

Thin air at the top of the atmosphere does not have enough molecules to create a high temperature as measured by a thermometer.

The Urban Environment

Figure 3.29

Measuring air temperature Thermometers: liquid-in-glass,

maximum, minimum, electrical resistance, bimetallic

ASOS Thermistors Infrared sensors

Urban Heat Island

Figure 3.30

Measuring air temperature Observation: Thermometers in the

shadeRadiant energy from the Sun in direct

sunlight increases the temperature recorded by a sensor.

True air temperature measured in the shade.