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Class #2: Seasonal and daily variations in temperature

Chapter 3

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Seasonal and Daily temperatures

Chapter 3

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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.

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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.

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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

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Class #2 July 8, 2010 12Stepped Art

Fig. 3-8, p. 63

Class #2 July 8, 2010 13Fig. 3-4, p. 60

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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.

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Why the Earth has seasons

• Seasons in the Southern Hemisphere– Opposite timing of Northern Hemisphere– Closer to sun in summer but not significant

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

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Local temperature variations

• Southern exposure: warmer, drier locations facing south. Implications for– Vegetation– Viniculture– Ski slopes– Landscaping– Architecture

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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°

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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

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Class #2 July 8, 2010 23Stepped Art

Fig. 3-13, p. 67

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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.

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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

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Daily temperature variations

• Protecting crops from cold– Cover– Smudge pots– Fans– Sprinklers

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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

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Air temperature data

• Daily, monthly, yearly temperature– Range: maximum minus minimum– Mean: average of temperature observations– Maximum: highest temperature of time period– Minimum: lowest temperature of time period

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Air temperature data

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

temperature variable.

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Air temperature data

• The use of temperature data– Heating degree-day: people heat when

temperature below 65°F– Cooling degree-day: people cool when

temperature above 65°F– Growing degree-day: temperature above of below

base temperature for specific crop

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Air temperature and human comfort

• Body heats through metabolism– wind-chill index– Hypothermia

• Body cools through emitting infrared energy and evaporation of perspiration

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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.

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Measuring air temperature

• Thermometers: liquid-in-glass, maximum, minimum, electrical resistance, bimetallic

• ASOS• Thermistors• Infrared sensors

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Measuring air temperature

• Observation: Thermometers in the shade– Radiant energy from the Sun in direct sunlight

increases the temperature recorded by a sensor.– True air temperature measured in the shade.