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Seasons. Henry Robinson WX201. Older ideas. Demeter and Persephone Tatiana and Maub. Winter, Spring, Summer, Fall. Solstice and equinox. Tilt of Earth. Earth’s spin axis (north/south axis) is tilted 23 ½ degrees relative to a normal to the earth’s orbital plane around the sun.

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Seasons

Seasons

Henry Robinson

WX201


Older ideas
Older ideas

  • Demeter and Persephone

  • Tatiana and Maub


Winter spring summer fall
Winter, Spring, Summer, Fall

  • Solstice and equinox


Tilt of earth
Tilt of Earth

  • Earth’s spin axis (north/south axis) is tilted 23 ½ degrees relative to a normal to the earth’s orbital plane around the sun.


Angle of sun
Angle of Sun

  • Sun is source of earth’s energy input.

  • Earth is a sphere, so angle to sun is a function of the latitude (and time of day).

  • Angle to sun determines how much the sun’s beam is spread out on surface


Low solar angle
Low Solar Angle

  • When sun is low in sky, beam of energy is spread out over a larger area, so a unit area gets less energy than when sun is high in sky.

  • Because earth is a sphere, areas near the poles have a lower sun angle, and less energy input.

  • With less energy input, poles are colder.


Earth s tilt
Earth’s Tilt

  • The earth’s tilt causes the sun’s position to change during the year. Sun is high in sky in summer and lower in winter. Daylight length is longer in summer and shorter in winter


Northern southern hemisphere
Northern/Southern Hemisphere

  • Tilt toward sun in Northern Hemisphere is tilt away from sun in Southern Hemisphere.

  • Summer in Northern Hemisphere is winter in Southern Hemisphere.


Length of day
Length of Day

  • Change in earth’s tilt will change the length of the day with the change in season.

  • Longer day will allow for more solar input.

Summer in northern

Alaska. Time series of

photos near midnight.



Solar radiation at earth s surface
Solar Radiation at Earth’s Surface

  • Atmosphere and clouds will reflect solar radiation. Thicker atmosphere will reflect more solar radiation.

  • Effective cloud cover will increase with lower sun angle because of reflections from sides of clouds.

  • Net effect is to reduce input solar radiation in polar regions.

Solar radiation on

June 21.


Distance from sun
Distance from Sun

  • Earth has an elliptical orbit. Earth is closer to sun in January and farther out in July.

  • Earth receives slightly more energy (6%) from sun in January as compared to July.

  • Makes NH winters slightly warmer than would be otherwise.


Precession of equinox
Precession of Equinox

  • In 13,000 years the Northern Hemisphere will receive 6% less energy during winter because of precession of equinox.

  • One of the causes of ice ages


Net energy budget positive reds will warm negative blues will cool
Net Energy Budget Positive (reds) will warm; negative (blues) will cool

Radiation budget forces seasonal

changes.


Annual energy budget
Annual Energy Budget

  • Tropics receive surplus of energy; poles receive a deficit.

  • Heat transport from tropics to poles.


Heat transport
Heat Transport

  • Ocean currents

  • North/south winds

  • Latent heat loss in evaporation and gain in precipitation.


Seasonal temperature variations

Average air temperature

near sea level in January

in degrees F.

Average air temperature

near sea level in July

in degrees F.

Seasonal Temperature Variations


Control of seasonal temperature variations
Control of Seasonal Temperature Variations

  • Seasonal temperature variations controlled by net energy input/loss, heat transport, and heat capacity (heat storage).


Net energy input loss
Net energy input/loss

  • North/south variation in energy input is small in summer and large in winter.

  • North/south temperature variations less in summer than in winter.


Clouds influence energy input
Clouds Influence Energy Input

  • Clouds reflect visible light, trap infrared energy.

  • Low clouds reflect more energy than they trap.

  • High cirrus clouds trap more infrared energy than the reflected solar energy.



Daily temperature variations
Daily Temperature Variations

  • Daytime warming

  • Nocturnal Inversions


Land water seasonal variations
Land/Water Seasonal Variations

  • Water has larger heat capacity than land.

  • Water has less annual temperature variations than land for same latitude.

Average air temperature

near sea level in January

in degrees F.


Variation in annual temperature
Variation in Annual Temperature

San Francisco’s temperature

is influenced by nearby ocean


Ocean current transport of heat
Ocean Current Transport of Heat

  • Gulf Stream transports warm water north into North Atlantic and warms western Europe.


Heating degree days
Heating Degree Days

  • 65 oF.-Mean Daily Temperature

  • Used to size heating equipment

Mean annual total heating degree-days in thousands of °F,

where the number 4 on the map represents 4000 (base 65°F).


Cooling degree days
Cooling Degree Days

  • Mean daily temperature –65 oF.

  • Used to size cooling equipment

Mean annual total cooling degree-days in thousands of °F,

where the number 1 on the map represents 1000 (base 65°F).


Summary
Summary

  • Angle of sun determines energy input.

  • Spherical shape of earth causes equator to receive more solar energy than poles.

  • Earth has a 23 ½ degree tilt of its spin axis relative to the plane of the earth’s orbit around the sun.

  • Tilt causes seasonal variation in input solar energy.


Summary cont
Summary (cont)

  • Tilt causes change in length of day.

  • Elliptical shape of earth’s orbit causes earth to be slightly closer to sun in January.

  • Annual heat budget of earth has tropic having an excess of energy and the poles a deficit.

  • Heat transport by ocean currents, winds, and latent heat.


Summary cont2
Summary (cont2)

  • Seasonal temperature variations controlled by net energy input/loss, heat transport, and heat capacity (heat storage).

  • North/south input energy variation more in winter than in summer.

  • Water has larger heat capacity than land causing less annual temperature variations.

  • Ocean currents keep Europe warm during winter.


Summary cont3
Summary (cont3)

  • Heating Degree day.

  • Cooling Degree day.


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