Seasons

1
Seasons

• Henry Robinson
• WX201

2
Older ideas

• Demeter and Persephone
• Tatiana and Maub

3
Winter, Spring, Summer, Fall

• Solstice and equinox

4
Tilt of Earth

• Earths spin axis (north/south axis) is tilted 23
  ½ degrees relative to a normal to the earths
  orbital plane around the sun.

5
Angle of Sun

• Sun is source of earths 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 suns beam
  is spread out on surface

6
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.

7
Earths Tilt

• The earths tilt causes the suns 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

8
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.

9
Length of Day

• Change in earths 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.
10
Total Solar Radiation at the Top of the
Atmosphere
11
Solar Radiation at Earths 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.
12
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.

13
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

14
Net Energy Budget Positive (reds) will warm
negative (blues) will cool
Radiation budget forces seasonal changes.
15
Annual Energy Budget

• Tropics receive surplus of energy poles receive
  a deficit.
• Heat transport from tropics to poles.

16
Heat Transport

• Ocean currents
• North/south winds
• Latent heat loss in evaporation and gain in
  precipitation.

17
Seasonal Temperature Variations
18
Control of Seasonal Temperature Variations

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

19
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.

20
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.

21
Specific Heat of Substances
22
Daily Temperature Variations

• Daytime warming
• Nocturnal Inversions

23
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.
24
Variation in Annual Temperature
San Franciscos temperature is influenced by
nearby ocean
25
Ocean Current Transport of Heat

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

26
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 65F).
27
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 65F).
28
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 earths orbit around
  the sun.
• Tilt causes seasonal variation in input solar
  energy.

29
Summary (cont)

• Tilt causes change in length of day.
• Elliptical shape of earths 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.

30
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.

31
Summary (cont3)

• Heating Degree day.
• Cooling Degree day.