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 • 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.

5. 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

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. 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

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 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.

10. Total Solar Radiation at the Top of the Atmosphere

11. 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.

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. 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

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 Francisco’s 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 65°F).

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 65°F).

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 earth’s 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 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.

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.