Controls of temperature

1. Controls of temperature

3. Theoretical constructs and models in science • Science uses idealized constructs unlikely to ever occur as a way to make comparisons and infer mechanisms • Models are similar: they are simplifications that are used to convey the essence of a concept or process

6. Blackbodies • Any object that is a perfect absorber of all radiation that strikes it and a perfect emitter of this radiation at its given temperature • No substances in nature are true blackbodies, but only approximations of them • Terrestrial surface of Earth approximates a blackbody in that it absorbs sunlight and reemits it as infrared radition

7. Blackbodies • Radiative equilibrium: when rate of absorption = rate of emission • Theoretical temperature at radiative equilibrium for Earth is 0 degrees F. • Observed temp: 59 degrees F • Reason: atmosphere is much less a blackbody than Earth’s surface. The atmosphere is not a black body. It is a selective absorber

8. Selective absorbers • Gases that selectively absorb radiation also emit radiation at that same wavelength (Kirchoff’s Law). • Natural and anthropogenic greenhouse gases • Water vapor (0-4%) • Carbon dioxide (0.04%)

9. Selective absorbers • Water vapor and carbon dioxide • Strong absorbers of infrared radiation, ielongwaveradition (LW) • Absorption of LW results in molecular motion and transference of kinetic energy to other atmospheric molecules (conduction) • Strong emitters of LW • Radiative transfer of LW to ground • Thus form a “blanket” of warming in the atmosphere

10. Atmospheric windows • Wavelengths between 8-11 micrometers are not absorbed by water vapor nor carbon dioxide

11. Atmospheric windows • However, clouds (liquid water droplets), are good absorbers in this range of wavelenths, especially low thick clouds like stratus. • Cloud bases radiate LW downward and block incoming shortwave • Temperature ranges are smaller with stratus deck • Higher nighttime temps and lower daytime temps

12. Radiative forcing at the global scale • Sum of atmospheric and surface properties that determine net radiation balance

13. Temp controls at any single point • Determined by: • Radiative forcing • Water availability • Sensible heat content • Latent heat transfer • Advective heat transport

14. Other controls on temp • Geographic factors are more contingent, variable over time and/or space

15. Temp controls : geographic factors

16. Temp controls : geographic factors What explains the packed isotherms and rapid decrease in temperatures at this location?

18. Altitude • As elevation increases, temperatures are cooler • Higher altitude: lower air pressure: fewer molecules to absorb LW radiation (i.e. more radiative cooling

19. Why are there large annual temperature ranges over interior Canada and Asia?

20. How would the annual temperature for Vancouver and Winnipeg differ?

21. Why is it cooler in south Florida?

22. Geographic location relative to sea and water • Maritime effect • Applies to locations near large bodies of water • Smaller temperature range • Continentality • Applies to locations distant from large bodies of water (i.e., landlocked) • Larger temperature range (Siberia: -70 F to 70 F annually)

23. Water buffers temperature extremes because it heats up and cools down more slowly than land.

24. Which hemisphere has the lower average annual temperature?

25. Cloud coverage • Cloudy conditions predominate: small temperature range • Clear conditions predominate: larger temperature range • Cloud type important • Stratus type clouds promote more cooling and smaller temperature range • Cirrus clouds promote warming

26. Cloud coverage

27. Proximity to ocean currents Cold currents stabilize the atmosphere. Warm currents destablize the atmosphere

28. Trends in temperature depend upon scale • Different trends in temperature develop at different temporal (time) scales

30. Fallacies of scale • Individualistic fallacy: extrapolating to the broad scale based on observations conducted at small, local scales • Ecological fallacy: making local-scale characterizations based on broad-scale observations.

31. What is the normal temperature? • 30 year average is standard • What would be a record setting high or low temperature depends upon the length of the record you have available. • Temperatures are often referenced to their departure from a climatic normal. Again, the length of the record that provided the average determines the amount of departure.