Controls of temperature

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

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

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

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%)

Selective absorbers

• Water vapor and carbon dioxide– Strong absorbers of infrared radiation, ie

longwave radition (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

Atmospheric windows

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

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

• Sum of atmospheric and surface properties that determine net radiation balance

Radiative forcing at the global scale

• Determined by:– Radiative forcing – Water availability– Sensible heat content– Latent heat transfer– Advective heat

transport

Temp controls at any single point

• Geographic factors are more contingent, variable over time and/or space

Other controls on temp

Temp controls : geographic factors

What explains the packed isotherms and rapid decrease in temperatures at this location?

Temp controls : geographic factors

Altitude

• As elevation increases, temperatures are cooler• Higher altitude: lower air pressure: fewer

molecules to absorb LW radiation (i.e. more radiative cooling

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

How would the annual temperature for Vancouver and Winnipegdiffer?

Why is it cooler in south Florida?

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)

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

Which hemisphere has the lower average annual temperature?

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

Cloud coverage

Proximity to ocean currents

Cold currents stabilize the atmosphere. Warm currents destablize the atmosphere

Trends in temperature depend upon scale

• Different trends in temperature develop at different temporal (time) scales

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.

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.