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Earth’s Climate System (part 2)
• revisiting the radiation budget
• heat capacity
• heat transfer
• circulation of atmosphere (winds)
• Coriolis Effect
• circulation of oceans (currents)
From last time:
Earth’s climate system
• climate driven by “solar energy”
• climate operates to distribute solar energy
across surface
Revisiting the radiation budget
energy in
=
energy used for warming
+ energy radiated back to space
Unequal distribution across Earth
Energy input & output averaged over year
Earth’s spin axis is inclined, so we get seasons
23.5o
Energy input by latitude & month
Radiation budget
energy in
=
energy used for warming
+ energy radiated back to space
Energy transferred to Earth:
Raises temperature, drives winds, ocean currents
Energy input & output averaged over year:
Excess heat in equatorial areas, heat deficit in polar areas
Average surface temperatures:
Higher in equatorial than polar areas
Response to seasonal forcing: temperature changes
Northern
hemisphere
Response to seasonal forcing:
average surface temperature changes over year
Response to seasonal forcing: albedo changes
(temperature-albedo feedback)
Ocean
Land
Northern hemisphere
Why does land temperature undergo bigger temperature
changes, and change more rapidly, than ocean
temperature?
Because of differences in “heat capacity”.
Heat capacity
-- quantity that measures the ability of a
substance to absorb heat
heat capacity = density x
cal / cm3
g / cm3
specific heat
cal / g
Heat capacity
• water has higher heat capacity than rock
• water has a greater ability to store heat
(it is a good “heat sink”)
• it takes more energy to raise temperature of water
than rock
Heat capacity
Heat capacity = Density x Specific Heat
(cal/cm3)
(g/cm3)
(cal/g)
For water:
1 g/cm3
1 cal/g
Ratios of heat capacities:
water : ice : air : land
= 60 : 5 : 2 :1
so water has a capacity to absorb heat that is 60 times
that of the land’s capacity fo absorb heat
On average, surface heats up more
at equator than at poles
• drives winds
in atmosphere
• drives ocean
currents
• strongly affects
climate (& weather)
Heat transfer
• heat flows from hot to cold
• heat transfer by various means
-- conduction
-- convection
-- radiation
• should get flow of heat from equator to poles
• heat imbalances drive winds, precipitation
patterns & ocean currents
Circulation of atmosphere (winds)
We get flow of air & heat from ground
upwards.
Why?
“Warm air rises, cold sinks”.
“Warm air rises, cold sinks”.
Because:
• most heating at
surface
• warm air has lower
pressure & density
than cold air
• lower density air
moves up, higher
density air moves
down
Wind
Uneven heating of atmosphere causes it to
move vertically & horizontally across
the ground.
Air that moves across surface is called
“wind”.
We get systematic wind patterns on planets.
Venus:
(1) rotation rate very slow (243 Earth days)
(2) get simple wind circulation pattern (northern and
southern Hadley Cells)
Hadley Cells
Earth:
(1) rotation rate fast
(2) get complex wind circulation pattern
owing to Hadley Cells + Coriolis Effect
Coriolis Effect
• apparent deflection of moving objects (e.g. air
masses, ocean currents) on planet caused
by planetary rotation
• deflection to right in northern hemisphere,
to left in southern hemisphere
In red:
apparent
path of
objects
moving
towards or
away from
equator
westerlies
easterlies
Flow of heat in atmosphere also determines
precipitation patterns.
“It rains most at the equator, and least in the
tropics (+- 30o latitude) and poles”.
Why?
“It rains most at the equator, and least in the
subtropics (+- 30o latitude) and poles”.
Because:
• Warm air can hold
more water vapor
than cold air
• When warm air rises,
it cools
• Equator has lots of
warm, wet, rising air
• Subtropics & poles
have dry, sinking air
desert belt
rain belt
desert belt
Circulation of oceans (currents)