Transcript Class7-Atmosphere

The Earth’s Atmosphere
The Atmosphere
Early Atm.
N2
CO2
H2O
H2S
HCN
…others
Present Atm.
N2 (78%)
O2 (21%)
Ar (1%)
CO2 (0.04%)
H2O (varies)
…others
Where’s the H and He?
The origin of the atmosphere
• Atmosphere formed early ( > 4 Ga)
– Formed by outgassing of the early Earth
• Volcanic gasses (H2O, CO2, SO4)
• H and He lost to space
• Around 3.8 Ga cooled enough to allow liquid
water (rain, oceans)
• CO2 dissolved in liquid water and was consumed
by early weathering
• N2 (and Ar) doesn’t react with water or rx and was
concentrated in the atmosphere
• O2 didn’t appear until ~ 3.5 Ga due to ---? Present
day O2 conc. wasn’t achieved until 400 mya.
An addendum
• Comets may have played a role
– In deliver in much of the H2O to the early earth
– Possibly in delivering organic molecules
• Including amino acids which survive impact…
• … and can even polymerize to form peptides
• This has raised the question: were the
building block for life delivered from
space?
The evolution of the atmosphere
• The increase of O in the atmosphere
allowed the formation of ozone - O3
O2 + photon -> 2 O
2O + 2O2 -> 2O3
• Ozone absorbs UV radiation and protects
the surface from UV light which destroys
cells
The Modern Atmosphere
Atmospheric Pressure
Hydrostatic Equilibrium
Figure 3.3
Profile of
atmospheric
temperature
What cools the atmosphere?
What heats the atmosphere?
Figure 3.2
Height
Temperature Inversion
Lapse Rate: 6.4°C/km
Nighttime
Temperature
Daytime
What creates the lapse rate?
Heating at the bottom:
• Increases temperature
• Decreases density
Less dense hot water rises…
• Displacing the cooler, denser
water at the top
Denser, cool water descends…
• Where it is heated
Convection in the atmosphere
Ideal gas law and Conservation of Energy
HIGH
LOW
Stable and Unstable
Atmospheric Conditions
-10°/km
0°/km
INVERSION
-6.4°/km
D Temperature/D Altitude = Run/Rise =1/slope
Figure 7.18
-6°/km
Cold Air Drainage
Temperature Inversion, Revisited
Large Scale Atmospheric
Circulation
If Earth Didn’t Spin
Large Scale Atmospheric
Circulation
Coriolis effect
Why does it rain in equatororial
rain forests?
First, we have to know something about water
• Absolute Humidity
– The amount of water in the air (g water/volume
air)
• Relative Humidity
– The amount of water in the air relative to the
total amount of water that the air can hold
– Usually expressed as a % the maximum amount
Absolute vs. Relative Humidity
Temperature Decreases
Relative Humidity Increases
Coastal Deserts
Coastal deserts
Dew Point
• When you reach saturation (RH = 100%)
because of changing the temperature (not
the absolute humidity), you’ve reached the
dew point
• Below the dewpoint, air is “super-saturated”
and the moisture will begin to condense out
Condensation in the Atmosphere
and Latent Heat
Convective Clouds
The cool dry air is no longer buoyant
It starts to subside
The air mass is now
drier and warmer
(at a specific altitude)
Energy from
the Sun
As water condenses, (latent)
heat is added back to the air
as sensible heat
Hits the dewpoint, starts to
condense on CCN
Rises, Expands, Cools
Warm, Moist Air
Heats the surface and causes evaporation (evapotranspiration)
Evaporation takes a lot of heat: Latent Heat of Vaporization (540 cal/g)
Why does it rain in equatororial
rain forests?
Why is it dry in midlatitude
deserts?