Understanding Our Atmosphere: Composition, Circulation, and Impact on Climate

2. What have we learned? • What is an atmosphere? • A layer of gas that surrounds a world • How do you obtain an atmosphere? • comet impacts. • outgassing by differentiation, volcanoes, • Why do atmospheric properties vary with altitude? • They depend on how atmospheric gases interact with sunlight at different altitudes.

3. The Atmosphere • The atmosphere is the envelope of gases that surround our planet • It is a very thin layer compared to the radius of the Earth • We refer to this layer informally as “air” • The majority of the energy that drives the atmosphere is from the Sun. • The atmosphere receives most of this energy from the surface. • Imbalances of energy drive the winds. Eventually, all energy degrades down to friction.

4. State of the Atmosphere • We commonly express the state of the atmosphere by measuring it using the following variables: • Pressure • Temperature • Wind • Humidity • Cloud cover • Precipitation type and amount • Visibility (distance one can see horizontally)

5. Atmospheric Composition Dry air (neglecting water vapor) is composed of the following gases: Nitrogen (N2) = 78% Oxygen (O2) = 21% Argon (Ar) = 1% Trace Gases Dust, bacteria, and other particulates

6. Atmospheric Composition • Trace Gases • Gases that are only found by examining a million or billion air molecules • Examples include: • Carbon Dioxide (CO2) = 340 ppmv • Neon (Ne) = 18 ppmv • Helium (He) = 5 ppmv • Methane (CH4) = 2 ppmv • Hydrogen (H2) = 0.5 ppmv

7. troposphere

8. stratosphere

9. Mesoshpere/Thermosphere

10. Atmospheric Circulation (convection) • Heated air rises at equator • Cooler air descends at poles Maximum Sun warming

11. Coriolis Effect

12. Coriolis Effect Coriolis effect deflects north-south winds into east-west winds

13. Coriolis Effect breaks upGlobal Circulation • On Earth the large circulation cell breaks up into 3 smaller ones, moving diagonally • Other worlds have more or fewer circulation cells depending on their rotation rate

14. Coriolis Effect Winds blow N or S Winds are diagonal Winds blow W or E Venus Earth Mars Jupiter, Saturn Neptune, Uranus(?)

15. Total Atmosphere Circulation

16. Hadley Cell • Air rises at the Equator due to high insolation at the surface (convection) • The belt of rising air and clouds is called the Intertropical Convergence Zone (ITCZ) • Large amounts of latent heat are released in the ITCZ • Air blows in at the surface to replace the rising air; these winds are the trade winds • Wind speeds beneath the ITCZ are low, and this zone is also called the doldrums • Air sinks in the subtropics as part of the Hadley Cell and forms the subtropical highs • The winds in the subtropical highs are also very slow, and this zone is called the “horse latitudes”

17. Polar Cell • Thermally-direct cell at high latitudes • Rising air occurs at a boundary between cold, polar air and warmer air in the mid-latitudes, called the polar front • Extremely cold air at the poles lead to surface high pressure • Winds blowing from the north to the south turn to blowing from the east due to the Coriolis force, so the surface winds are called the polar easterlies

18. Ferrel Cell • This cell is thermally-indirect (hot air sinks and cold air rises) • The upper branch of the Ferrel Cell predicts easterly winds aloft, but they are observed to westerly • Even though the complete cell doesn’t exist, the rising air at the polar front, the sinking air at the subtropical high pressures, and the correct surface winds exist • Surface flow from the south turns to the blowing from the west, resulting in our wind zone of the “prevailing westerlies” • The mid-latitudes is a very complex region, with many secondary circulation features (storms) present