The Earth’s Atmosphere,

Climate and Weather

• Of what is the Earth’s atmosphere made?

• Where did the earth’s atmosphere come from?

• Has the Earth’s atmosphere always been the same?

• Is there evidence that life forms have affected the atmosphere?

• What is the Ozone layer and what does it do?

• Can we expect the atmosphere to change in our lifetimes?

• What causes atmospheric pressure?

• How does atmospheric pressure change with altitude

• What are the different layers of the atmosphere and on what basis are they defined?

• How is heat get from one place to another?

• What is the Greenhouse Effect and what causes the seasons?

Questions

Spheres Timescales

extraterrestrial

Cosmos billions of years

terrestrial

Lithosphere millions of years

Pedosphere 100s to 1000s of years

Hydrosphere days to 1000s of years

Atmosphere minutes to 1000s of years

Biosphere seconds to 1000s of years

Noösphere seconds to 10s of years

Atmospheric Composition

Present Atmosphere

Nitrogen (N2) 78%

Oxygen (O2) 21%

Argon (Ar) 1%

Carbon Dioxide (CO2) 0.039% >> 390 ppm

Water (H2O) 0 to 4%

Present Atmosphere

The Hole in the Ozone Layer!It appears in the winter

Where did the present atmosphere come from?

Or,

Why is there air?

Formation of the Earth’s Atmosphere

First Atmosphere was like the Solar Nebula• Mostly Hydrogen and Helium• Would have been quickly lost because the Earth’s gravity is not great enough to either hold H or He at this temperature

A second atmosphere was produced by gases from volcanoes and comets from outer space. CO2 concentrations were probably very high.

Photosynthesis in particular changed this atmosphere by adding oxygen

Other changes continue today

Earth’s first atmosphere (H and He) was similar to that of Saturn’s

moon: Titan

In the early Earth, gases would emanate from volcanic

eruptions like this one on the big island of Hawaii

Secondary Atmosphere - Outgassing

Initially Composed of:• Ammonia (NH3)• Hydrogen (H2)• Carbon dioxide (CO2)• Methane (CH4)• Water vapor (H2O)• Nitrogen (N2)•But NO OXYGEN!

No Oxygen!

Evidence:• No Rust Minerals

• Conglomerates made of Pyrite and Uraninite(these would rapidly oxidize and dissolve today)

Witwatersrand conglomerate, South Africa

(Middle Archean)

photosynthesis

CO2 + H2O CH2O + O2Carbon dioxide Water Carbohydrate Oxygen

Where atmospheric Oxygen comes from

Photosynthesis and Respiration

CO2 + H2O CH2O + O2

Photosynthesis Requires energy input

aerobic Respiration Gives off energy

NASA/Goddard Space Flight Center, The SeaWiFS Project and GeoEye, Scientific Visualization Studio

SeaWiFS Biosphere: 3 years

GlobalProd_Seawifs.mpg

First a look at photosynthesis in the modern world and then back to the early Earth…..

Atmospheric EvolutionPrimary - Captured from Solar Nebula

- Mostly Hydrogen and Helium- Quickly Lost

Secondary - Formed by Outgassing by Volcanoes

Tertiary - Modified by Life (addition of Oxygen)

The Oceans rain down as the Earth cools

However, every once in a while…

Oceans were vaporized by large impacts

But eventually things settled down a few billion

years ago and we ended up with modern Oceans

IG4e_02_01

Quick radiation review

Electromagnetic Radiation

- Everything above -273˚C (absolute zero, 0 Kelvin) emits radiation

- Hotter objects emit more energy at shorter wavelengths

- Colder objects emit more energy at longer wavelengths

Figure 2.2, p. 53

Shortwave and Longwave Radiation

The hot sun radiates at shorter wavelengths that carry more energy,

Energy absorbed by the cooler earth is then re-radiated at longer wavelengths, as predicted by Wein's law.

Electromagnetic Radiation

Figure 2.2, p. 53

Sun

Earth

Two Faces of Solar Energy

Ultra violet Visible

10 meters

Lethal Energy source for life forms

Ultraviolet rays absorb ozone (O3)in the stratosphere

Ozone is good in the stratosphere but bad in the troposphere

Atmospheric Pressure• Molecules bumping into an object create a

force on that object

• Pressure is the force applied per unit area

P = Force/Area; where gravity is the force in atmospheric pressure

Which box below is exerting the greatest pressureupon the ground?

1 kg 1 kg

Pressure and Density of the Atmosphere

• Gravity holds most of the air close to the ground

• The weight of the overlying air is the pressure at any point

Vertical Structure

The world is a big place, but the atmosphere is very shallow. Consider …

• In Billings, about 12% of the mass of the atmosphere is below our feet

• At the top of Beartooth (~11000 ft), you are above ~30% of the atmosphere’s mass

• You are closer to outer space than to Bozeman!

Temperature Scales

• In the US, we use Fahrenheit

• Celsius (centigrade) is a scale based on freezing/boiling of water

• Kelvin is like centigrade, but starts at absolute zero (-275°C)

Atmospheric Soundings

Helium-filled weather balloons are released from over 1000 locations around the world every 12 hours (some places more often)

These document temperature, pressure, humidity, and winds aloft

The atmosphereis layered accordingto its temperaturestructure

In some layersthe temperatureincreases with height

In others it decreaseswith height or is constant

Why?

… “pause” is a level

… “sphere” is a layer

Vertical Thermal Structure

• Heated in thermosphere by O2

and N2 absorption

• Heated in stratosphere by ozone absorption

• Heated from below by latent and sensible heat fluxes

Lower in the Atmosphere

We live here

What layer is that ?

Transfer of Heat

• Conduction - Where molecules transfer energy by coming into contact with one another.

• Convection - Where a fluid moves from one place to another, carrying it’s heat energy with it.– In atmospheric science, convection is usually associated

with vertical movement of the fluid (air or water).

– Advection is the horizontal component of the classical meaning of convection.

• Radiation - The transfer of heat by radiation does not require contact between the bodies exchanging heat, nor does it require a fluid between them.

Conduction – Direct Heat Transfer

Conduction of heat energy occurs as warmer molecules transmit vibration, and hence heat, to adjacent cooler molecules.

Warm ground surfaces heat overlying air by conduction.

Radiation– heat transferred even in a vacuum!

Heat driven convection

1. Bottom water is warmed

2. It expands an is therefore less dense

3. It rises to the surface

4. and then spreads out to the sides

5. Cooler water at the sides sinks

Inside of stars

Inside of the Earth

In the lower troposphere during about every summer day

Convective thunderstorms caused by surface heating, rising buoyant plumes, and the release of

latent heat in clouds

A convective thunderstorm

‘convective’ tornadoes

Hurricanes

Convection in a hurricane

Mid Latitude Cyclones

Rising air in the center of a low pressure zone is the driving force in most storms

Temperature, Density, and Convection

Heating of the Earth’s surface during daytime causes the air to mix

Warmed from Below

Solar radiation passes first through the upper atmosphere, but only after absorption by earth's surface does it generate sensible heat to warm the ground and generate longwaveenergy.

This heat and energy at the surface then warms the atmosphere from below.

Molecular Structure of Water

Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.

water moleculeice

Molecular Structure of Water

Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.

ice

Energy associated with phase change

80 calories 100 calories 540 calories!

The Earth’s Radiation Balance

Incoming Energy = Outgoing Energy

(absorbed sunshine)(area) = (thermal loss)(area)

S(1-a)pr2 = s T4 (4 pr2)

Solve for TT = -18° C; (0°F)

The radiative equilibrium temperature

The equilibrium surface temperature should be -18°C (~0°F) .

But, the observed surface temperature is

+15°C (~60°F).

Why?

We have an atmosphere

Solar Radiation

30% reflected

~50% absorbed by the surface

20% absorbed by the

atmosphere

Greenhouse Effect

Heat radiated by the Earth’s surface is absorbed by gases in the atmosphere and

reradiated back to the Earth

Carbon Dioxide (CO2) ConcentrationsCombustion of fossil fuels (coal and oil) by humans has increased concentrations of some greenhouse gases

Scattering: Why is the sky blue?

• Sunlight is scattered by air molecules

• Shorter wavelengths (green, blue, violet) scattered more efficiently

• Unless we are looking directly at the sun, we are viewing light scattered by the atmosphere

• Finally, blue dominates over violet because our eyes are more sensitive to blue light

Why are Sunsets Red?• The sun appears fairly white

when it’s high in the sky

• Near the horizon, sunlight must penetrate more atmosphere and therefore, there is more scattering of blue and other shorter wavelengths

• The orange sunset occurs because longer wavelengths, such as orange and yellow, make it through

• If there are a lot of particulates, yellow will be scattered as well

Seasons & Sun's Distance

Earth is 5 million kilometers further from the sun in July than in January, indicating that seasonal warmth is controlled by more than solar proximity.

Figure 3.1

Solstice & Equinox

• Earth's tilt of 23.5° and revolution around the sun creates seasonal solar exposure and heating patterns

• At solstice, tilt keeps a polar region with either 24 hours of light or darkness

• At equinox, tilt provides exactly 12 hours of night and 12 hours of day everywhere

Solar intensity, defined as the energy per area, governs Earth's seasonal climate changes

A sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heat source than a beam impinging directly.

Seasons & Solar Intensity

Hours of daylight and Latitude

Midnight Sun

The region north of the Arctic Circle experiences a period of 24 hour sunlight in summer, where the Earth's surface does not rotate out of solar exposure

Take home message

• Seasons are regulated by the amount of solar energy received at Earth’s surface, which depends upon:

– angle at which sunlight strikes Earth’s surface.

– how many hours the sun shines per day.

• Seasons are NOT due to the elliptical nature of the earth’s orbit, i.e. changes in distance from the Sun.