Lecture 1: Introduction to the Atmosphere(Chapter 1 in Ackerman&Knox, Chapter 1 in Hartmann)

• The Earth System– The Atmosphere is an extremely thin shell compared to solid Earth.

• Gravity and Gases – Atmospheric composition• Trace gases and aerosols

– CO2, H2O, methane, CFCs, aerosols

• Atmospheric pressure and density – the static atmosph.– hydrostatic relation and the ideal gas law – Pressure and altitude

– Vertical structure of the atmosphere Introduction to weather maps (read on your own)

Time zones (read on your own)

Facts about the Earth

• The distance from the surface to the center of the Earth is about 6400 km.

• Almost all the atmospheric mass is confined to the lowest 100 km (about 2% of the distance to the center)

• The Earth (including atmosphere) rotates once per day*Atmospheric motions are primarily horizontal (especially in the zonal direction)

*Vertical displacements of air parcels have important implications for precipitation.

• The distribution of land/ocean is very different between the two hemispheres.

• The atmosphere is well mixed up to about 80 km height.

Weather and Climate

*Weather is the instantaneous state of the atmosphere in a region as described by a certain set of variables. (Key word is instantaneous)

*Climate is the “average weather” in the region as described by the same set of variables averaged over a certain period of time, but also the variations and extremes with the probabilities that the extremes will occur. (Accumulated over many years.)

Satellite image of continental US Visible image (on 24 June) measures reflectance

Fig. 1.2

Notice the difference in land/ocean distribution between the Hemispheres.Overall, 70% of surface is covered by water.

Atmospheric composition

• Gravity keeps the atmosphere from escaping to space• If gravity acted alone, the heaviest molecules would be

at the surface, the lightest near the top• Weather processes stir up the atmosphere, keeping it

well mixed• Nitrogen and oxygen make up 99% of the atmosphere• Still those elements are not very interesting in the study

of the atmosphere• Carbon dioxide and water vapor are far more interesting

– CO2 is increasing in concentration – H2O (the gas) is very variable, 0-4%. H2O exists in all 3 phases

The trace gases are the interesting ones. Also aerosols (dust particles) such as sea salt, fine soil, smoke and soot, pollen, ash & dust from volcanic eruptions, meteoroids.

0.038 (0.028) (0.00008)

(0.)

preindustrial

/

CO2 moves between the atmosphere, ocean and lands. Let’s focus on atmospheric sources/sinks.

Fig. 1.4

The Keeling curve of atmospheric CO2 concentrationNotice the increase and the superposed annual cycle

The hydrologic cycle describing water in the atmosphere (in all three phases) and how it enters and leaves the atmosphere.Water connects the surface (land or ocean) to the atmosphere.

IR satellite composite image from the same day as before.IR measures temperature of the surface. The whiter the cloud, the colder it is and the higher in the atmosphere it is.Note: ITCZ, extratropical cyclones in SH, convection over China.

Methane has not been steadily increasing such as CO2

Fig. 1.8

Fig. 1.9

CFCs are extremely stable gases

Aerosols: dust, smoke, salt, ash, etc.

Vertical structure in the atmosphereDensity (rho) as a function of height above the surface

Vertical structure in the atmosphere

• What about pressure?• Hydrostatic equation: balance between pressure gradient

force and gravity.– dp/dz = - rho g

• Ideal gas law:– p = rho R T

Let’s go to the board!

z = - H ln (p/p0), where H is scale height and is only constant if T is constant.In other words, p = p0 exp(- z/H)

-

Pressure (p) as a function of height

Symbols on the weather map: 4 different fronts

Symbols

Fig. 1.17