Nature and Composition of the Atmosphere
I.	Atmosphere: envelope of gases and suspended particles which surrounds the earth, held to it by the force of gravity. It spins with the earth.
II.	The Earth/Atmosphere System: the atmosphere is intimately linked with the earth's surface, not just by gravity, but also by a host of energy and material exchanges. It would be difficult to talk about the atmosphere in isolation from its interaction with the surface, since many of the atmosphere's properties are linked to the surface below.
III.	Systems: to appreciate the linkages among spheres of the physical environment, the concept of a system is very useful.
	A.	system -- a set of inter-related components, linked by flows of energy and/or matter.
		1.	Energy -- the ability to do work or produce change
		2.	Matter -- physical substance (stuff), made of atoms/molecules. Comes in three states: gas, liquid, solid
	B.	A system can be defined at different scales
		1.	global scale: the whole earth/atmosphere system. e.g. global atmospheric circulation.
		2.	regional scale: hydrologic processes in a drainage basin. e.g. storm flow, erosion, and deposition in the Oconee River system.
		3.	local scale: gas exchange in a leaf (carbon dioxide, oxygen, water).
	C.	open vs. closed systems
		1.	open systems -- characterized by exchanges of energy and/or matter with surroundings. Flows occur among elements within the system, but there are also external exchanges, as well. Open systems are very common in the physical environment.
			a.	e.g., global earth/atmosphere system is open with respect to energy. It is constantly entering our atmosphere from the sun (solar radiation) and constantly being dissipated back to outer space from the earth (terrestrial radiation).
			b.	e.g., water in a lake -- enters by precipitation and input drainage from points higher in the watershed, leaves by evaporation, output drainage through underlying rocks and laterally to lower points in the watershed.
		2.	closed systems -- no exchanges with surroundings, entirely self-contained. Closed systems are very rare in nature. Hard to isolate a system from all external influence.
			a.	e.g., global earth/atmosphere system is virtually closed with respect to matter. Expect for occasional meteors coming in and some hydrogen atoms escaping, the earth's substance is finite. Things like water or nitrogen are not exchanged with outer space. We must make do with what we have.
IV.	Weather: the day-to-day condition of the atmosphere. Hot days, cold days. Wet days, dry days. Weather is constantly changing because the atmosphere is constantly redistributing energy, moving it from regions of surplus to regions of deficit.
V.	Climate: the long-term, normal condition of the atmosphere at different times of year. Generally characterized by variation in monthly mean temperature and precipitation.
	A.	Athens summers: hot and muggy, not much change from day to day, most rain from thundershowers
	B.	Athens winters: mild to cool, lots of day-to-day change, most rain from cyclonic storms and frontal passages.
VI.	Composition of the Modern Atmosphere
	A.	Air is not a single chemical entity. It is a mixture of gases, most of them uncommon. We will only learn the common or energetically significant ones.
		1.	The mixture is well homogenized from surface to 80 km height-the homosphere. (Zone of most concern to us in this course)
		2.	Above that height, different gases are stratified by molecular weight-the heterosphere.
	B.	In homosphere, most gases occur in constant proportions. Ordered by volume occupied, these gases are:
		1.	Nitrogen (N2) 78%
		2.	Oxygen (O2) 21%
		3.	Argon (Ar) 0.9%
		4.	Carbon dioxide (CO2) 0.035% (350 ppm)
		5.	Ozone (03) Trace
	C.	The main gas that occurs in variable proportions from day to day in the homosphere is:
			Water vapor (H20) 0 - 4%
(Humidity is a measure of atmospheric water vapor content)
	D.	Nitrogen is crucial to life on earth (the biosphere), but not generally in its gaseous form. Must be converted to solid fertilizers by bacteria (or lightning, humans). Nitrogen is not very important in atmospheric energy balance.
	E.	Molecular oxygen is critical to the biosphere, but not very energetically significant, as far as atmospheric energy transfers are concerned.
	F.	Argon is an inert gas.
	G.	Carbon dioxide and water vapor are very significant energetically, because they absorb many wavelengths of earth, or terrestrial radiation. This helps trap heat in the atmosphere (see greenhouse effect later).
	H.	Ozone is very significant energetically, because it absorbs harmful ultraviolet radiation.
		1.	Ozone is concentrated well above the earth's surface (30-50 km), where it shields the biosphere by absorbing about 90% of incoming UV radiation.
		2.	Recent concern over ozone depletion by chlorofluorocarbons (CFCs) from human sources (freon, injected into styrofoam), and its role in elevating melanomas (skin cancer) and birth defects (genetic errors) is a "hot" topic.
V.	How was the modern atmosphere formed?
	1.	Most of it accumulated from volcanic eruptions over eons. The level of oxygen was very small until about 400-600 million years ago.
	2.	What enriched the oxygen content to levels that sustain modern terrestrial life?
		a.	Photosynthesis - the primary energy capturing device in the biosphere. Green plants can convert solar energy to chemical energy.
CO2 + H20 --sunlight--> (CH2O)n + O2
		b.	Carbohydrates are the energy base of most living things. Plants and animals consume food, combine it with oxygen we breath to reverse photosynthesis. We call this respiration. It is the source of our metabolic energy. It also, as a byproduct, has allowed molecular oxygen to accumulate in the atmosphere.
		c.	We are beholden to green plants for our energy source (food) and our key to unlocking the trapped solar energy (molecular oxygen). Green plants are the energetic base of the biosphere, without them we cannot survive.
VI.	Vertical structure of the atmosphere
	A.	Pressure changes with height.
		1.	The air does weigh something. Gas molecules are not weightless. They are attracted to the earth by gravity.
		2.	Air pressure is the weight of the atmosphere, the downward gravitational force exerted by a column of air.
		3.	The density of air is greatest at the surface, and decreases with height. Likewise, air pressure decreases progressively with height, quickly near the surface, more slowly in the upper atmosphere. (There is less air above you to exert a downward force, or weight.)
		4.	Mean surface air pressure at sea level is about 1000 millibars (actually 1013.2 mb). At about 5.8 km height, you have passed through about ½ of the atmosphere, air pressure drops to 500 mb.
	B.	Temperature changes with height. We use direction of temperature change with height to define four layers of the atmosphere.
		1.	Troposphere. Lowest layer of the atmosphere. 0-15 km. (Height varies with latitude and time of year - thicker where warmer.)
			a.	Weather, clouds, life primarily confined to this thin zone.
			b.	Normally, temperatures decrease with height in the troposphere, because the earth absorbs solar radiation and acts as a heating element for the air above. About 6.5°C/km.
			c.	Occasionally, temperatures will increase with height in a small layer of the tropopause. We call this a temperature inversion.
			d.	At the top of the troposphere is a cold, isothermal layer (constant temp) called the tropopause. About -55 to -60°C.
		2.	Stratosphere. From about 20-50 km above earth's surface. The zone of ozone concentration. Temperatures increase with height. Near 0°C in the isothermal zone atop the stratosphere, called the stratopause.
		3.	Mesosphere. From 50-80 km above earth's surface. Temperatures decrease with height. Topped by mesopause.
		4.	Thermosphere. Above 80 km (same zone as heterosphere). Temperatures increase with height, individual molecules are rare, but bombarded by unfiltered solar radiation.