The origin of our earth's atmosphere is still subject to much speculation.
One theory seems fairly certain; that when the earth was formed some
five billion years ago, it was probably too hot to retain any of the atmosphere
it had to begin with. Based on our knowledge of gases in the universe, this
first atmosphere probably consisted of helium, hydrogen, ammonia and methane.
If we assume that volcanoes five billion years ago emitted the same gasses as
they do today, the earth's second atmosphere probably consisted of water vapor,
carbon dioxide, and nitrogen. These gasses were expelled from the earth's
interior by a process known as outgassing.
The vast amounts of water vapor expelled by the volcanic earth resulted in the
formation of clouds which, in turn, produced rain. Over a period of thousands of
years, the rain accumulated as rivers and lake and ocean basins.
The water reservoirs acted as sinks for the large amounts
of carbon dioxide (as they do today) and through chemical and biological
processes became locked up in sedimentary rocks as limestone.
The nitrogen, which is not very chemically active continued to accumulate in the atmosphere.
While oxygen currently makes up 21 percent of the atmosphere, there was only a trace in the air when life first appeared on the planet. The single-celled bacterium dwelling in the oceans did not need oxygen to live. Oxygen first appeared in the environment when early bacteria developed the ability to split water molecules apart by harnessing the energy of sunlight - a key part of photosynthesis. Photosynthesizing organisms produced the oxygen that accumulated over geologic time.
These processes acting sequentially and simultaneously appear to have produced
the delicate balance of 78% nitrogen (N2) and 21% oxygen (O2) we observe
today.
Relative Composition of the Atmosphere
The relative composition varies from place to place on the surface of the Earth. The reason for this variation is the presence of aerosols and water vapor - both of which vary widely in amount:
Arosols are tiny liquid droplets, such as fog, or tiny solid particles, such as ice crystals, smoke, sea salt crystals, dust, and volcanic emissions, suspended in the air.
Humidity is the amount of water vapor in the air.
Clouds
The study of clouds, where they occur, and their characteristics, may well be the
key to understanding climate change.
Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth. Figure 2. low clouds
High, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the
outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface of the Earth. Figure 3. high clouds
Whether a given cloud will heat or cool the surface depends on several factors,
including the cloud's height, its size, and the make-up of the particles
that form the cloud.
The balance between the cooling and warming actions of
clouds is very close although, overall, cooling predominates.
The verticle distribution of temperature determines to a large degree the exchange of air in the verticle direction.
As an unsatuated mass of air rises, it expands and cools. The dry adiabatic lapse rate is the decrease of temperature with altitude when heat is neither gained nor lost. Adiabatic processes occur without the addition or subtraction of heat from an external source.
The moist adiabatic lapse rate is the way temperature drops in a saturated mass of air. The moist adiabatic lapse rate is always less than the dry rate because of the addition of latent heat of rising air above the level of condensation.
Clouds form when the air rises and becomes satuated in response to adiabatic cooling.
