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Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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I. Atmospheric chemistry: 78% Nitrogen, 21% Oxygen, with remaining 1% various other gasses, including Argon, Carbon Dioxide.
 II. Atmospheric divisions: Troposphere: surface to 5 or 10 miles high; 95% of total gas in atmosphere at this level. Most weather processes occur in this layer. Temperature drops with increasing altitude.

Stratosphere: extends above troposphere to 30 miles. Protective ozone shield occurs in this layer. Temperature remains essentially constant with increasing elevation.  III. Solar Radiation and the Atmosphere: Light is electromagnetic radiation which occurs at wavelengths we can perceive (the visable spectrum). However, this radiation can have much lower (radio and infrared radiation) or higher (ultraviolet, x-rays) wavelengths. The bulk of the sun's energy emissions occur from infrared through ultraviolet wavelengths.

Solar radation interacts with the atmosphere by:

Absorption, in which it is absorbed by the atmosphere or land, increasing its temperature.

Reflection, in which it is bounced back into space off of earth or clouds or snow . The amount of radiation reflected off of earth is termed albedo.

Scattered, in which it is dispersed by atmospheric molecules in all directions. This creates blue color of sky.

The amount of energy absorbed by atmosphere is influenced by the chemicals in the atmosphere. Some atmospheric gasses allow solar energy in visable wavelengths to pass through, while blocking infrared wavelengths. This is important, as most of the energy reflected away from Earth is in the infrared wavelengths. This process allows solar energy to become trapped in the atmosphere, keeping it warm.  IV. Variations in solar energy inputs Global solar radiation budget:

31% reflected or scattered back into space

23% absorbed by atmosphere

46% absorbed by earth surface

Changes with altitude: As most of solar energy is absorbed by Earth, air temperature will increase as it gets closer to Earth

Changes with location: Because sun is more directly overhead, and becasue the light passess through less atmosphere before striking the Earth, equatorial areas receive more solar radiation than the poles, making them warmer.

Changes over time:

In summer, northern Hemisphere is tilted toward the sun, making it more overhead, increasing solar radation input.

In winter, northern Hemisphere is tilted away from sun, decreasing solar radation input.
   V. Global circulation patterns. Weather is created by these differences in solar energy inputs.

Here's how it works:
  Air will rise at the equator as it is warmer there

Air will sink at the poles as it is colder there.

The sinking air will create a vacuum, which is replaced by air moving up from the equator. Thus, in a simple world warm and cold air should continually cycle from equator to pole.

But, world not simple. For instance, by constantly spinning on its axis, the Earth breaks up this simple convection cell into three smaller ones, with these touching at 30o (latitude of Gulf Coast) and 60o (southern Alaska and Hudson's Bay).

At 30o N and at poles, air is sinking from both sides. As cool air holds less water vapor, this air will be dry when it reaches the ground. Most of the deserts on the Earth (Mohave, Sahara, Atacama, Australian, Polar) occur at either 30o N or 30o S or at poles.

At equator and 60o N, air is rising from both sides. As this warmer air cools, it creates clouds and precipitation. This is why the equator and boreal forests are wet places.

The location of these junctions change over a year, moving poleward in summer, and equator-ward in winter.

These cells are responsible for westerly flow of winds in mid-latitudes, and easterly flow of winds in equatorial latitudes
 

V. Climate change. The climate changes over all scales of observation (from days to millions of years)

A number of factors can alter the Earth's climate:
  (1) Intensity of solar emissions

(2) Transparency of atmosphere

  If less energy can pass through air (due to volcanic ash, dust and smoke clouds), the planet will cool

If transparency to infrared changes (from increased CO2, CH4, etc.), the planet will warm

  (3) Changes in the Earth's orbit. This first proposed by Serbian mathematician Milutin Milankovitch. There are three main ways in which the Earth's orbit changes in relation to the sun:  Whether Earth is closest to sun in Northern Hemisphere summer or winter (23,000 year cycle)

How tilted the Earth's orbit is on its axis (41,000 year cycle)

How eccentric the Earth's Orbit is (100,000 year cycle)

Ice ages occur when: Earth is closest to sun in Northern Hemisphere summer, when the tilt is greatest, and when the orbit is most eccentric

Interglacials occur when Earth is closest to sun in Northern Hemisphere winter, when tilt is least, and when orbit is most circular.

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Created 2 September 2011, Last Update 02 September 2011

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