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Weather and Climate

The current temperature, wind speed and direction, precipitation, humidity, and cloud cover constitutes a weather report. The study of weather is meteorology. Weather as it varies over the course of years constitutes climate. The study of climate is called climatology. The study of climate over thousands of years is called paleoclimatology.

The greenhouse effect

The greenhouse effect is the primary determinant of Earth's average temperature and therefore of climate. An understanding of climate depends on an understanding of the greenhouse effect. [1]

If an astronaut digs a hole on the Moon and measures the temperature at the bottom of the hole, the reading will be -18°C. During the day (which lasts 14 earth days) the surface will be much hotter and at night it will be much colder. The temperature in the hole reflects the average temperature. It is the distance to the sun that determines the average temperature. Any object in the vicinity of the earth will have an average temperature of -18°C. The object might be black or white or big or small. Only the distance to the sun matters. Any object in space gets very cold unless warmed by the sun.

The significance of the greenhouse effect

The average temperature of the Earth is not a cold -18°C. It is a pleasant 15°C. The Earth is 33°C warmer because of the greenhouse effect. Without the greenhouse effect, Earth would be an ice world and there could be no life. It is important to understand the greenhouse effect and to do that you need to know some physics.

Electromagnetic radiation

Electromagnetic radiation is ubiquitous and part of every day life. Rainbows, radio waves, and x-rays may seem very different, but they are not. They differ only in wavelength. They are all completely described by Maxwell's equations. The elegance and terseness of the equations are apparent, but the mathematics is subtle and advanced.

The wavelength of electromagnetic radiation can vary enormously. Gamma rays have wavelengths smaller than the width of an atom. The radio waves used to communicate with submarines are many kilometers long.

The Electromagnetic Spectrum

Wavelength	Common Name	Uses	Atmosphere
1 to 10 nanometers	gamma rays	cancer therapy	opaque
10 to 100 nanometers	X-rays	cancer therapy	opaque
100 to 1000 nanometers	Sunlight
100 to 400 nanometers	Ultraviolet	suntans, sunburns	partly opaque due to ozone
400 to 700 nanometers	Visible light		mostly transparent
1 to 10 microns	Near infrared	night vision goggles	partly opaque due to GHG
10 to 100 microns	Far infrared		opaque
100 to 1000 microns			opaque
1 to 10 millimeters			opaque
1 to 10 centimeters	microwaves	radar, microwave ovens	mostly transparent
10 to 100 centimeters	microwaves	cell phones	transparent
1 to 10 meters	ultra high frequency (UHF)	television,	transparent
10 to 100 meters	very high frequency (VHF)	FM radio, television	transparent
100 to 1000 meters	short wave	AM radio, long distance radio	transparent
1 to 10 kilometers	long wave	"atomic" clocks	transparent
10 to 100 kilometers	extra long wave (ELF)	submarine communication	transparent

Examine the above table and especially note the following:

Human eyes can sense only a tiny fraction of the electromagnetic spectrum. (Some insects can see ultraviolet. Some snakes have a special organ that can "see" infrared.)
Wavelengths shorter than 400 nanometers can break chemical bonds and kill cells. (This is good for cancer therapy, killing pathogens, and preserving food. It is bad when it causes sunburns and skin cancer.)
The atmosphere is partly opaque to ultraviolet and infrared while it is transparent to visible light. (However, clouds, fog and haze block visible light at times.)

Blackbody radiation

Everyone is familiar with blackbody radiation although they would not use these words. Everything emits blackbody radiation unless its temperature is at absolute zero. The intensity and wavelength of the radiation depends on temperature. Hold your hand in front of your face and notice that you can sense the warmth of your hand. That is because your hand emits blackbody radiation. You can't see it because the wavelength is too long for human eyes.  Look at the heating element in the toaster the next time you make toast. It's hotter and so you can see it radiating at 600 nanometers which is another way of saying it glows orange. The sun is hotter still; it is white hot. A major part of the blackbody radiation of the sun is visible light.

Greenhouse gasses (GHGs)

All gasses found in the atmosphere are transparent to visible light. Some gasses found in the atmosphere are opaque to certain wavelengths of infrared. These are the gasses that cause the greenhouse effect. The most important naturally occurring greenhouse gasses are water vapor, carbon dioxide and methane. Water vapor is sometimes not considered to be a GHG because water vapor can lead to heating and cooling. Condensed water vapor in the form of clouds reflect sunlight from their topsides. The reflected light is energy lost to space and therefore does not warm the Earth. The Sun's energy does not heat the cloud and the Earth below is shaded. The net result of water vapor can be cooling.

The atmosphere also contains anthropogenic (human made) greenhouse gasses. All refrigerants and solvents made from fluorine and chlorine are potent greenhouse gasses. Almost all refrigerators and air conditioners made since 1928 use freon, the trademarked name of certain compounds of fluorine, chlorine, and carbon. One kind of freon, known as freon 12, has been outlawed. Scientists discovered freon 12 was destroying the ozone layer. Freon 12 has high chemical stability which means that it persists in the atmosphere for many years. Yet chemical stability is what makes freon 12 desirable as a refrigerant.

These compounds have a collective name. It is CFC (chloro-fluoro-carbon.) No one in 1928 had any idea that CFCs posed a threat to the entire Earth. The truth is that they are a threat in two unrelated ways. Certain CFCs destroy ozone in the upper atmosphere. All CFCs are potent GHGs. It is certainly fortunate that these dangers were recognized in time.

Another example of an anthropogenic GHG is sulfur hexafluoride. It is widely used in electrical equipment and metallurgy. Its global warming potential is 23,900 times more than carbon dioxide--more than any other gas. Any sulfur hexafluoride that escapes becomes a near permanent part of the atmosphere as it will persist for thousands of years. The amount of sulfur hexafluoride in the atmosphere is not contributing significantly to global warming at this time. But its continued use should be considered carefully.

An explanation of the greenhouse effect

Enough physics has been presented to allow an explanation of the greenhouse effect.

Visible light from the sun travels through Earth's atmosphere. The energy in the visible light is converted to heat on the surface of the Earth. The same energy is emitted from the surface of the Earth as black body infrared radiation. The incoming and outgoing energy balances when the Earth is at -18°C. However, some of the outgoing energy is reflected back to the Earth by greenhouse gasses in the atmosphere. This increases the balance point to 15°C.

Earth

Visible light (yellow arrows) from the sun warms the earth during the day. The warmed earth emits infrared (red arrows) in all directions. Greenhouse gasses in the atmosphere (blue) reflect part of the infrared back toward the Earth, resulting in global warming.

Sun

[1] http://www.grida.no/climate/vital/foreword.htm Vital Climate Graphics are based on the reports of the Intergovernmental Panel on Climate Change (IPCC), which was established by the United Nations Environment Programme and the World Meteorological Organization. This is authoritative and very well done. If you read only one link, read this one.