Weather refers to atmospheric conditions (e.g., of temperature, humidity, precipitation, wind characteristics, and atmospheric pressure) in a particular region at a specific time. Because weather conditions vary from place to place and time to time, it usually doesn't make sense to talk about "global weather" or weather over very large regions or time intervals.

Climate refers to averages of these atmospheric conditions over a relatively long interval. With climate, the region of interest could be the entire Earth - global climate.

But you knew that.

The purpose of this presentation is to outline the fundamental parameters of global climate on a global scale.

Q: Why do weather and climate happen? That is, why doesn't the atmosphere just lie motionless.

A: Because energy is being poured into the climate system. For all practical purposes, the light of the sun and Earth's rotation are the only sources of this energy.

Let's start with something simple

Terry Pratchett's Discworld series envisions Earth as a disk on the back of four giant elephants who are, in turn supported by the Great A'Tuin, a giant turtle. If the sun were suspended directly above the disc, warming all regions equally, climate would be simple.
Atmospheric stratification: Even in the Discworld, however, it wouldn't be absolutely straightforward. In terms of atmospheric pressure, the atmosphere grades upward without obvious breaks, but as we've seen, the atmosphere has four distinct zone, each characterized by a distinct thermal gradient:

The layers:

Even in the Discworld, we would see such stratification. But why?

Remember the Greenhouse effect? Solar energy usually reaches the Earth's surface in the form of visible light. The Earth absorbs about 70% and reflects about 30%.

This absorbed energy: Hot objects reradiate energy as infrared radiation (a.k.a. radiant heat), however many atmospheric gasses that are transparent to visible light (allowing the energy in) are opaque to infrared, preventing this energy from radiating back into space. The result is that the atmospheric gasses opaque to infrared are warmed up. (For our purposes, infrared and heat are the same thing.) These are greenhouse gasses. Foremost among them, water vapor.

  • Troposphere: (0 - 15 km) Warmed by: With increasing elevation, these heat sources diminish, causing temperature to drop roughly 6.5 deg. C for every kilometer.

  • Stratosphere: (15 - 50 km) Solar ultraviolet radiation impinges on the stratosphere. This radiation causes the dissociation of O2 molecules which recombine as O3, ozone, which forms an ozone layer. Happily, ozone is opaque to ultraviolet light. As a result, the biosphere is protected from ultraviolet radiation that would otherwise render Earth's surface sterile. However, as ultraviolet light strikes the stratosphere, its energy is absorbed by the ozone, which heats up, radiating infrared. At higher elevations, there is more UV radiation.

  • Mesosphere: (50 - 90 km) Becomes cooler with increasing altitude for the same reasons as the troposphere. In this case, however, heat comes from the stratosphere, not Earth's surface.

  • Thermosphere: (90 km and above) The "air" is so thin that it mixes with the solar wind, the flow of very hot charged particles from the sun, and is heated by it.

    Why is there an upper limit to convection? Consider the relationship between heat and pressure - heat of compression. When a volume of gas is compressed, the energy of the work of compression is transferred to the gas molecules, increasing their thermal energy. Common practical applications of this include:

    The converse of heat of compression is the cooling that results from decompression. If a location on the planet's surface is particularly warm:

    Because this cooling occurs even if there is no transfer of heat to surrounding air, it is termed adiabatic cooling. (Adiabatic means "without transfer.") This is the minimum rate at which rising air cools. In practice, there is usually some heat transfer to surrounding air, also.

    Adiabatic lapse rate: The rate at which rising air cools adiabatically. As long as the surrounding air cools faster with increasing height than the adiabatic lapse rate, that parcel will continue to rise. Typically, the adiabatic lapse rate is slightly less than general cooling in the lower atmosphere. Once the adiabatic lapse rate exceeds the general rate of cooling, the parcel will no longer rise. This places an upper limit on convection and defines the top of the troposphere.

    Albedo: But there is a small complication: An object's reflectivity is its albedo. High-albedo objects (clouds, glaciers, sea ice, etc.) reflect solar energy back into space as visible light. Low-albedo objects (oceans, forests, etc.) absorb it and reradiate it as heat. Water vapor, the ultimate greenhouse gas, condenses well below the tropopause (the transition from the troposphere to the stratosphere) forming clouds. These reflect sunlight back into space, while the water vapor in and beneath them traps heat from the surface. Thus, clouds amplify the tropospheric gradient.

    Earth's energy budget: Taking all of this into account, we can describe Earth's overall energy budget. On average, solar energy arrives at a rate of 342 watts (i.e. Joules/sec) in the form of visible and ultraviolet light. It's fate:
    • 3% absorbed by stratosphere
    • 17% absorbed by troposphere
    • 50% absorbed by Earth's surface
    • 30% reflected back into space as albedo.
    In the graphic at right, the width of the colored line is scaled to the percentage of incoming radiation at that moment.

    Thermal radiation: But this isn't the end. Absorbed energy gets reradiated at longer infrared (IR) wavelengths. Thus, at any moment, solar energy is passing through the Earth system both as direct solar radiation and as reradiated infrared radiation of energy that had been received from the sun previously. This leads to the paradoxical effect that at any moment, the amount of energy being radiated at infrared wavelengths greatly exceeds the amont being received from the sun. In the graphic at right, the width of the colored line is still scaled to the percentage of incoming radiation at that moment.

    Inconstant sun: As far as the instrumental record tells, the sun's irradiance doesn't change much. There is an 11 year sunspot cycle in which the sun's energy output fluctuates by 0.15% - a small fluctuation compared to other climate parameters. There is speculation that over the long term, the sun's output has actually varied, however, with climatic consequences for Earth, but the topic is enigmatic.

    Note: Don't confuse cycles in solar output with changes in the Milankovich Cycles - Earth's orbital parameters that have forced major long-term climate changes.

    Atmospheric circulation: But sunlight does more than heat the atmosphere, it causes it to circulate. Atmospheric circulation is driven by two things:

    Latitudinal differential in incoming sunlight: The amount of solar energy received per unit area of Earth's surface is a function of the angle at which the light strikes. The most concentrated energy is delivered to equatorial regions whereas polar regions receive very little. Resulting circulation is, in part, the atmosphere's attempt to equalize that heat distribution.

    The Coriolis effect: Even if we lived on some Discworld, where solar energy is evenly distributed, that world's rotation would influence the movement of any moving air mass. Conversely, if the Earth didn't rotate, air and water currents would travel in straight lines. The combination of rotation and inertia, however, causes currents to veer onto circular courses.

    Global convection cells: The interaction of the latitudinal differential and Coriolis force breaks global circulation into three convection cells per hemisphere, forming globe girdling belts:

    • Hadley cells: form adjacent to the equator as strong sunlight warms equatorial air, which rises to the top of the troposphere and spreads north and south.
    • Polar cells: form as very cold air descends near the poles and spreads southward.
    • Ferrel cells: occur in the middle latitudes as a result of interactions between Hadley and polar circulation.
    The cells meet in broad zones roughly 30 and 60 deg. north and south. These zones are the location of the tropical and polar jet streams, respectively - rapid high altitude air westerly air currents.

    When we translate the schematic above into three dimensions, we see that on Earth's surface we perceive the atmospheric circulation cells as latitudinal wind zones, with easterly trade winds at the bottom of the Hadley cells, westerlies with the Ferrel cells, and polar easterlies at high latitudes.

    What is being moved by these currents?
    • Air (duh)
    • The heat that it carries
    • Water vapor.

    But note: most of the water vapor in these cells condenses and precipitates out long before it reaches the top of the troposphere. As a result, global atmospheric circulation implies a latitudinal zonation of humidity and aridity. Consider the Hadley cells. Warmer air can hold more water vapor than cool air, so the most humid air is found in the equatorial tropics. As this air rises to form the Hadley cells, it quickly cools and its humidity precipitates out. The result is a belt of rising air with persistent clouds and rainfall, the intertropical convergence zone (ITCZ) (above). By the time the air of the Hadley cells descends to the surface, it is extremely dry. Consequently, land regions near 30 deg. host many of the world's largest deserts:

    At this point, note that we have gone a long way toward describing Earth climate systems using a small number of parameters:

    We will put some fine points on this picture next week. For now, lets ask what climate systems would be like if we varied these parameters by looking at other worlds of the Solar System.

    What would Earth's atmosphere look like if there were no oceans?

    In a word, it would look like Venus

    They call Venus Earth's "evil twin." Barely smaller than Earth, it's geologic features are similar to what we see here. Because it's size, internal heat, and gravity are similar to Earth's, as are many of its surface features.

    So Venus' geology is slightly different from earth's, but its surface conditions are radically different.

    Effective temperature: The ideal temperature at which a planet precisely re-radiates all of the energy it receives from the sun. Effective temperature is primarily a function of:

    However, to know incoming solar radiation, we must also consider albedo, the amount of radiation reflected back into space. Clouds and surface features like ice caps have high albedo.

    Incorporating albedo into estimates of effective temperature, we get:

    Makes sense. Mars is farther from the sun than Earth, and Venus has much greater albedo. For Mars, it also tracks mean surface temperature well. But for Venus and Earth, mean surface temperature is:


    Earth is 33 K warmer than expected. Venus is 500 K warmer. This difference is because of the greenhouse effect

    How did this come to be?

    Venus, unlike Earth, has essentially no carbon cycle. On Earth,

    But on Venus there is no hydrosphere, so CO2 just gathers in the atmosphere.

    Icing: Did I mention that the clouds are made of droplets of sulfuric acid? (Now breathe deeply.) Follow this link for a fictional but realistic cinematic imagining of Venus' surface. BBC's Voyage to the Planets - Venus.

    What if Earth were much smaller with lower gravity and a weaker magnetic field?

    Consider Mars.

    The evidence: Mars has a proper permanent (?) atmosphere. This atmosphere is mostly CO2, but is thin. At 6.3 x 10-3 bars, it is roughly 1/100th the thickness of Earth's.

    But now the problem: There is evidence that Mars once had a much more substantial atmosphere. Where has it gone?

  • Liquid water: Considerable physical and chemical evidence indicates that early in its history, liquid water flowed across Mars. How frequently or for how long is open to debate.

  • Ice: Mars' average temperature is 218 K (-55 deg C), however temperatures vary from 140 K (-133 deg. C) in polar winter to 300 K (27 deg. C, 80 deg. F) in equatorial summer. Mars has permanent polar water-ice caps. The temperature contrast between these ice caps and adjacent warmer ground drives much of Mars' weather. Recent explorations have revealed that considerably more ice is buried beneath thin layers of soil across much wider regions of the planet, or occasionally pooled on the surface.

    So, the water in Mars' atmosphere, at least, seems mostly to have frozen.

    But this begs the bigger question: Why did Mars once have rivers and now have, at most, tiny ephemeral flows of groundwater?

    Problem I - Gravity: Mars is a small world (escape velocity 5.0 km/s). Thus, even though it is colder than Earth, we expect its atmosphere to leak into space faster. That wouldn't be a problem if its atmosphere were being replenished by gasses from volcanic eruptions.

    During its first two billion years, Mars was volcanically very active. Even now, it boasts the largest volcanoes in the Solar System. Whether these volcanoes ever so much as burp today is a pressing question, but in all probability, we have missed seeing major volcanic activity there by over half a billion years. So, it looks like Mars' atmosphere is not being replenished.

    Problem II - Magnetism: But simple leakage is not Mars' only problem. Both Earth and Mars are close enough to the Sun to experience a strong solar wind. On Earth, this wind is deflected by magnetic fields generated by Earth's liquid core. Does Mars have a global magnetic field like Earth's to deflect it? At one time it may have. Recent measurements of weak remnant magnetism preserved in its rocks reveal a pattern of geomagnetic reversals like those of Earth. But being smaller, Mars cooled off faster and its core froze. Magnetic patterns are only evident in rocks that are over 2.5 billion years old. Maybe in its early history (the same age as its wet surface) Mars' atmosphere was protected from the solar wind. For most of its history, however, it has not been, so that the solar wind has slowly stripped gasses from its upper reaches. In short, Mars is slowly being undressed by the Sun.

    What if Earth's atmosphere had a really different composition?

    Consider Saturn's cold moon Titan

    Physical specs:

    Titan's dense atmosphere resembles Earth's in some ways: But then it gets strange. It contains a complex photochemical smog that makes is opaque in visible light. Thus, the Voyager spacecraft saw a featureless orange ball. With its infrared imaging and and synthetic aperture radar, the Cassini probe has shown that Titan has a complex surface geology including: All in all, Titan seems like an odd reflection of Earth, but a few "octaves" lower - i.e. at a temperature (-94 K (-179 deg. C)) where water is a rock and methane and ethane flow as liquids.

    Titan's idiosyncrasies:

    Titan paradox: But wait! Titan does not have a magnetic field. Why does it have an atmosphere 1.5 times as dense as Earth's while Mars' is only 0.01? Answers:

    All of these are global issues influencing global climate. Earth, however, has geography - a highly varied surface with land and oceans. This fact adds the next layer of complexity to global climate, which we will take up next week.