GEOS 114 MIDTERM REVIEW
Introduction to the Atmosphere
Know the difference between weather and climate. Weather is the state of the atmosphere at a given time and place. It changes
continuously. Climate is more than just average weather. Climate also describes extremes, variations, and the probabilities that such
departures from “normal” will take place.
Important elements of weather and climate: air temperature, humidity, type and amount of cloudiness, type and amount of
precipitation, atmospheric pressure, wind speed and direction.
Know the four spheres of the Earth system: atmosphere, lithosphere, hydrosphere, biosphere
Atmosphere: the thin, life-giving blanket of air surrounding the planet
Lithosphere: the rigid outer layer of Earth that includes the crust and the uppermost part of the mantle.
Hydrosphere: the water portion of the planet.
Biosphere: includes all life on earth.
All four spheres interact and exchange energy and material. The ultimate source of energy for the system is the Sun. Heat from the
Earth’s core is an almost negligible source of heat for the atmosphere except for the odd volcano.
Atmospheric composition: if you take out the super variable components of the atmosphere, you’re left with Nitrogen, Oxygen, and
Argon (over 99% of the atmosphere).
Know the importance of carbon dioxide, that it’s a greenhouse gas, and is leading to the warming of the lower atmosphere.
Variable components: water vapor (very important for mostly obvious reasons), aerosols (important for what they can do to light and
heat, as well as health effects), and ozone.
Water vapor: varies from near 0 to 4% of the atmosphere, important source of latent heat
Aerosols: liquid and solid particles suspended in the atmosphere. Also important in forming condensation nuclei for formation of
raindrops.
Ozone: a form of the oxygen molecule composed of three oxygen atoms instead of the usual two. It’s very VERY reactive, so you
wouldn’t want to breathe it no matter where you are. Ozone is a very important atmospheric gas. Ozone absorbs UV, the energy
deposited to the stratosphere is what makes stratospheric temperatures rise with altitude instead of the decrease seen in the
troposphere. Chlorofluorocarbons (CFCs) release chlorine into the stratosphere, which destroys ozone and leads to the ozone hole. IF
the Montreal protocol is followed, this should be a solved problem.
Earth’s current atmosphere is NOT the one that it started with. The primordial atmosphere was blown away by the solar wind when
the Sun’s nuclear furnace first ignited. The secondary atmosphere was then formed by outgassing from the molten rock on the
surface, and would have consisted largely of water and carbon dioxide. Once the Earth cooled sufficiently, water would have
condensed to form oceans, and carbon dioxide would have dissolved in that water, eventually forming rocks. At that point, the
atmosphere would have been largely nitrogen with a little argon (that comes from radioactive decay, incidentally). Oxygen only
comes later from life. Exactly how much oxygen was present at what time is still a matter of debate.
Height and structure of the atmosphere.
Pressure drops off with altitude since atmospheric pressure is simply the weight of the atmosphere above a given point. Half the
atmosphere lies below an altitude of 5.5 km. 90% of the atmosphere lies below 16 km.
Temperature falls off with altitude at first, but has considerable structure. The four layers of the atmosphere (when the atmosphere is
divided up by temperature—there are other methods that yield other kinds of layers) are the troposphere, stratosphere, mesosphere,
and thermosphere. The borders between these layers are named after the layer below: tropopause, stratopause, mesopause.
Troposphere: the lowest layer, where we live, weather happens. Ordinarily, it is well mixed. Temperature falls off with increasing
altitude, on average, at the environmental lapse rate, 6.5°C/km (3.5°F per 1000 feet). When a reversal of this pattern occurs, and the
temperature increases with altitude, a temperature inversion exists. The tropopause is the boundary between the troposphere and the
stratosphere. Its height is much greater in the tropics (~16 km) than at the poles (~9 km).
In the stratosphere, temperature increases with altitude, due to absorption of UV light by ozone. The stratosphere is also home to the
ozone layer, between about 15 km and 30 km (if the average tropopause height is 12 km). The stratosphere extends from the
tropopause up to a height of about 50 km—the height of the stratopause.
Mesosphere: in the third layer, temperatures again decrease with height until about 80 km above the surface, where the temperature
again flattens out—the mesopause.
Thermosphere. Here, the temperature again rises with altitude, due to absorption of highly energetic UV light by oxygen and
nitrogen. Though the temperature eventually rises to something like 1000°C (at which point it levels off, figuring that hot enough is
hot enough), because there just isn’t much atmosphere to carry the heat, one would not feel terribly hot. Basically, the reasoning here
is the same as why one can take being hit by a small drop of boiling water coming off a pot on a stove, but immersing oneself in a
boiling cauldron would be a bad idea, to put it mildly—there’s a lot more water in the cauldron to transfer heat to you than there is in
the droplet. Likewise, in the thermosphere, it’s hot, but there just isn’t much air to carry the heat.
If, on the other hand, we define atmospheric layers in terms of composition, we get two different layers: the heterosphere and the
homosphere.
Homosphere: the lower, uniform layer of the atmosphere up to about 80 km.
Heterosphere: the upper, tenuous atmosphere, where atmospheric composition varies considerably with height. We have four layers:
the first dominated by molecular nitrogen, the next dominated by atomic oxygen, the next dominated by helium, and the last
dominated by hydrogen atoms. Why all the variation? They separate out according to molecular weight, so the heavier stuff stays
closer to the surface and the lighter stuff ends up higher. But why would that happen? Above about 80 km, interactions between
molecules become relatively unimportant, so molecules behave more like individual ping pong balls than a viscous fluid. Since they
don’t interact, they don’t hang together, so everything separates out.
If we decide to divide up the atmosphere according to ionization, we end up with two layers: the uncharged inner layer and the outer
layer, between about 80 km and 400 km, called the ionosphere. In the ionosphere, atoms and molecules are readily ionized by
absorption of energetic ultraviolet light from the Sun. Below 80 km, not much of the extreme UV necessary for ionization can get
through, and even if it does, ions can reunite with each other relatively quickly. Above 400 km, there just isn’t much of anything
around, so while it might all be ionized, it can’t do much. There are three separate layers within the ionosphere, and, since the
ionization is driven by the Sun, there is considerable day/night (diurnal) variability. Activity in this region leads to the aurora
(borealis and australis—the northern and southern lights, respectively). These are generally caused when particles from the Sun
shower down on the Earth’s atmosphere along magnetic field lines (hence the tendency to be located around the magnetic north and
south poles). Collisions between these particles (particles in the atomic/nuclear/high energy physics sense, not the atmospheric
science sense!) and atomic oxygen and nitrogen produce the spectacular colors seen in the northern lights.
Heating Earth’s surface and atmosphere
It’s all about the Sun here! The Sun provides well over 99.9% of the energy that reaches the Earth’s surface and drives the Earth’s
weather. This energy gets recycled in millions of different ways, so don’t try to tell us that the source of a hurricane’s energy is the
Sun. While that’s technically true, there’s a LOT going on between when the Sun’s rays hit the surface and when a hurricane forms.
Let’s put it another way—the Sun’s there all the time, but hurricanes aren’t, so something else must be going on.
This energy is not deposited equally at the Earth’s surface; the amount received varies with time of year, time of day, and latitude.
This unequal heating drives winds and ocean currents.
Revolution refers to the Earth’s motion in its yearly elliptical orbit around the Sun. Rotation is the daily spinning of the Earth on its
axis. The average distance between the Earth and the Sun is 150 million km. In reality, that distance varies from 147 million km (in
January) to 152 million km (in July). Note that the Northern Hemisphere is actually closer to the Sun in winter!
Know how the seasons work—. It’s the result of being tilted toward the Sun (summer) or away from the Sun (winter). The effects:
an overhead Sun deposits more energy per square meter than a Sun lower in the sky; the day is longer in summer than in winter,
giving more time to warm the surface; and when the Sun is lower in the sky, its rays must travel through more of the atmosphere
before reaching the Earth’s surface, so less radiation reaches the surface.
Earth’s orientation—make sure you understand, which describe how the Earth’s orientation to the Sun changes through the year.
Know what the Tropic of Cancer, Tropic of Capricorn, Equator, and Arctic Circle are and why they lie at those latitudes. Know
the summer solstice, winter solstice, autumnal equinox, and spring equinox. Where are the Sun’s rays directly overhead on each
of these days.
Summary: seasonal fluctuations in the amount of solar energy reaching various places on Earth’s surface are caused by the migrating
vertical rays of the Sun and the resulting variations in Sun angle and length of daylight.
Energy is the ability to do work. Energy is neither created nor destroyed, but can be changed from one form to another. Work is this
class…(  )…work is loosely described as moving something some distance when opposed by a force. So no work is done when an
air hockey puck slides around, but work is done when you push a car uphill—your pushing is being opposed by the force of gravity,
and the energy is stored as potential energy. Work is also done in dragging a bag of rocks across the ground, but in that case, all the
energy is dissipated by friction. The two major types of energy are kinetic energy (energy of motion) and potential energy (stored
energy). Potential energy is what you have a lot of when you push the car to the top of the hill. Kinetic energy is what you end up
with when you forget you left the car in neutral and it starts to roll away. Kinetic energy is also what you end up with when your car
slams into a telephone pole, though it’s a bit harder to see because what’s moving is the molecules in your smashed car, and the
crashed phone pole.
Heat is a form of energy, and is a distinct concept from temperature. If you dump heat into an object, its temperature will usually go
up unless you’re doing something like melting an ice cube, in which case the ice melts and it’s the phase change that is soaking up all
the heat. Heat is related to the total kinetic energy of all the molecules in the ice cube, while temperature is related to the average
kinetic energy of all those molecules. So a teaspoon of water taken out of the ocean has the same temperature, but a LOT less heat
than the entire ocean. Heat always moves from higher temperature to lower. That’s actually part of how temperature is defined.
Mechanisms of energy transfer: conduction, convection, and radiation.
Conduction is the transfer of heat between two objects in contact. Air is a poor conductor of heat. That’s part of why a quilt keeps
you warm—it preserves a nice air pocket through which heat does not readily flow. Conduction isn’t terribly important in the
atmosphere except for getting heat from the surface to the very base of the atmosphere.
Convection is the transfer of heat by movement or circulation within a substance, and only takes place in fluids. Convection is the
dominant means by which heat goes from the Earth’s surface to the atmosphere. Meteorologists use the term “convection” to describe
vertical motions, though it can also be used to describe horizontal motions in other fields. Naturally, we cannot leave well enough
alone, so we use the term advection to describe horizontal movements of air that carry heat, carbon monoxide, bees, or anything else
on the winds.
Radiation is the transfer of heat from one object to another without any medium in between. Since this is the only way to transfer
heat through the vacuum of space, it is the dominant method by which the Sun’s energy reaches us. The spectrum is a good thing to
know. For atmospheric science, the most relevant forms of radiation are infrared, (IR) visible, and ultraviolet (UV).
The wavelength of radiation determines its energy. Shorter wavelengths are more energetic than longer ones.
Laws of radiation:
1. All objects above –273°C (absolute zero 0K) emit radiation.
2. Hotter objects radiate more energy per unit area than do cooler objects
3. The hotter the radiating body, the shorter the wavelength of peak radiation.
4. Objects that are good absorbers of radiation are also good emitters at that wavelength.
The Earth’s atmosphere readily transmits visible light from the Sun to its surface, and is very efficient at absorbing the infrared
radiation trying to escape from the Earth’s surface.
Know Wien’s displacement law (λmax = c/T) and the Stefan-Boltzmann Law (E = σT 4).
Fate of incoming radiation: radiation may be absorbed, transmitted, reflected, or scattered. See figure slide #31 in solar radiation
power point for a globally averaged indication of the fate of solar radiation.
Reflection: light bounces back from an object at the same angle at which is encounters a surface. The fraction of radiation that is
reflected by a surface is its albedo. The average albedo for the Earth’s surface is about 31%. Freshly fallen snow may have an albedo
over 90%. See Table 3.1 (slide #20 in PowerPoint solar radiation).
Scattering: (think about a small particle here, not a large object) produces a large number of weaker rays, traveling in different
directions. Most of the energy is scattered forward Know how scattering leads to blue skies (p. 71).
Check out figure 3.20 and be sure you understand it using the discussion on p. 71-72. In particular, be sure you know what the Sun
does, what the Earth does, and how the atmosphere interferes with a perfectly decent system like that. Know how water vapor, ozone,
and oxygen are important in absorbing the Sun’s rays. Know that there’s an atmospheric window in the infrared, and what that means.
Know that the atmosphere is largely transparent to visible light which heats the surface, and that, in turn, warms the atmosphere.
Water vapor and carbon dioxide are the principal absorbers of the Earth’s outgoing radiation. Know that “shortwave” refers to what’s
coming in from the Sun and “longwave” refers to what the Earth is putting out.
The role of the Earth’s atmosphere in warming the Earth’s surface is called the greenhouse effect. This is a good thing, since without
it, conditions would be 30°C colder!
Know how clouds affect the temperature at the surface by reflecting the Sun’s radiation and absorbing & reradiating the Earth’s
radiation.
Know how the heat budget of the Earth’s atmosphere. The specific numbers aren’t anywhere near as important as the concepts!!
Latitudinal heat balance. and understand how it generates the seasons. Know why the surpluses and deficits are there and what they
mean. Averaged over the whole year, the latitudes between 36°N and 36°S receive more solar energy than is lost to space, but the
region where that is the case migrates with time of year.
Temperature
The daily mean temperature is calculated either by taking the average of 24 hourly temperature measurements or by adding the
maximum temperature to the minimum temperature and dividing by two. The temperature range is just max T minus min T. Monthly
mean temperature is calculated by averaging the daily means. Annual mean temperature is calculated by averaging the 12 monthly
means. The annual temperature range is calculated by taking the difference between the warmest and the coldest monthly mean
temperatures
An isotherm is a line of equal temperature. Know how they work, what they mean, and how to read maps of them. There is some
guesswork involved in drawing them on a map. Closely packed isotherms indicate a steep temperature gradient.
Five temperature controls):
1. differential heating between land and water
2. ocean currents.
3. altitude
4. geographic position
5. cloud cover and albedo
Differential heating between land and water—why is water so much slower to heat up? High heat capacity
1. water is highly mobile, so convective movements mean that water mixes to some depth (6 meter daily variations and 200-600
meter yearly variations). Soil and rock are nowhere near as mobile (10 cm daily; 15 m annually).
2. land is opaque, so solar radiation can’t penetrate; water is clear so several meters are heated.
3. The specific heat of water is more than triple that of land.
4. evaporation from water is considerably higher—another place for heat to go.
The climates of St. Louis, MO and San Francisco, CA are two good examples of the effects of water, as are the climates in the
Northern and Southern Hemispheres.
Ocean Currents can carry lots of heat far away from their source. The best example is probably the Gulf Stream and its extension,
the North Atlantic drift, which keep Europe considerably warmer than it would otherwise be. Likewise, the cool California current
keeps the West Coast of the U.S. cooler than it might otherwise be.
Altitude. Higher places are cooler than lower places. Temperatures drop by 6.5 oC per 1000 m in the free atmosphere, but not quite as
much at the ground, since the air at the top of a high mountain is still warmed directly by the mountain’s surface—heat doesn’t have to
work its way up through a few thousand meters of atmosphere to get there.
Geographic position. This irritatingly vague category refers to the geographic position relative to a large ocean. This is clearly very
closely related to the land/water issue. If you’re right next to the water, sure, your temperatures will be moderated, but if the wind
consistently blows from the water to you, your climate will be considerably more moderate than if the wind generally blows from you
out over the ocean. Hence, Seattle’s climate is more moderate than New York City’s.
Cloud cover and albedo. Cloudy days are cooler than sunny ones; cloudy nights are warmer than clear ones. Clouds keep in the heat
from the ground, but don’t let the Sun in, so they produce a more moderate climate. In London and Seattle, they consider this
“charming”. Note that both cities adore hot beverages laden with caffeine. Snow cover just makes things colder, since it reflects so
much of the Sun’s energy back into space.
Be sure you understand the world distribution of temperatures. ocean currents, continents, altitude,—it’s all in there. In particular,
note how things change from January to July and how ocean currents and continents shift the patterns of isotherms.
Understand daily and annual temperature variations. Daily variations are obviously driven by the Sun’s position in the sky and
understand how it works. In particular, why the maximum temperature isn’t when the Sun is directly overhead, but a bit later.
Note the three factors that affect the magnitude of daily temperature changes: variations in Sun angle, location relative to an ocean
(and winds), and clouds.
Depending on what the weather is doing on a given day, these nice daily variations may go right out the window. If a cold front rolls
through, then the maximum and minimum temperatures will occur at a time determined by the front’s passage.
The annual temperature variation also has a funny lag, in that the maximum temperatures tend to occur weeks after the Sun is at its
highest position in the sky. The reasons are sort of similar, though not really, since heat transport from the tropics is a bigger issue.
Understand how a liquid-in-glass thermometer, and a bimetal strip work.
explanation, but the simple physics of it are good to understand.
We’re not looking for a detailed quantum mechanical
Understand why thermometers must be placed inside an instrument shelter
Know the Fahrenheit, Celsius, and Kelvin temperature scales and how to convert from one to the others.
Heating degree days, cooling degree days and growing degree days are all useful for understanding what the heck the guys in the
back of the newspaper are talking about. There are about a billion of these kinds of statistics which will allow you to determine the
growth rate of lettuce, insects, and various other lower life forms.
The heat index is generally a lousy indicator of how hot it is. Tell someone in Phoenix that your heat index was 110 the other day and
they’ll laugh at you. People in Minneapolis don’t talk about wind chill. It’s just too depressing. Note that neither wind chill nor the
heat index has anything to do just how quickly a brick will warm up or cool down—they’re really just human-based misery indices
that allow people from Alabama and Alaska to quantify just how much more their weather stinks than yours. See
http://www.nws.noaa.gov/om/windchill/index.shtml for an explanation. And no, I do not expect you to memorize these suckers! .
Know how to contour an isobar map
Know how to get weather information from a map given symbols and weather data