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 4-11 and 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 30%. 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. 61).
Check out figure 3.28 and be sure you understand it using the discussion on p. 69-70. 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 Winnipeg, Manitoba and Vancouver, British Columbia 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 maxiumum 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! .
Moisture and Atmospheric Stability
The continuous exchange of water between the oceans, the atmosphere, and the continents is called the hydrologic cycle or water
cycle. See the figures in the text, power point and the one I drew on the blackboard, and understand how this works. It is, of course,
powered by the energy of the Sun.
Transpiration is the process by which plants release water to the atmosphere—plant sweat, in essence.
Estimates over N. America suggest that 6 times more water is carried through the air than through all the rivers combined. A cool,
though perhaps utterly useless fact. Here’s a more useful one: over continents, precipitation exceeds evaporation; over oceans, the
opposite is true.
A calorie is the amount of head required to raise the temperature of one gram of water one degree Celsius. Another, slightly less
useless fact. The Calories you see quoted on food packages are actually kilocalories. The capital “C” is supposed to distinguish the
two.
Under certain circumstances, a calorie of heat will not raise the temperature of an object at all. This happens when the object
undergoes a change of state—a phase change. Know the terms melting, freezing, evaporation, condensation, sublimation, and
deposition. Know how much heat is involved in each process and whether that heat is absorbed or released.
Humidity is a general term used to describe how much water is in the atmosphere. The terms you should know are: absolute
humidity, mixing ratio, vapor pressure, relative humidity, and dew point.
The vapor pressure of any atmospheric constituent (usually water vapor in this course, but in reality, everything has a vapor pressure,
even iron) is that part of the total atmospheric pressure attributable to that component. This is also called the partial pressure of
water vapor. In equilibrium, the same number of water molecules arrive on the surface of a puddle as leave it, at which point we say
that the air is in saturation, and the vapor pressure of water is then the saturation vapor pressure.
The saturation vapor pressure is temperature dependent—at higher temperatures more water can exist in the vapor phase. This is the
origin of the statement “warm air holds more water than cold air”. It really has nothing to do with the air holding on to anything, since
the nitrogen and oxygen molecules pretty much don’t care what water does.
Relative humidity is the ratio of the actual amount of water vapor in the atmosphere to the amount of water required for saturation.
(what is there divided by what could be there) Pick your units for one and make sure they match the units of the other. 100%
humidity doesn’t necessarily mean that there’s a lot of water around. If it’s really cold, 100% humidity may not involve much water.
Relative humidity changes in only two ways: either by changing the water content or by changing the temperature. If the amount of
water in the atmosphere increases, RH increases; if the amount of water decreases, RH decreases. If the temperature goes up, the
saturation vapor pressure increases, so the RH drops; if the temperature drops, the saturation vapor pressure decreases, so the RH
increases. Be sure you know how to calculate relative humidity.
Relative humidity changes naturally as a result of: daily temperature changes resulting from the Sun’s changing position in the sky,
temperature (and moisture) changes as the result of air moving horizontally from one place to another, and temperature changes
caused by vertical motions in the atmosphere. .
The dew point temperature or dew point is widely used by meteorologists. If you cool an air mass until it reaches saturation, the
temperature at which that occurs is the dew point. Unlike relative humidity, the dew point is a measure of the actual water vapor
content of the atmosphere. The dew point is not a linear representation of water vapor content; a dew point of 25°C means the
atmosphere contains twice as much water vapor as a dew point of 15°C and four times as much water vapor as 5°C.
Humidity can be measured using a psychrometer, a hair hygrometer, or an electric hygrometer (usually a hygristor—the electrical
resistance changes with humidity).
Adiabatic temperature changes. If you don’t get this, you’ll never get clouds. If you raise an air parcel off the ground, it will cool
by expansion at a rate of 10°C/km—the dry adiabatic lapse rate—until the air parcel reaches saturation and cloud formation begins.
The level at which condensation begins is the lifting condensation level. Once condensation begins, we have a source of heat as the
latent heat of condensation is released from the air parcel. Because it depends on the amount of water being condensed, the wet
adiabatic lapse rate varies between about 5°C/km and 9°C/km.
Know the four lifting processes: orographic lifting, frontal wedging, convergence, and localized convective lifting (unequal
surface heating). .
A rain shadow desert is the result of orographic lifting and being on the leeward side of a mountain.
The real weathermaker is atmospheric stability. If the atmosphere is sufficiently stable, there aren’t going to be clouds around no
matter how hard you try. Be sure you understand absolute stability (environmental lapse rate less than the wet adiabatic lapse rate),
absolute instability (environmental lapse rate greater than the dry adiabatic lapse rate), and conditional stability (environmental
lapse rate between the wet adiabatic lapse rate and the dry adiabatic lapse rate). Be sure you understand what a temperature
inversion is.
and understand what kinds of clouds you would expect on stable vs. unstable days.
Instability is enhanced by: intense heating of the lowest layer of the atmosphere, heating of an airmass from below as it passes over a
warm surface, general upward movement of air as caused by any of the lifting processes mentioned, and radiational cooling from
cloud tops.
Stability is enhanced by: radiational cooling of the Earth’s surface after sunset, the cooling of an airmass from below as it passes over
a cool surface, and general subsidence (sinking motion) within an air column.
Forms of Condensation and Precipitation
For condensation to occur, the air must be saturated and there must be a surface of some kind onto which the water vapor can
condense. Near the ground, this can be grass, a tree, or your hair. Up higher in the atmosphere, you need condensation nuclei (CN).
Without CN, that air parcel will have to be dramatically supersaturated to condense anything out. CN are microscopic smoke, dust,
salt, and acid particles. The most effective ones are hygroscopic (water-loving).
The most important cloud-forming process is adiabatic cooling as the air rises.
Collision-coalescence: get a few monster drops in a storm and they start grabbing up the little ones. This process occurs in warm
clouds where there are no ice crystals anywhere. This one is pretty simple. You get a monster droplet that starts with a salt particle or
something similarly hygroscopic so it attracts plenty o’ water. Then that drop starts picking up other droplets. The process gets out of
control if our megadrop starts falling—then it picks up other little ones on the way down. Updrafts really help this process because
they recirculate the bigger drops through the cloud so they can suck up more little droplets. for why rain drops can’t get much bigger
than 5 mm in diameter.
Air pressure—understand units; barometer & how it works 1 newton/m2 = 1 pascal; 100 Pa = 1 millibar, mb;
how does a mercury barometer work?—there’s actually a vacuum above the mercury, so it’s the air pressure
that supports the column, not the vacuum that sucks it up the tube.
Airplanes use an altimeter based on a barometer to measure altitude.
Barometric measurements taken at different locations are corrected for altitude so we can see horizontal
variations in pressure without having to deal with the complexities of mountains, hills, etc.
Ideal gas law: pressure = temperature x density x constant. So both temperature and density can affect air
pressure. In the atmosphere, density is often more important, so high pressure areas are usually associated with
higher density.
The pressure at any given altitude is equal to the weight of the air directly above that point. So as you go up in
the atmosphere, there’s less air above you, and therefore the pressure drops. The boiling point of water is the
temperature at which the vapor pressure of water equals atmospheric pressure—since the atmospheric pressure
is lower in, say, Denver, than here, that is why the boiling point of water is only 95°C in Denver, while it’s
100°C here.
Wind is the result of horizontal differences in air pressure, and blows from high pressure to low pressure.
Winds on the Earth’s surface are dictated by the balance of three forces: the pressure gradient force (to get
things started), the Coriolis force, and friction.
Pressure gradient force—a big pressure drop over a short distance will generate much stronger winds than a
small drop over a long distance. Always directed at right angles to the isobars.
Isobars—lines of equal pressure on a map. Understand how these work!! The tighter the lines, the higher the
pressure gradient, and the stronger the winds will be in that area
The underlying cause of pressure differences is (ultimately) unequal heating of the surface.
Sea breeze—understand how daytime heating generates a breeze from the sea to the shore, and nighttime
cooling can generate a breeze in the opposite direction. Understand the circulation that is set up as a result.
Hydrostatic equilibrium is balance between pressure and gravity, and explains why the high pressure at the
Earth’s surface doesn’t just blow the atmosphere off into space.
Cyclonic flow is the air flow around a cyclone—a low pressure system: counterclockwise in the Northern
Hemisphere and clockwise in the Southern Hemisphere.
Anticyclonic flow is the air flow around an anticyclone—a high pressure system: clockwise in the Northern
Hemisphere and counterclockwise in the Southern Hemisphere.
The consequence of horizontal convergence at the surface in a low pressure system means that the air must rise
at the center. So a low pressure system tends to draw air in at the surface, lift it up, and spit out out at the top.
It’s like a fountain. High pressure systems do the opposite: they draw air in at the top, push it down in the
center, and blow it out at the surface. Lows have upward movement (lift), while highs are associated with
downward movement (subsidence). Both lows and highs are 3-dimensional systems, so if the divergence or
convergence aloft aren’t there, the high or low won’t be very strong and may not last long. This is called upper
level support, and is very important in intensifying or weakening cyclones and anticyclones.
Because cyclones lift air, they tend to be associated with rain and poor weather, while highs tend to suppress
cloud formation and lead to clear, sunny weather.