A Quick Look at the Atmosphere and Climate

The thin, light blue line around the Earth is about all there is of the Atmosphere.

Thermosphere – Very high temperature from absorbance of solar radiation. (But very little heat because of the near non-existence of air molecules to hold it!) Mesosphere -- Generally very cold and very thin air. Stratosphere – Relatively warm air because absorbance of solar radiation creates abundant ozone, which absorbs much energy. Troposphere -- ~90% of the air is in the troposphere. This is the only layer thick enough to breathe (at the bottom) and it is the layer in which all weather takes place.

Because of the confining pressure of the gas above, the bottommost part of the atmosphere (the troposphere) is the most densely packed. We have seen that air is compacted by immersion in water, but it is also compacted by immersion in itself. As you get higher – up in the mountains or in an airplane – the confining pressure is lower because there is less air above you. In very high mountains it can be difficult to breathe, and “mountain sickness” can result from lack of adequate oxygen. Airliners that fly above a certain level have pressurized cabins to ensure adequate breathing air, which does not exist outside the airplane. If the cabin pressure is lost, little masks attached to pressurtized oxygen drop out of the overhead area so you can keep breathing. (What, didn’t you listen to the flight attendant?)

1000+ km ~80km ~50 km ~12 km

~30km

99% of air!

Ground Surface

The Sun radiates electromagnetic energy across the entier spectrum, from high-energy/short wavelength cosmic and x-rays to low energy/long wavelength radio waves. Most of these wavelengths do not reach even the top of the troposphere because they are absorbed and ionize atoms in the upper layers – particularly in the “ionosphere” (most of the thermosphere and the upper part of the mesosphere). This is what creates ozone, for example. Ozone is particularly good (but not perfect) at absorbing UV radiation, so only a little of that gets to the troposphere. Visible light (almost all wavelengths), about half of the Infrared (heat – IR) and a little UV are all that reach the troposphere, but that layer is almost perfectly transparent to them, so they pass right through and reach the surface. (Clouds, dust, and gasses absorb, on average, about 20% of it, and reflect about 25% of it back to space, so on average about 55% of the total incoming energy actually makes it to the ground). That 55% that gets to us is about half visible light and about half IR, with a tiny (but potentially dangerous) smidgeon of UV. About 5% of that is reflected from the Earth’s surface, leaving about 50% for us. This energy is absorbed by the Earth and re-radiated. All of what is radiated is heat – IR.

Air is transparent to light, but not to IR. The various gasses in the atmosphere are very good at absorbing IR, which is why only about half makes it through the atmosphere coming in. Some of this IR passes through the troposphere but most is retained, warming the air when the sun is actually shining. Of course, when the sun goes down the absorbance and re-radiation stop, but enough heat remains in the atmosphere to keep us from freezing to death before the sun comes back! Some gasses, CO2 in particular, are much better at absorbing the heat than others – they are less transparent to it. These are called greenhouse gasses. The dominant gasses in the atmosphere, N2 in particular, retain some heat but not nearly as mush as the greenhouse gasses. Glass is also transparent to light but not to heat. Light coming through the closed windows of a car is absorbed by the interior, re-radiated as heat, which can then not get back out. It builds up inside making the car hotter and hotter until its exterior radiates heat at the same rate as light is absorbed. Obviously the amount of heat the atmosphere absorbs will depend upon how much light the Earth below it absorbs. So why is it hot at the equator and cold at the poles?

15° 15° 30° 30° 60° 60°

90°

The diagram below shows a part of the same sheet of black construction paper taken with light from a source beyond it taken at four different angles. When light reflects off a surface the angle at which the reflected light rays reach our eye is the same as the angle of incidence. Notice that the paper looks less black as the angle of incidence decreases. Why is this?

You have certainly noticed another manifestation of this phenomenon driving down the road. Far off ahead of you there appears to be water on the road – a mirage. No matter how fast you drive that “water” stays the same distance away from you. Most people think this has something to do with the hot pavement, but it is simpler than that. (The hotter the pavement the more shimmery the “water”, but that’s not what creates the illusion in the first place.) Next time you see this notice that as a car approaches you from the other side of the “water” you see a blob of whatever color the car is crossing the “water”. The same thing happens as you approach a sign – you get a blob of yellow if it’s a caution sign or red if it’s a stop sign. This is a clue about what’s happening and what you are seeing. What causes this illusion?

Minimum amount of reflectance

(90°)

More reflectance

(60°) Even more reflectance

(45°) A lot of reflectance

(30°) ~100% reflectance

(~0°)

As the angle of incidence of light on a surface gets smaller the amount of light reflected gets greater. There is, in fact, a direct correlation between the two – a mathematically definable relationship. The “water” that you see on the highway ahead of you is really a reflection of the sky. The poor, blobby reflections of approaching cars or distant signs is the best observational evidence supporting this fact. The edge of the “water” is at that distance ahead of you where the angle of incidence is such that there is 100% reflectance of the light off the surface. 100% of the light striking the road and coming to your eye at that angle of incidence is reflected. Similarly, the paper in the earlier slide looked lighter and lighter as the angle of incidence declined because you see more and more of it reflected at the lower angles.

~100% reflectance

15° 15° 30° 30° 60° 60° 90°

4 lux 9 lux 31 lux 64 lux

We can semi-quantify this relationship with a simple experiment. We could do it better with a more controlled one. With a light meter that reads in lux (picture at left) in a dark room I did a very rough experiment, the result of which results are shown below. Notice that the meter reads higher as the angles of incidence and reflection decrease – more light is being reflected. (Shining the light directly on the light meter from the same distance as the other readings were taken gives a value of about 350 lux so not much of the light is reflecting off the black surface even at low angles.)

So what? What does how much light a surface reflects have to do with climate?

Well, reflected light doesn’t really have anything to do with it, but ask yourself this: what happens to the light that isn’t reflected? A certain amount of light leaves the flashlight. Some of it reflects off the surface and returns to the light meter. Clearly the amount is not the same for different angles of incidence/reflection so something else must be happening to the light – happening in inverse relationship to the reflectance.

What might that other thing be?

Remember this experiment?

What happened to the missing 426 lux that didn’t reach the sensor through

the red filter?

Similarly, what happened to all the red light that didn’t reach the

balloon and my hand 3m down in the water?

Again, similarly, why does the balloon look red? Sunlight is white, not red. When we look at the balloon all we see is red. What happens to the other colors in the sunlight?

In every case the light is absorbed – it excites the molecules of the substance absorbing it and is then radiated by those substances as heat. And remember – the more light something absorbs the more heat it radiates. A white car interior doesn’t get as hot as a black one if it’s closed up and in the sun. The white reflects most of the light (which passes back through the glass as easily as it got in in the first place) and the black absorbs it and converts it to heat (which does not escape through the glass.) So let’s propose that the temperature piece of climate may have something to do with absorbance. A place that absorbs more light is able to radiate more heat into the atmosphere, where it can be stored as heat in gas molecules. A place that absorbs less light will create less heat to radiate into the atmosphere for storage.

Ray path

Albemarle Is. (no shadow) Ray

path

Chippewa Falls, WI (1m shadow)

Center of Earth

Equa

tor (

0°)

30°

45°

New Iberia, LA (.5775m shadow)

Ray path

Remember our flat- Earth

hypothesis?

Angle of incidence = 45°

Angle of incidence = 60°

Angle of incidence = 90°

Angle of incidence = 0°!

Albemarle Is.

Chippewa Falls, WI

Equa

tor (

0°)

New Iberia, LA

Angle of incidence = 45°

Angle of incidence = 60°

Angle of incidence = 90°

Angle of incidence = 0°!

North Pole

15° 30° 60° 90°

More More Reflectance = Absorbance = Colder Air Warmer Air

This is why the climate is hot at the Equator and cold at the poles. The higher angle of incidence at the Equator means that less light is reflected and more absorbed. More absorbance means more heat to transfer to the air. The lower angle of incidence at the poles makes for more reflectance and less absorbance. This means there is less heat generated and radiated to the air. At every latitude the amount of heat generated is intermediate – there is a gradient of temperature between the two extremes.

Albemarle Is.

Chippewa Falls, WI

Equa

tor (

0°)

New Iberia, LA

North Pole

More More Reflectance = Absorbance = Colder Air Warmer Air

Here is the point more graphically, in two senses.

Max Absorbance of Light = Max Radiation of Heat

Min

Abs

orba

nce

of L

ight

=

Min

Rad

iatio

n of

Hea

t

0° (equator) Latitude 90° (pole)

Amou

nt

While we’re playing with balloons, think about these. Why do they fly? You’ve been told why – because warm air rises. But why does warm air rise? As we add energy to air the molecules have two ways of dealing with the additional energy. As we discussed earlier the electrons can jump up a shell, but heat will not do this – its energy is inadequate.

What is added in this case is, actually, just heat. A gas burner and fan are used to heat the air at what will eventually be the bottom of the balloon and blow it into the envelope. As the air

inside gets warmer the balloon begins to rise and finally takes off. What’s happening?

As the air gains heat its molecules absorb that heat, causing them to move faster. The electrons spin faster, the atoms get more “jiggly” and they push against each other more, forcing more space between them. You can’t see it, but cooler air leaks out the bottom of the balloon because there is no room for it as the warmer air expands. That’s right – the warm air begins rising even before the balloon inflates and gravity forces it upward in the same way it forces a boat to rise when you step out of it – by pulling colder air beneath it. In this case though the balloon envelope keeps the warm air from Going very far and the open bottom lets the cooler air escape. Eventually there is less air (fewer air molecules) in the balloon than outside of it! The air inside is, in other words, less dense than The air outside. Now as gravity pulls the denser outside air beneath the balloon it also forces the balloon to rise.

The result of the differential heating of the atmosphere is that it undergoes convection. The air would rise where it was warmest and sink where it was coolest, moving from one end of that system to the other to replace the air “ahead” of it. Slightly cooler air would be flowing in to replace rising warm air at the Equator, slightly cooler air flowing in to replace that, and so on back to the pole.

Gravity keeps the air from escaping to space. At the same time, as the air rises to the top of the troposphere the pressure decreases and the air cools – like the air coming out of a spray can.

Warmest Air Rises at Equator

Coldest Air Sinks at

Poles

Equator

The convection would work as shown – transferring excess heat

at the equator to the poles. (Much would radiate into space

from the winds aloft as well.)

If the Earth did not rotate there would be a single convection cell in each hemisphere, moving air from the equator to the poles and back

like this:

Equator (0°)

Because the Earth does rotate the Coriolis effect causes the winds to follow a different path. They are

deflected to the right in the Northern Hemisphere and to the left in the Southern. By the time they have

crossed about 30° of latitude they have turned to move almost parallel

to the latitude lines.

This is what causes “prevailing winds”. At a given latitude the wind

is more likely to blow from one particular direction rather than

others.

~30° N

~30° S

~60° N

~60° S

90° S

90° N

EASTERLY

(TRADE)

WINDS

WESTERLY WINDS

WESTERLY WINDS

POLAR FRONT

POLAR FRONT

DOLDRUMS

Equator (0°)

The deflection also established three convection cells instead of

only one.

Where air rises the atmospheric pressure tends to be low and

where it sinks the pressure tends to be high.

Rainy belts go along with low pressure zones, deserts with

high pressure zones.

~30° N

~30° S

~60° N

~60° S

90° S

90° N Cold Desert

Cold Desert

Warm Desert

Warm Desert

Cool Coniferous Forest (Taiga)

Cool Coniferous Forest (Taiga)

Hot Tropical Rainforest

HIGH

HIGH

HIGH

HIGH

LOW

LOW

LOW