OVERVIEW:

We will discuss the size, scale, and components (including the Earth) of the Solar System; the solar constant and electromagnetic radiation (the mechanism by which the Sun heats the Earth); the orbital oddities and axial tilt of the Earth (seasonal forcing mechanisms); the rotation of the Earth and the Coriolis acceleration; the "3-cell" theory and associated wind belts; and finally, the Polar Jet.

OUTLINE:

1. Some important definitions:

1.1. Meteorology: The study of the dynamical, physical, and chemical condition of the atmosphere.

Micro-meteorology:	.le. 10 kilometers.
Meso-meteorology:	.ge. 10 to .le. 1000 kilometers.
Synoptic meteorology:	.ge. 1000 kilometers.

1.2. Weather: The observable consequences of the state of the atmosphere, in terms of:

Wind - Direction (frommey; dT) and speed (knots, m/s).

Visibility - the greatest distance that can be seen along the horizon circle (miles, meters).

Present weather - precipitation and surface-based phenomena, such as fog and haze.

Sky condition - describes the dome-shaped region above the horizon in terms of cloud type and amount, or vertical visibility in the case of dense fog.

Temperature (dC).

Dew point - the temperature to which the atmosphere must be cooled at constant pressure and moisture content in order to induce saturation (dC).

Pressure - the weight of the atmosphere above a unit area of the Earth's surface (hPa or millibars).

1.3. Hypothesis: An initial scientific "guess" about how a given physical process occurs, usually based on logical deductions from first principles and a knowledge of how similar processes occur.

1.4. Theory: An experimentally confirmed, well-documented scientific explanation for a given physical process. A "theory" is an explanation, and is synonymous with a "law of nature." Some types of theories are:

1.4.1. Descriptive: Physical principles applied to explain observations

1.4.2. Kinematic: Purely mathematical.

1.4.3. Dynamical: Derived from "first" physical principles, such as Newton's Laws. (Also mathematical.)

2.1. The Solar System began as a cloud of gas and dust more than 10 billion years ago. The protocloud contained gas that had previously been part of other stars, and was enriched with elements heavier than hydrogen.

2.2. By some combination of light pressure and gravity, the cloud began to collapse. The shockwave from a "nearby" supernova may also have played a role in the collapse.

2.3. By about 5.5 billion years ago, the cloud had collapsed into a rotating disk with a large central bulge. The internal pressure in the cloud's central bulge, brought about by its own gravitational pull, caused the process of thermonuclear fusion to begin. The Sun was born. The ignition of the young Sun drove most of the ices out of the inner Solar System. Gravitational eddies in the cloud's disk began to coalesce into smaller bulges -- which became planetesimals -- asteroid-sized bodies of undifferentiated ice and dust.

2.4. By about 4.5 billion years ago, the planetesimals had collided and formed the major planets (including the Earth) and the asteroids. Some of the protocloud's ices became the massive atmospheres of the planets of the outer Solar System. Thus the outer Jovian planets became gas giants. The inner terrestrial planets remained small, rocky bodies with thin atmospheres. The comets -- the remainder of the protocloud's original ices -- were driven far beyond the orbit of Pluto, and formed the Oort Cloud.

2.5. The Moon was created when a Mars-sized body collided with the partly differentiated primordial Earth. The collision caused a huge volume of relatively light, continental-type rock to be thrown into orbit around the Earth, where it eventually coalesced into an independent body: The Moon. The Mars-sized body was completely destroyed and absorbed into the primordial Earth.

3.1. The four terrestrial inner planets of the Solar System are Mercury, Venus, Earth, and Mars. The Earth is the largest of these in terms of mass and volume, and also has the highest mean density. The Earth is also the only terrestrial planet in the inner Solar System with a significant moon. Mars has two moons, but these are tiny, asteroid-sized bodies. Pluto (the outermost planet) is also sometimes classified as a terrestrial planet, in spite of its tremendous distance from the Sun. Pluto has a large moon of its own called Charon.

3.2. The four Jovian outer planets of the Solar System are Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest of these in terms of mass and volume. Neptune has the highest mean density. All four have complex systems of moons and rings. Jupiter has four moons that, if moved into the inner Solar System, would qualify as terrestrial planets: Europa, Callisto, Ganymede, and Io. The same can be said for Saturn's Titan and Neptune's Triton.

3.3. The asteroids -- also called the minor planets -- lie in a belt between the inner planets and the outer planets. There are probably hundreds of thousands of these rocky bodies, the largest of which is Ceres -- approximately 1000 kilometers in diameter. Some are in highly eccentric orbits with low points beneath the orbit of Earth and high points above the orbit of Mars: These are called Earth-crossing asteroids.

4.1. The Earth's original atmosphere consisted of a thin veil of gases that accreted onto its surface from the original protocloud. This was lost following the Solar ignition.

4.2. The Earth's second atmosphere came from volcanic eruptions and outgassing of volatiles while the proto-Earth slowly solidified. This was a Jupiter-like atmosphere, and completely poisonous to anything living on the Earth now.

4.3. Life began in the early oceans, as early as 300 million years after the early Earth cooled. The earliest life was a primitive cell that learned how to carry out photosynthesis -- a plant. These plants slowly built up the free oxygen in the Earth's atmosphere.

4.4. The Earth's current atmosphere is chemically unstable, and is a product of the living things on the planet. Plants consume carbon dioxide and release oxygen -- holding the oxygen level much higher than it would be without plants. Animals consume the oxygen and return carbon dioxide. Without plants, all the oxygen in the atmosphere would quickly be lost via chemical reactions with rocks. The Earth's atmosphere would come to chemically resemble Mars' atmosphere.


Current chemical composition of the Earth's atmosphere.

COMPONENT	PERCENTAGE (MASS)
Nitrogen	~76
Oxygen	~23
Water Vapor	~1 - 4
Trace Gases (Mostly Argon)	~1

5.1. Energy escaping from the Sun's visible surface (called the photosphere) is primarily in the form of visible and ultra-violet radiation. This radiation streams outward equally in all directions, arriving at the orbit of the Earth 8 3/4 minutes later.

5.2. The spherical shape of the Earth causes solar radiation to strike its surface at an angle that varies with latitude. Low latitudes (near the equator) receive radiation head on. High latitudes (near the poles) receive radiation at an oblique angle.

5.3. The mean Solar Flux at the top of the atmosphere is about 1350 watts/m², but the slope of the Earth's surface causes this radiation to be distributed over a small area of the Earth's surface near the equator, and a large area of the Earth's surface near the pole. Additionally, sunlight arriving at low latitudes travels through the relatively thin portion of the atmosphere that lies directly overhead, while sunlight arriving at high latitudes travels along a line almost parallel to the Earth's surface before reaching ground level, and is therefore attenuated. Bottom line: The polar surfaces receive about half the amount of solar radiation per unit area as the equatorial surface.

5.4. Albedo, incoming and outgoing radiation.

5.5. Internal transformations and storage of energy.

6.1. The Earth's orbit is slightly eccentric, thus the Earth is closer to the Sun at some times and further from the Sun at others.

6.1.1. The low point of the orbit is called the perihelion -- when the Earth is 147 million kilometers from the Sun. This currently falls on January 3rd.

6.1.2. The high point of the orbit is called the aphelion -- when the Earth is 152 million kilometers from the Sun. This currently falls on July 3rd.

6.2. The axis of the Earth is currently tilted 23 1/2 degrees from "vertical." This causes the northern hemisphere to be tilted toward the Sun during half the year, and tilted away from the Sun the other half of the year. This is the mechanism responsible for the seasons.

6.2.1. Spring Equinox: March 21-22. The Sun rises in the east and sets in the west. For both hemispheres, the day is the same length as the night.

6.2.2. Summer Solstice: June 21-22. The Sun rises in the northeast and sets in the northwest. For the northern hemisphere, the day is longer than the night.

6.2.3. Autumnal Equinox: September 21-22. The Sun rises in the east and sets in the west. For both hemispheres, the day is the same length as the night.

6.2.4. Winter Solstice: December 21-22. The Sun rises in the southeast and sets in the southwest. For the northern hemisphere, the day is shorter than the night.

6.3. The Arctic and Antarctic Circles: 23 1/2 degrees equatorward from the north and south poles, respectively. The Arctic Circle is the latitude above which the Sun does not rise above the horizon between the Autumnal Equinox and the Spring Equinox. The Antarctic Circle is the latitude above which the Sun does not rise above the horizon between the Spring Equinox and the Autumnal Equinox.

6.4. The Tropic of Cancer and the Tropic of Capricorn: 23 1/2 degrees poleward of the equator in the northern and southern hemisphere, respectively. The Tropic of Cancer (23 1/2 degrees north) is the latitude at which the Sun is directly overhead on the Summer Solstice, and the Tropic of Capricorn (23 1/2 degrees south) is the latitude at which the Sun is directly overhead on the Winter Solstice.

7.1. The Earth's surface becomes very hot beneath the intense, concentrated solar energy falling on the equator.

7.1.1. The lowest part of the atmosphere is heated from below by the Earth's surface, causing it to expand upward and become thinner. Strong upward vertical currents are established in the atmosphere above the equator.

7.1.2. The thin air is not as heavy as the cooler air to the north and south of the equator, thus a permanent area of low pressure is established on the equator. (Pressure is the weight of the atmosphere on a unit area of the Earth's surface, as in "pounds per square inch.")

7.2. The Earth's surface gets very cold near the poles, where the solar energy strikes at a sharp angle.

7.2.1. The cold polar surface cools the lower atmosphere. A kilogram of cold air fills a smaller volume than a kilogram of hot air (think of hot air expanding, as in a hot air balloon), so the volume of the polar atmosphere shrinks and the air becomes thicker.

7.2.2. The polar atmosphere's shrinking volume sets up downward vertical currents -- just the opposite of the situation above the equator. Air sinks above the poles.

7.2.3. The thick air at the poles is heavier than the warmer air at lower latitudes, thus a permanent area of high pressure is established on the north and south poles.

7.3. If the Earth did not rotate on its axis and if it had a uniform surface, a relatively simple flow would set up between the polar highs and the equatorial low. One can imagine a closed circuit:

7.3.1. Air sinks at the poles due to cooling (because only weak sunlight reaches the Earth's surface there).

7.3.2. High pressure sets up on the poles, and the cold air is pulled along the Earth's surface by gravity toward the low pressure at the equator.

7.3.3. Because of the intense sunlight and high surface temperature at low latitudes, air arriving at the equator is heated from below. The heated air expands and rises.

7.3.4. The heated air travels poleward in the upper atmosphere and cools as it reaches the poles. From there it sinks vertically, completing the circuit.

8.1. "Coriolis" or the Horizontal Deflection Force (HDF) causes flow to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere.

8.1.1. The HDF is caused by the imbalance between translational speed and angular speed.

Translational speed is the speed of an object in miles per hour. The Earth rotates on its axis in an eastward direction once every 24 hours. The circumference of the Earth at the equator is about 24,000 miles, so an object fixed to the Earth's surface at the equator is moving eastward at 1000 miles per hour (x 24 hours = 24,000 miles). But the circumference of the Earth at 45 degrees north is only about 17,000 miles, so an object fixed to the Earth's surface at 45 degrees north is only moving eastward at about 710 miles per hour (x 24 hours = ~17,000 miles).

Angular speed is the speed of an object in degrees per hour. The Earth rotates through 360 degrees in 24 hours, so the angular speed of the Earth is 15 degrees per hour. Because the Earth is solid, this is the same at all latitudes.

8.1.2. Air moving from the north pole to the equator is deflected toward the west. Let's say we have a "parcel" of air just south of the north pole. Its angular speed is 15 degrees per hour, but its translational speed is very small. (The circumference of the Earth at the pole is very small, so the parcel barely has to move at all to make a complete circuit each day.)

If the parcel starts moving southward, it will encounter land surfaces that are moving eastward at higher translational speeds -- the circumference increases as you move from the pole to the equator, so the eastward translational speed of the Earth's solid surface increases.

The parcel is not attached to the Earth's surface, and has the smaller eastward translational speed that it began with at the higher latitude, so the land surface beneath it outruns it.

The solid Earth has a constant angular speed, and the air parcel has a constant translational speed.

8.1.3. Air moving from the equator to the north pole is deflected toward the east. Let's say we have a "parcel" of air on the equator. Its angular speed is 15 degrees per hour, and its translational speed is 1000 miles per hour.

If the parcel starts moving northward, it will encounter land surfaces that are moving eastward at lower translational speeds -- the circumference decreases as you move from the equator to the pole, so the eastward translational speed of the Earth's solid surface decreases.

The parcel is not attached to the Earth's surface, and has the higher eastward translational speed that it began with at the lower latitude, so it outruns the land surface.

8.2. The simple Hadley cell circulation (described above) is broken up into three small cells in each hemisphere. This is called (appropriately) the Three-Cell Circulation.

8.2.1. Permanent low pressure is on the surface at the equator and 60 degrees north. Permanent high pressure is on the surface at 30 degrees north and on the pole (90 degrees north).

8.2.2. Wind flows from the Polar High (90 degrees north) southward to the Subpolar Low (60 degrees north), and the HDF deflects it to the west. These permanent winds are called the Polar Easterlies. (Winds are named according to the direction they come from.)

8.2.3. Wind flows from the Subtropical High (30 degrees north) northward to the Subpolar Low, and the HDF deflects it to the east. These permanent winds are called the Prevailing Westerlies.

8.2.4. Wind flows from the Subtropical High southward to the Equatorial Trough, and the HDF deflects it to the west. These permanent winds are called the Trade Winds.

9.1. A sample 300 millibar polar projection chart is here.

9.2. A sample North American 300 millibar chart with plots is here.

9.3. A close-up of the North American 300 millibar chart is here.

LAB:

1. Introduction to upper-air charts. (Analysis guides are here, here, and here.)

2. Analysis of 300-millibar isotachs; identification of the Polar Jet over North America.

HOMEWORK:

1. Read Lutgens and Tarbuck chapters 1 and 2, and chapter 7 (pp. 192 - 205).

2. Graphical interpolation exercise.

3. Be sure to bring all items in the "other required materials" list to the next meeting.