Quiz Format
In-class Quiz #4 will be administered after student weather forecast validations, near the beginning of the period set aside by the University for a final exam, which is 1:30 pm to 4:00 pm. It will be closed book, closed note, and closed computer (and other electronic devices).
You should be able to complete the quiz in 20-30 minutes, but if you need more time you can take it. Like the other in-class quizzes, it could be worth 3.33% of your course grade (if it is retained after the worst of your four in-class quizzes is automatically dropped.)
The question(s) will be short answer, short essay, and/or simple meteogram or weather map analysis and interpretation. The question(s) might test nothing more than basic factual knowledge, but they might also test conceptual understanding, reasoning
ability, and perhaps your ability to communicate your understanding and reasoning.
When you finish the quiz, you will be asked to complete a standard course evaluation.
Topic Coverage
Topics eligible for coverage on the quiz consist primarily of topics addressed in lecture, in lab sessions (not including lab activities themselves but including background information provided for them), and in reading assignments, mostly since Quiz #3.
(The mechanics of preparing and submitting forecasts for the ongoing forecasting assignment will not be addressed by this quiz.)
Some topics addressed in lecture and/or lab include (follow links to handouts and supporting materials for details).
- Proof, Confirmation, Disconfirmation, and Disproof: Reasoning from Evidence in Science
- Drawing and Interpreting Contour Maps
- Gradients and Temperature Advection [PDF file]
- Forces that push on air and determine wind patterns
-
Some Important Points about Winds and Pressure
- Illustrations of the Coriolis effect
- Animation of the combined effects of the pressure-gradient and Coriolis forces alone
- winds aloft (where there is no friction with the earth's surface) blow so that the pressure-gradient force and the Coriolis force approximately balance each other
- these winds blow roughly parallel to pressure contours (isobars), rather than perpendicular to them, as one would expect if the pressure-gradient force (PGF) were the only force pushing on air
- Additional effect of the force of friction between wind and the earth's surface (relevant only near the earth's surface)
- friction slows the wind down, prevents the Coriolis force from turning the wind as much as it would without friction
- as a result, the wind blows across isobars at an angle, not parallel to them (PGF plus Coriolis) or perpendicular to them (PGF alone),
- General Circulation of the Atmosphere (global temperature, pressure, and wind patterns, especially at low latitudes)
- Figure: Hadley cells and trade winds (low latitudes)
- northeast trade winds (in the tropics in the Northern Hemisphere) and southeast trade winds (in the tropics in the Southern Hemisphere) blow very persistently and converge near the equator (the Intercontinental Convergence Zone, or ITCZ)
- where air near the earth's surface converges, it must go somewhere--namely, up
- where air is forced to rise, it expands under lower pressure, cools, and clouds form
- the result is a relatively persistent band of cloudiness (daily thunderstorms, in fact) around the globe near the equator, visible on infrared satellite images most of the year
- these clouds produce rainfall that by the end of the year totals more than almost anywhere else on the earth; the earth's tropical rainforests are located there
- Weather Patterns at Midlatitudes: Connecting the Dots (PDF file)
- relation between temperature in the lower troposphere (near the earth's surface) and pressure in the middle and upper troposphere ("aloft"), relative to other areas at the same altitude
- what pattern of winds aloft that the pressure pattern aloft creates (polar jet stream, directly above the polar front)
- features of the polar jet stream (troughs and ridges)
- location of midlatitude cyclones (a type of storm) in the jet stream pattern
- just ahead of "tongues" of cold air, "protruding" equatorward, just behind "tongues" of warm air "protruding" poleward, along the polar front
- characterized by bands of clouds along cold and warm fronts
- Weather patterns at midlatitudes: connecting more dots
- the polar front, a feature of the temperature pattern in the lower troposphere at midlatitudes, exists because the sun strikes the earth at lower angles (making it less intense) at higher latitudes than at lower latitudes
- low sea-level pressure areas tend to develop along the polar front
- the pressure difference between the center of a low pressure area and surrounding areas pushes air into motion toward the low pressure center, but the Coriolis effect (in which the earth rotates beneath the moving air, making it appear to deflect to its right) and friction modify the wind pattern
- the wind pattern around low-pressure areas near the surface is (in the Northern Hemisphere) an inward, counter-clockwise spiral
- these winds blow in a region of large temperature gradient (the polar front), so temperature advection occurs on the west and east sides of the low pressure area
- on the west side of a low, winds blow from the north, bringing cold air southward
- on the east side of a low, winds blow from the south, bringing warm air northward
- temperature advection by the winds blowing around a low pressure area causes the location of the polar front to shift, southward toward the equator on the west side of the low and northward toward the pole on the east side of the low, creating "tongues" of cold and warm air, respectively
- the advancing cold tongue creates a cold front (a zone of large temperature gradient that moves from colder toward warmer air), and the advancing warm tongue creates a warm front (a zone of large temperature gradient that moves from warmer toward colder air)
- both fronts emanate from the center of the low, at least at first (before the cold front catches up with the warm front and an occluded front forms, which emanates from the center of the low pressure area)
- air converges along fronts and is forced to rise, encounters lower pressure and expands and cools to form clouds
- hence, fronts are often characterized by long bands of clouds along them
- these clouds can often produce precipitation
- the collective pattern of temperature (including fronts as features of the temperature pattern), a low pressure area, counter-clockwise wind pattern around the low (in the N. Hemisphere), and cloudiness and sometimes precipitation along the fronts, is a type of storm called a midlatitude cyclone
- the jet stream aloft follows the polar front, so when the polar front develops wobbles as "tongues" of cold and warm air advance, so does the polar jet stream (creating troughs and ridges in the pattern aligned with tongues of cold and warm air in the troposphere below it)
- midlatitude cyclones therefore form along the leading edge of troughs in the jet stream (and hence at the trailing edge of tongues of cold air in the lower troposphere), at the trailing edge of ridges in the jet stream (and hence at the trialing edge of tongues of warm air in the lower troposphere)
- El Niño/La Niña and impact on weather patterns (PDF file)
- NOAA's El Niño/La Niña animations (real data)
- Visualization of observations of an El Niño event (real data)
- by adding (El Niño) heat to, or removing (La Niña) heat from, the lower troposphere in the central and eastern Pacific Ocean near the equator, El Niño/La Niña events modify the temperature contrast beween low and high latitudes, thereby modifying the location and intensity of the polar front at midlatitudes, which can modify the location and strength of the jet stream and hence the path and strength of midlatitude cyclones
