Introduction for Instructors



5 Jan 2012

Instructor Manual

USPS® Weather Course

(Wx 2008 and Revised Edition 2012)

This revised Instructor Manual has been prepared to assist in teaching the USPS® Weather Course. The course builds on a long USPS tradition of presenting a comprehensive meteorologically (scientifically) oriented course designed for recreational boaters. The 2012 Revised Edition of the Student Manual has both text and graphic additions and enhancements, but its basic structure and contents remain the same as the Wx2008 course.

• The Weather Course can be taught with a mix of the original Wx2008 and the revised edition Wx2012 Student Manuals.

• The final closed-book examination remains the same.

This Instructor Manual is not be the traditional black & white hard copy with all the thumbnail slide images. Instead it is a part of the instructor CD that contains the PowerPoint Presentation with slides and complete notes. Anecdotal evidence indicates that more and more instructors are not using the hard copy thumbnail images, but rather review the slides and notes using PowerPoint. The idea is to reduce costs by not printing a hard copy version. If after trying this approach, instructors still prefer a hard b/w copy with the thumbnail images, we will format and print some. Please let us know what you think after teaching the course.

Three Appendices are a part of this manual. Two of them deal respectively with Upper-Air Charts and Sounding Analysis Diagrams. Instructors should read them. They are designed to provide instructors with additional background information to increase their comfort level in teaching various topics relating to atmospheric dynamics. The third relates to Clarifications and Corrections that pertain to the original Wx2008 Student Manual, but not to the revised edition Wx2012 one. It is provided in case some students are using the Wx2008 manual.

About the Weather Course

(Wx2008 and the WX2012 Revised Edition)

Student Manual

The course has one Student Manual, is taught in a one-module format, and has one closed-book examination at the end. The topics are organized into eight chapters. The course is designed to be completed in 10 classroom sessions, but each instructor will determine the pace of the course.

The course can be taught with a mix of Wx2008 and Wx2012 manuals. The old paragraph numbers have been carried over to the revised edition, but many page references are different. New material in the Wx2012 manual is contained in additional paragraphs that are indicated by an “a” or a “b” following the paragraph that has been carried over.

• Atmospheric Dynamics: Basic physical processes, surface and upper-level winds, convective vertical air movement, and how they affect each other are the bases for explaining what often appear to be unrelated weather systems and events. Since at least the W 90 weather course of two decades ago, atmospheric dynamics has always been an integral part of the course. In fact, many of the graphics were created to illustrate these fundamental dynamic relationships.

• Websites: The Weather Course puts Internet-based “weather stations” (e.g., the NWS websites) with their radar and satellite images and weather analyses/reports/forecasts at the center of the forecasting process. The manual also includes in each chapter pertinent references to the NWS’s online weather course–JetStream.

• Tradition: The course continues the USPS emphasis on surface maps and ends with the traditional forecasting exercises based on the NWS Daily Weather Maps included with the course materials. In the manual the section on individual weather observations continues to receive its well-deserved full treatment and the section on Folklore with its marine sayings–a long-time favorite–has been retained.

• Climate: Except for a brief discussion in the first chapter, the study of climate is not included. In a ten-session weather course, the subject and the closely related one of climate change cannot be adequately handled. Actually these topics add little (if anything) to a student’s understanding of shorter-term weather patterns and events–the focus of the USPS weather courses.

• Level of Difficulty: Over the years the weather course in its various editions has been regarded as one of the more difficult of the USPS advanced and elective courses. The new manual faces up to this challenge not by avoiding or burying difficult topics (Coriolis Effect, upper and lower air flow relationships, adiabatic rates, etc.), but rather by meeting the challenge head-on with clear text and supporting graphics.

The QuickGuide

• Weather Seminar: Students who have taken a USPS Weather Seminar (Onboard Weather Forecasting or its successor – Basic Weather and Forecasting) already have the Captain’s QuickGuide as a part of the seminar materials. While the Weather Course deals much more comprehensively with all the subjects covered in the QuickGuide, it is a particularly convenient waterproof pocket reference for use in making weather observations outside the classroom–an activity that instructors should encourage.

• Optional QuickGuide Orders: The QuickGuide is neither required nor necessary to answer homework or examination questions. As optional material, instructors should order QuickGuides no later than the third week of the course so they arrive by the time the last chapter–Chapter 8 on Forecasting–is covered.

About Teaching the Weather Course

Classroom and Equipment

Classroom sessions are based primarily on PowerPoint® presentations. Consequently, a classroom and equipment suitable for the effective use of PowerPoint continues to be essential. In addition, access to the Internet in the classroom while by no means necessary is a big plus (and not just for teaching Chapter 8–Part 1, Professional Forecasting). NWS or other weather web pages can be used throughout the course to connect classroom topics with timely radar and satellite images, current reports/forecasts, and actual prevailing weather conditions.

Student Computers

The class may well have one or more students who do not use computers or have Internet access. Each could be paired with a student who is computer facile and has such access to work together outside the classroom, particularly when Chapter 8–Part 1 is covered. Alternatively, the instructor could hold a session or two for those students.

Challenging Topics

Effective teaching always requires preparation. Instructors who “wing it” are not nearly as effective as they like to think they are. The Weather Course, more than most, requires getting your head back into the subject before entering the classroom. This is especially true for topics that students find more difficult. Weather topics that present special teaching challenges are:

• Supersaturation, Supercooling, and the Bergeron Process: These related subjects (Supersaturation – par.29a, Supercooling – par.57-58, and the Bergeron Process – par.59-60 explain some counterintuitive meteorological conditions (relative humidity greater than 100%, dew point higher than air temperature, liquid water drops and droplets colder than 32° F). It is not essential that students master these topics and they will not be tested on them. Instructors, however, will be more confidents in dealing with humidity and precipitation if they understand them.

• Coriolis Effect: For many students the Coriolis Effect is a difficult concept to understand. Instructors should thoroughly familiarize themselves with the subject as covered in Chapter 2 (par. 41-45 and the figures on page 29). To provide more assistance the manual contains not only the traditional explanatory description and graphics but also a new way of explaining and illustrating it. Since nothing turns on it, the manual glosses over the distinction some make between describing the Coriolis Effect as a force or as an effect.

In any event, instructors should not get bogged down in the explanations since for the purposes of the course students need only know that (i) the Coriolis Effect causes the wind to curve to the right in the Northern Hemisphere and (ii) the effect increases with latitude.

• Winds Aloft: The subject of wind flow aloft involves three fairly complex topics.

o Wind Speed and Direction Aloft. The first relates to the relationship between air temperature, the pressure gradient force and wind speed/direction aloft. Instructors should pay particular attention to the discussion and graphics in Chapter 2 (par. 50-55 and the figures on page 33).

o Surface and Upper-Air Flow. The second topic relates to the relationship between upper-air flow and surface winds. The key concept is convergence and divergence both at the surface and aloft. Instructors should fully understand the explanation and graphics in Chapter 6 (par. 76-86 and the figures on pages 120 and 123).

o Upper-Air Charts. The third topic is the difference between surface and upper-air charts and the concept of a constant pressure surface. A succinct description is given in Chapter 8 (par. 35 and the figures on pages 160 and 161). Appendix 1–Upper-Air Charts –provides instructors with some additional background information about Upper-Air Charts and their uses—particularly the 500 mb chart.

• Vertical Air Movement: While the general effect of rising and sinking air on cloud formation and precipitation is fairly straightforward, explaining vertical air movement itself is not. The explanation involves three related topics covered in Chapter 4: Lifting Processes (par. 14-15); Lapse Rates (par. 16-22); and Types of Stability (par. 23-31). The graphics on pages 66-69 are designed to assist the instructor in teaching and students in learning the fundamentals of vertical air movement.

• Mid-latitude and Tropical Storm Comparison: Key to understanding the dissimilarities between mid-latitude (extratropical) and tropical storms is the fundamental difference in their structures (cold core and warm core). Both a descriptive comparison of the two types of storms and related graphic presentations of their cores are set forth in Chapter 7 (par. 41 and the figures on page 140).

• Sounding Analysis Diagrams: A brief description of Sounding Analysis Diagrams and an example of one kind (the Skew-T Plot) are given in Chapter 8 (par. 36 & 74-75 and the figure on page 170). A complete understanding of these thermodynamic diagrams requires professional training that is well beyond the scope of this course. In dealing with the Skew-T Plot instructors have a choice. They can:

o simply refer to the Skew-T Plot as a graphic tool used for vertical analysis of the atmosphere (its counterpart being surface maps used for horizontal analysis) and then quickly move on; or

o use the Skew-T plot as a basis to review and reinforce the basic lapse rate and stability concepts covered in Chapter 4. (pages 66-70).

In any event instructors should have some familiarity with the Skew-T Plot so that at a minimum they can answer student questions about it. Appendix 2–Sounding Analysis Diagrams–provides information both to answer the most likely questions and to use the plot for review.

• Flexibility: Instructors have a fair amount of flexibility in teaching the Weather Course because the homework questions and final examination appropriately focus on the key facts and principles. Instructors are encouraged to take advantage of this flexibility to:

o Customize presentations for particular geographical areas.

o Reflect the particular interests of individual instructors.

o Emphasize/de-emphasize the more technical explanations in the manual.

o Determine the amount of time devoted to weather websites.

o Vary the amount of time devoted to particular map exercises. For example, while instructors might choose to spend a fair amount of classroom time on the “Using Maps to Forecast” Questions (Exercise IV), they might de-emphasize the “Drawing a Prognostic Chart” (Exercise III).

PowerPoint Presentation

Revised Power Point Presentation

In connection with the publication of the revised student manual, the PowerPoint presentation also has been updated with the addition of new slides and some minor changes in sequence. The revised presentation can be used with both the original and the revised student manuals.

Organization of Slides

There is an introductory set of five slides designed for use at the beginning of the first class. All the remaining slides are organized into the eight chapters of the manual. Within each chapter the slides are divided into two groups.

• Wx08 /Wx12 Slide Group: The slides in the first group are designated “Wx08” or “Wx12” to indicate which slides have been added with the Revised Edition of the Student Manual. This group of integrated slides is a complete presentation of the material covered (and closely follows the organization of topics) in the chapter. The group includes figures in the manual as well as some supplemental slides. The slides begin each chapter with a title slide and a class activities/demonstrations slide—the latter being mainly for the benefit of the instructor. Most instructors will probably hide it. Each chapter presentation ends with a chapter summary.

• Wx02 Slide Group: After the Wx08/Wx12 slides there is another group of slides that is designated Wx02. These “hidden” slides are legacy slides from the predecessor Wx2002 course that did not make the cut for this course. They are included only as an additional convenient resource for instructors. Unlike the Wx08/Wx12 slides, the slide notes have not been edited.

Restraint

As mentioned above, instructors have a great deal of flexibility in teaching the Weather Course and customization of presentations is encouraged. The addition of about 200 Wx02 slides to the 400 Wx08/Wx02 slides that already cover all the material enhances this flexibility, but at a potential price. There is a risk that instructors will use too many slides—the so-called lingering “death by PowerPoint.” Wx02 slides should be used at most sparingly. Despite this risk they were added because they include some old favorites and can be hidden so easily. The Instructor CD comes with them already hidden.

• NOTE: Instructors should use the “Hide Slide” feature to avoid showing a slide that they do not intend to use. The slide will not show in the normal sequence of the slide show. The hidden slide can be unhidden by simply clicking on “Hide Slide” again. Remember that you can present any slide (whether hidden or not) in the active presentation at any time by typing the number of the slide and pressing “Enter.” That slide will immediately show. To return to the normal presentation, enter the number of the slide last shown.

Homework and Final Examination

Student Review

• Chapter Summaries and Homework: Student review for the final examination is facilitated by the summaries at the end of each chapter and the organization of homework questions by identified topics. The questions (i) have been selected to reinforce the more important principles in each chapter, (ii) are straightforward (no tricks), and (iii) avoid non-essential obscure subjects and facts. To make preparation for the final examination easier, Appendix B repeats all the homework questions and answers. The appendix is similarly organized by chapter and topic.

• Weather Scenarios: After the Chapter 7 regular homework questions, there is a new section in the revised student manual. It has practice questions relating to weather scenarios (pages 152-153). For any students who are using the Wx2008 Student Manual, Instructors should reproduce these pages so they will not be disadvantaged when they take the exam.

Final Examination

Except for some minor updating, the final examination is the same examination used with the Wx2008 student manual. In fact, going forward the same examination will be given to students regardless of which student manual they are use.

• Examination Questions: The final examination is a typical 100-question USPS multiple-choice, closed-book test. About 80% of the exam questions will be based on homework questions. Unlike some of the homework questions, there will be no test questions that have the compound answers “all of the above”. The final examination will only include questions on the material in the first seven chapters. The material covered in Chapter 8 is reinforced through map drawing and analysis exercises.

• Tips for the Examination: While the test will only cover the material in the first seven chapters, there is obviously some overlap in the topics dealt with in these chapters and Chapter 8–Forecasting. If a topic is covered in one of the first seven chapters it may be on the exam even though it is also dealt with in Chapter 8. What follows are some points of clarification and some guidance for instructors and students.

o Temperature Conversions: There will be no exam questions that require students to convert Fahrenheit degrees to Celsius degrees or vice versa (i.e., no questions like Chapter 1 homework questions 10 and 11 will be on the exam).

o Station Models: Unlike exams for previous weather courses, station model questions will deal only with temperature, dew point, pressure, pressure tendency and wind speed and direction. There will be no questions about other symbols such as precipitation, cloud type or cloud cover. These additional station model elements are covered in Chapter 8 but only in connection with the use of the Daily Weather Maps.

o Mid-Latitude Storm: Continuing a USPS weather course tradition, every exam will include verbatim the ten questions in the Chapter 6 homework based on a frontal mid-latitude storm figure (questions 20 – 29).

o Scenarios: While there will be two of the customary “forecasting” scenarios on the exam (e.g., “Your cruiser is 50 miles east of….”), the analysis required to answer the three questions based on each scenario will not require any of the additional information contained in Chapter 8. Each scenario will be based upon one of the following weather events or patterns: warm front; cold front; thunderstorm squall line; advection fog; or waterspout.

A significant number of students are missing questions based on a scenario that involves a warm front pattern in the winter assuming that winter precipitation must involve a cold front. Some others are confusing an approaching warm front (indicated by a darkening sky with lowering layered–stratus-type clouds) and an approaching cold front or squall line (indicated by a line or wall of dark clouds and static or lightning).

Appendix 1

Upper-Air Charts

In many ways upper-air charts should be easier to understand than regular weather surface maps. Upper-air charts are just like regular topographical maps with each contour line depicting an equal altitude. In a regular topographical map (such as one used by hikers) the Earth’s surface (the ground) is mapped. In an upper-air chart the surface being mapped is a mathematical surface defined as a surface of equal air pressure. Because everywhere on the surface the atmospheric pressure is equal the surface is called a “Constant Pressure Surface.”

Students easily confuse contour lines (lines of equal altitude) on an upper-air chart with the isobars (lines of equal pressure) drawn on a weather surface map, especially since instructors (as does the manual) typically analogize isobars to contour lines. Unfortunately sometimes the analogy is carried further and highs are analogized to hills with concentric isobaric rings of increasing high pressure and lows are analogized to valleys with isobaric rings of decreasing low pressure. The Hill-Valley analogy can really confuse students.

The Student Manual avoids this confusion by (i) directly relating isobars on a surface weather chart with a graph of comparable atmospheric pressures (see figure on page 27) and (ii) showing an illustration of a constant pressure surface together with the upper-air chart that depicts it (see figures on pages 160–161).

Constant Pressure Surfaces

Upper-air air pressure data at different altitudes acquired by radiosondes provide the input for plotting constant-pressure surfaces. The information comes from weather stations throughout the world and is collected at 0000 and 1200 UTC [Universal Coordinated Time (UTC) = Greenwich Mean Time (GMT) = Zulu Time (Z)]. Based on the timing of this collection process, upper-air charts are produced twice daily. Selected pressure levels (e.g., 850 mb, 500 mb, etc.) are plotted on their own charts and so each chart represents the topography of one particular mb surface.

The altitude of each constant pressure surface varies from place to place because of differences in the temperature of the air below it. The drop of air pressure with altitude is more rapid in cold air masses than in warm air masses. Put another way, because raising the temperature of air reduces its density, greater altitudes are required for warmer air to exhibit the same drop in pressure as colder air. For example, the 500 mb level is at a lower altitude where the air below is colder and at a higher altitude where the air below is warmer.

Not surprisingly the contours of an upper-air constant pressure surface show a global scale gradual slope downward from the warm tropics to the colder polar latitudes. A NWS source for these charts is . Compare these charts with the aviation wind speed and wind streamline charts at . These latter charts do not have contour lines.

Contour Lines

Most importantly, upper-air observations have established that upper-air winds almost parallel height contour lines. In addition, the charts typically have wind arrows that show wind speed as well as direction. Much can be learned, however, from just the contour lines themselves.

• Contour Line Spacing: Where contours are closely spaced (a steep height gradient) winds are strong and where contours are far apart (a weak height gradient) winds are weak; the steeper the height gradient, the stronger the horizontal air pressure gradient and the stronger the wind.

• Contour Line Curvature: Contour lines exhibit both cyclonic (counterclockwise in the Northern Hemisphere) and anticyclonic (clockwise in the Northern Hemisphere) curvature. These curves correspond to the troughs and ridges in the westerlies. At the center of a ridge the air column is relatively warm (contour heights are higher). In contrast, at the center of a trough, the air column is relatively cold (contour heights are lower).

• Contour Line Heights: Similar to station models there are rules for converting the printed data on an upper-air chart to actual amounts. Unfortunately the rule for converting printed height information to height in meters varies with the particular pressure surface chart. Just in case a student happens to ask, here are the rules for each chart:

o 850 mb Chart–the first digit is a 1 and is omitted

o 700 mb Chart–the first digit is a 3 or 2 and is omitted

o 500 mb Chart–the last digit is a 0 and is omitted

Example: the NOAA Higher-Level Chart figure on page 32 has the 0 omitted. In contrast, the illustration on page 161 has already added it.

o 300 mb Chart–the last digit is a 0 and is omitted

o 200 mb Chart–the first digit is a 1 or 0, the last digit is a 0, and both digits are omitted

Different Upper-Air Charts

Because upper-air charts at different pressure levels depict the wind flow at those levels, they are used for somewhat different purposes. Set out below are some highlights:

• 850 mb and 700 mb Charts: These charts represent constant pressure surfaces at altitudes that range around 5,000 ft. (1 mile) and 10,000 ft. (2 miles) respectively. They are particularly useful for showing thermal (warm and cold) and moisture advections. The 700 mb chart is used in the mountainous west part of the country where surface elevations are high. The 700 mb wind flow indicated by the contour lines steer air mass or single cell thunderstorms.

• 500 mb Charts: These charts represent constant pressure surfaces at altitudes that range around 18,000 ft. (3.4 miles). The charts show upper-level steering winds, the vorticity (curving/spinning flow) associated with rising and sinking air, and the trough and ridge patterns responsible for the development and displacement of surface weather systems.

• 300 mb and 200 mb Charts: These charts represent constant pressure surfaces at altitudes that range around 29,000 ft. (5.5 miles) and 39,000 ft. (7.4 miles) respectively. The charts are particularly useful in locating the polar jet stream. The 300 mb chart is mainly used in the winter when the jet stream is lower and the 200 mb chart is used in the summer when the jet stream is higher.

The 500 mb Chart

The 500 mb Chart is especially useful for forecasting purposes because it is such a powerful surface weather prediction tool. Some of the indicators and patterns that meteorologists especially look for are:

• The 564 Decameter Line: The line marks the area with the greatest mixing of cold dry air and warm moist air – an area of great low pressure development. Known as the “storm track line” it is often shown as a thicker contour line. Surface storm tracks usually lie 300 to 600 nm north of and track parallel to this 5640 meter contour.

The worst weather is usually poleward with the highest winds and seas.

• Long Waves: Long waves (wavelengths of 2000 to 3000 nm) with northward and southward amplitudes circle the globe in the mid-latitudes. They are easier to see on Polar charts than Mercator projections. The waves appear to stand still, move slowly to the east for a period of time, and then breakdown or retrograde (move westward). A specific long wave pattern tends to last 10 days or more without significant fluctuation. Long waves determine the overall weather or storm track (e.g., drought, storminess, abnormal temperatures).

• Short waves: Shortwaves range in size from 500 to 1500 nm and have a life cycle of less than a week. Use the 5400 m contour to locate the short wave’s mid-point. Short waves tend to move rapidly from west to east passing through the longer waves. They tend to flatten or enhance as they do so.

When short waves and long waves are in phase they enhance one another; when out of phase they flatten. Sometimes short waves move north or south; they “cut-off” from the main contours that then shift north.

For the development of a mid-latitude low-pressure system, it is necessary for the short wave trough axis in the vertical to tilt back towards cold air (lower heights) with altitude.

• Zonal Flows: A zonal flow pattern means contour lines are arranged west to east. Zonal flow patterns tend to be unstable and short lived. They often break down into a more amplified pattern fairly rapidly. When a transition from a zonal to a more amplified or “meridional flow” pattern is taking place, a strong surface low of storm or higher force will usually develop.

• Meridional Flows: In a meridional flow pattern, the contours have more amplitude (north-south orientation) than in a zonal flow pattern. Meridional flow patterns tend to move cold air south and warm air north when compared to a zonal flow pattern. Surface lows and 500 mb short waves will move more toward the north or south along a meridian than in a zonal pattern. The entire meridional flow pattern may move from west to east over a period of days.

• Blocking Ridges: A high amplitude ridge that blocks the west to east progression of the upper level “Westerlies” is aptly named a “blocking ridge”. Large open longitude space between height contours of the same value that cover up to sometimes 30° of longitude is a characteristic of a block. There is very little airflow penetrating the area.

Blocks may last 10 days or more. Generally short waves will be steered northward over the blocking ridge. Closed lows that do dig southeastward of a block tend to be fairly strong. (e.g., surface wind of 45 – 50 kts.)

• Cut-off Lows: If a 500 mb meridional flow pattern becomes amplified enough, an upper-level low may form in the southern boundary of the upper-level “westerlies” and become “cut off”. These cut-off lows may be accompanied on the surface by strong winds perhaps with showers and thunderstorms.

Cut-off lows occur most in the spring and fall when the upper level westerlies migrate north and south, respectively. They tend to remain stationary and persist for several days – sometimes up to two weeks. They either gradually weaken or are picked up by the upper-level westerly flow when the pattern changes.

Appendix 2

Sounding Analysis Diagrams

Typical TV and web weather reports and weather course textbooks – the USPS Weather Course Student manual is no exception – are filled with horizontally oriented surface weather maps and radar and satellite images. The vertical oriented Sounding Analysis Diagrams, however, that professionals also use are given (if at all) only cursory treatment. While the manual briefly describes them and contains an example of one type–the Skew-T Plot–they are well beyond the scope of the course. Instructors, however, should be familiar with them.

Sounding Analysis Diagrams (just like surface weather maps and upper-air charts) are ways of organizing data. In this case the information that is plotted on them is “sounding” information: data (temperature and humidity) arranged by pressure level (altitude) above a certain place on the ground. Sounding Analysis Diagrams are aptly described as thermodynamic–i.e., they deal with heat/temperature and the related movement of rising and sinking air parcels. These diagrams can take different forms. The National Weather Service uses the one called the “Skew-T Log P Plot” (the short form name is the “Skew-T”). An example of this kind of plot is given on page 170 of the manual.

This memo walks the reader through the Skew-T Plot describing the way the diagram is set up, what the items on the chart represent, and how to interpret it. While not complicated, the only way to make sense of what follows is to refer to the plot on page 170 as you read along.

Skew-T Log P Plot:

The Skew-T plot is a graph with a horizontal Celsius temperature scale across the bottom going from cold on the left to warm on the right much like the Stability Figures in Chapter 4 (pages 68-69). There is, however, one big difference. The temperature scale is skewed to the right going up the chart (hence the name “Skew-T”) with isotherm lines at a 450 angle so they run in straight lines from lower left to upper right. In the example on page 170 the isotherms are shown as dashed, straight, faint yellow lines with the 0 C and -200 C isotherms highlighted as dashed blue lines.

The vertical axis has a scale showing decreasing pressure levels with increasing altitudes. Both millibars for pressure (black numbers) and kilometers for height above the surface (red numbers) are shown. Selected pressure levels are depicted with straight black horizontal lines. The scale is an inverse logarithmic one (hence the “Log P” part of the name). The combination of this scale and the skewing of the isotherms allow data up to the top of the troposphere to fit on one diagram and be easily analyzed.

Using the Skew-T Plot in the manual (page 170) as an example, the following information is displayed on the plot:

• Temperature: The thick solid red line represents the reported actual vertical temperature profile of the atmosphere at various pressure levels and heights. The red line depicts what is called the “Environmental Lapse Rate” in the manual.

• Dew Point Temperature: The thick solid green line represents the actual dew point profile of the atmosphere at various pressure levels and heights.

• Wet Bulb Temperature: The thin blue line represents the temperature that would result from evaporation cooling.

• Dry Adiabats: Solid yellow lines curved from lower right to upper left represent dry adiabatic lapse rates for a theoretical rising parcel.

• Wet Adiabat: A dotted brown curved line shows the wet adiabatic lapse rate of a theoretical rising parcel from what was deemed the most unstable parcel level. Some Skew-T charts show the entire grid of wet adiabats like the dry adiabats shown in the example.

• Lifting Condensation Level: Labeled “LCL” in a tan color shows the level at which a forced lifted parcel would reach saturation (cloud base height).

• Level of Free Convection: Labeled “LFC” in a tan color shows the level where a rising parcel would become buoyant (warmer than the surrounding air).

• Equilibrium Level: Labeled “EL” in a tan color shows the level where a rising parcel’s temperature would again equal the surrounding air and the parcel would no longer be buoyant. The parcel, however, could still continue to rise due to its momentum.

• Maximum Parcel Level: Not shown in the example, but it would be labeled “MPL” to show the maximum theoretical height that a parcel could rise above the EL due to its momentum.

Reading the Plot:

The dry adiabats and the wet adiabat establish the temperature “paths” of rising parcels. An easy way for instructors to get (and help students to become) familiar with the Skew-T Plot is to review the example in the manual using the following observations as a guide:

• The red air temperature line (the environmental lapse rate) shows a temperature inversion at about the 800 mb level (the red line curves sharply to the right).

• The temperature inversion places a lid on rising air from the surface so that the air at the surface is likely to get hotter than it otherwise would without the lid.

• The air in the low-level inversion layer is drier (the green dew point line curves sharply to the left).

• The wider the distance between the dew point line and the temperature line at any given altitude the lower the relative humidity and the less likely condensation will occur (clouds will form) in the absence of rising parcels.

• The temperature and dew point lines are very close around the 800 mb pressure level so any further cooling at that level could produce clouds even without rising parcels.

Assume the brown dashed line represents the temperature path of an actual rising parcel of air. Note the following:

• The parcel is rising from the surface because it is warmer than the surrounding air (the brown line is to the right of the red temperature line).

• Nearer the surface the parcel is cooling at the dry adiabatic rate (the brown line parallels the yellow dry adiabat lines).

• When the parcel reaches the LCL (lifting condensation level), however, the shape of the brown line changes because the parcel cools more slowly with elevation. The reason is the parcel is now saturated and cooling at the slower wet adiabatic lapse rate.

• When the parcel crosses the red temperature line for the first time it reaches the inversion layer where it is cooler than the surrounding air (the brown line is to the left of the red line). It will stop rising unless there is a lifting force that forces it further up (perhaps the buoyancy force of hotter air below it).

• When the rising parcel crosses the red temperature line the second time it has reached the LFC (the Level of Free Convection- the buoyancy point) where it is warmer than the surrounding air (the brown line is again to the right of the red line). The parcel will continue rising so long as the brown line remains to the right of the red line (i.e. the parcel continues to be warmer than the surrounding air).

• When the rising parcel crosses the red temperature line for the third time at the EL (the Equilibrium Level) it is no longer warmer than the surrounding air and will no longer continue rising due to its own buoyancy.

• At high altitudes the brown dashed line representing the wet adiabatic lapse rate develops a shape like the curve of the dry adiabatic lapse rates. The rates are almost the same because at the higher colder altitudes the parcel contains little moisture for generating latent heat to slow its cooling rate.

• On the right hand side of the diagram wind speed in knots and direction are given at different pressure (altitude) levels using the familiar barbs used by station models.

A few additional details about the diagram should be pointed out, particularly since a student may ask what they are.

• The red “(375m)” is the height of the surface (ground) above mean sea level in meters. In contrast, the red numbers that are not in parentheses are the heights in kilometers above the ground.

• “MU” before “LFC” and “EL” indicate that these levels were calculated on the basis of the characteristics of the most unstable theoretical parcel found in the lowest 300 mb layer.

• “ML” before “LCL” indicates that this level was calculated using the mean (average) conditions in the lowest 100 mb layer.

While there is some difference of opinion over which type of theoretical parcels should be used in forecasting thunderstorms, the ML parcel is preferred for estimating the height of the cumulus cloud base in the late afternoon.

Stability Analysis:

In forecasting severe weather meteorologists are not only interested in the lifting forces that can initiate convection but also the instability factors that can sustain it. One of those key factors is what professional meteorologists call “CAPE”–the convective potential energy that would be available to power strong updrafts. In the example on page 170 the large area (a somewhat elliptical shape) bounded by the LFC, the EL, the red temperature line and the brown wet adiabat is a graphic representation of that potential energy–the larger the area of CAPE then the greater the possibility of thunderstorms.

A large CAPE indicates that storms will build very quickly. Remember, however, that CAPE is only potential energy that will be converted into kinetic energy (upward movement) only if a parcel rises sufficiently to enter the region of CAPE.

The opposite of CAPE is CINH–convective inhibition. A parcel forced into a CINH region will tend to sink. In the manual’s example there is a small CINH area (a triangular shape) at about the level where the temperature inversion begins, bounded by the brown wet adiabat line to the left and the red temperature line to the right. A rising parcel in this region will be cooler than the surrounding air and therefore have negative buoyancy. The small size of the CINH indicates that it is a relatively weak lid.

While additional data would be needed to make a forecast, the Skew-T example indicates that severe afternoon thunderstorms are a distinct possibility.

Appendix 3

Wx2008 Student Manual

Clarifications and Corrections

[not applicable to the Revised Edition Wx2012 Student Manual]

At the time of writing this appendix, the Wx2008 course had been taught for over three years so we have the benefit of comments from instructors throughout the country. There are some substantive matters that should be clarified, refined, or corrected. For the most part, they are fairly technical and tangential to the main principles that are the course’s focus. Instructors should deal with them as they see fit.

An errata sheet has not been included with the Wx2008 Student Manuals. Instructors should copy this Appendix 3 and give them to any students who are using the original Wx2008 Student Manuals rather than the new 2012 Revised Edition ones.

Chapter 1:

• Lapse Rate

Lapse rates are changes in temperature with altitude.

Page 18 Ques 20: To accurately reflect this definition, replace the question with the following: “On average, the temperature in the lower atmosphere:” Make the same change on page 216.

Chapter 2:

• ITCZ

Textbook figures illustrating global scale pressure and wind patterns (like the ones on pages 20 and 21 of the manual) typically show the ITCZ and the Doldrums as a broad band centered on the equator. While these simplified versions are quite useful for illustrative purposes, the varying widths and locations of the ITCZ and the Doldrums are actually much more complex.

Page 20 Par 11: To eliminate text inaccuracies about the width and locations of the ITCZ and the Doldrums, in the first sentence replace “roughly within 100 north and south” with the phrase “in the general vicinity.”

• Semi-permanent Highs and Lows

The major semi-permanent highs and lows that affect weather in the continental United States are:

o the Aleutian Low that disappears in the summer;

o the Icelandic Low;

o the Pacific High;

o the Bermuda High;

o a high over the continent in the winter; and

o a low over the southwest in the summer.

Page 22 Par 20: Replace the list in this paragraph with the above list.

Chapter 3:

• Dew Point and Persistence

When the air temperature falls below the dew point, the dew point normally is lowered.

Page 50 Par 26: The last sentence does not take into account the above relationship between air temperature and dew point. Replace this sentence with the following: “The dew point does not change during the day in the way that relative humidity does. Meteorologists say that dew point is more “persistent” than relative humidity. This “persistence” makes dew point a far more useful humidity measurement than relative humidity in describing air masses.”

• Supersaturation and Humidity

When supersaturation occurs the relative humidity can be higher than 100% and the dew point can be higher than the air temperature. The text does not deal with supersaturation and these consequences.

Page 51 Par 30: To cover supersaturation and its relationship to humidity, replace the entire paragraph with the following two paragraphs:

“For water vapor to become water droplets (condensation) or ice crystals (deposition) there must be some kind of surface on which the water vapor molecules can stick. On the Earth’s surface there are all sorts of such surfaces (e.g., lakes, oceans, the ground, plant leaves). In mid-air the necessary platforms for condensation, freezing or deposition to occur are tiny nuclei. Typical condensation nuclei, for example, are sea salt, and particles of sand, dust or smoke. In the absence of these nuclei, supersaturation can occur. When the air is supersaturated, the relative humidity is higher than 100% and the dew point is higher than the air temperature.

“The regular relationships among air temperature, dew point and relative humidity in the absence of supersaturation are:

o the dew point is lower than or equal to (but not higher than) the air temperature;

o the dew point spread is the difference between the air temperature and the dew point;

o the smaller the dew point spread, the higher the relative humidity; and

o when the dew point spread is zero (the dew point = the air temperature), the relative humidity is 100%.”

Page 57 Par 64: To take into account the possibility of supersaturation with the relative humidity being higher than 100%, in the 6th bullet delete the phrase “or is below.”

• Fog and Wind Speed

The wind speeds most favorable to the development of fog are 2 to 3 knots for radiation fog and 5 to 15 knots for advection fog.

Page 61 Ques 19 & 20: To clarify the ambiguous reference to speed in these questions insert “wind” before “speed”; make same changes on page 221.

Chapter 5:

• Sun Pillars

Sun Pillars are caused by ice crystal reflection–not refraction.

Page101 Par 74: To eliminate the reference to refraction, replace the last sentence with the following: “They, however, are caused by ice crystal reflection–not refraction.”

Chapter 7:

• Tropical and Subtropical Storms

While storms in the tropics and subtropics are typically single air mass without fronts, there are also hybrid storms (now called “Subtropical Storms–see page 138 par 42) that have fronts. Also some “subtropical” regions experience mid-latitude weather with frontal storms during parts of the year (e.g., Florida in the Winter).

Page 131 Par 10: To eliminate the unqualified generalization that storms in the tropics and subtropics are single air mass without fronts, eliminate the last sentence.

• Intertropical Convergence Zone (ITCZ)

Accurately defining and describing the locations of the ITCZ is a fairly complicated matter. The actual front where the Northeasterly and Southeasterly trade winds converge is much narrower (up to only 300 miles) than the 200 width for the ITCZ stated in the text.

Page 132 Par 12: To eliminate inaccuracies in the description of the ITCZ, replace the 2nd and 3rd sentences with the following: “In the Atlantic Ocean, for example, the ITCZ can migrate northward as far as 150 North in the Northern Hemisphere’s Summer and southward to just north of the Equator in the Northern Hemisphere’s Winter.”

Chapter 8:

• Ensemble Forecasts and Spaghetti Plots

The text should (i) explain the relationship between the Spaghetti Plot figures and Ensemble Forecasts, and (ii) make it clear that different forecasting computer runs may use not only different initial data but also different models.

Page 154 Par 26: Replace the last two sentences with the following:

“Sometimes different models also are used. If the results of multiple runs (so-called “Spaghetti Plots” when graphically depicted) are quite different or disorganized the forecast is not considered reliable. If there is a uniform pattern to the results the forecast is considered more reliable. See Spaghetti Plot figures showing hypothetical plots of the Polar Jet Stream.”

• Canada Weather Radio

All the Canadian Weather channels can be received using the same radios that receive NOAA Weather Radio broadcasts.

Page 162 Par 69: Replace the 1st sentence with the following sentence: “Canada has its own “Weather Radio” that uses seven VHF channels with the same operating frequencies as NOAA, and so the same VHF receiver can be used to receive both Canadian and NOAA weather forecasts.”

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download