PREFACE

Some stolid educators believe that the mind is like a cup and that only "fluff" prevents it from being filled with pearls of knowledge. Motivated by this philosophy, they present basic meteorology as an encyclopedia whose entries must be memorized and mastered.
  We believe that the mind is more like a sieve which, unless enough "fluff" is added, the pearls of knowledge are filled and filled but never retained. It is this model of the human mind that motivated us to take great care in creating this new edition. By interjecting cultural allusions, anecdotes, and, yes, even humor to act as "fluff," we firmly believe that students will retain pearls of knowledge about meteorology long after the course has ended.
  For example, in order to explain the lag in temperature between the summer solstice and the warmest time of the year in July and early August, one elementary text, which took an encyclopedic approach to presenting meteorology, states that "the troposphere's temperature takes time to adjust to the changing solar energy input." In this textbook, we take an alternative approach, using the anecdote of heating up cold pizza in a preheated oven to teach the notion of seasonal lag. Yes, we readily admit that the "fluff" of connecting the idea of heating cold pizza in an oven to the summer lag in temperature requires words that stolid educators maintain are a waste of time and space. We submit, however, that, by appealing to an experience common to students' lives, they will retain this pearl of knowledge long after all encyclopedic explanations fade from memory.
  Though invoking cold pizza may lend the impression that this textbook forsakes scientific rigor for metaphor and anecdote, rest assured that it does not. For example,
the occlusion of a mid-latitude low-pressure system is usually described as the process by which "the cold front catches up with the warm front." For the first time in an elementary textbook, we will show that it is the reshuffling of upper-air divergence that causes occlusion to occur, rendering the tired explanation of occlusion as the "cold front catching up to the warm front" as merely an effect rather than a cause.
  A World of Weather is a textbook. It is also a laboratory manual. Real-life examples and non-traditional problems that use weather data from previous storms support our hands-on approach. These exercises make our book a valuable teaching tool for introductory courses at both large and small universities, including colleges that do not have a meteorology program and offer only one course in weather. Though the text is streamlined for college students with a non-meteorology major, we firmly believe that A World of Weather will also serve as a well-rounded foundation for students intending to major in meteorology.
  Fundamentally, we want students to be good weather consumers. They will be bombarded by all types of weather information throughout their lives via television, the Internet, radio and the written word. Some of it will be scientifically accurate; some of it will be fuzzy and misleading. In order to discriminate between accurate and misleading information, students will need to retain what they learned about basic meteorology.
  We believe that our textbook will give students lifelong pearls of knowledge.

Lee Grenci
Jon Nese

HOW TO USE THIS BOOK

Dear Student of Meteorology:

Welcome to A World of Weather! To prepare you for your tour through meteorology, we want to offer a few travel tips to make your experience more worthwhile.
  The first thing you should know is that this book is written in a non-traditional style. We purposely abandoned the standard encyclopedic format that is used in most scientific texts. Missing from this book is the typical parade of facts that occupy space without explanation or example. Instead, we try to paint mental pictures through the use of anecdotes and metaphors to help you more easily relate important meteorological concepts to everyday, real-world experiences. In this way, your learning will be cemented in the foundation of personal experience.
  Please note that we don't want you to simply memorize a litany of facts. Rather, we want you to see meteorology for what it truly is-a set of conceptual keys that can unlock the doors of understanding.
  We forewarn you that the text is riddled with puns and humor. We firmly believe that the process of learning should take place with an "air" of excitement and fun. As teachers of meteorology, our duty is not only to impart knowledge and understanding, but to nurture an enthusiasm for the science that we love and respect. We hope that our passions are contagious!
  Though A World of Weather hails as a text, it is also a lab manual, containing a diversified portfolio of problems. After reading a given chapter, we suggest first turning to the review questions to test your understanding of basic concepts. Then, laboratory problems will afford you the opportunity to have "hands-on" experience with weather data, in some cases analyzing the greatest storms in recent history (such as The Blizzard of '93 and the May 3, 1999 tornado outbreak in Oklahoma).
  So fasten your learning seat belts! It's time to get going! Have a great trip!

Points of Interest on Your Trip

Cover Material

Front Cover...A panorama of visualizations of the earth.
Back Cover...Hurricane Linda approaching Baja California in September 1997.
Inside Front Cover...contains common unit conversion formulas and a description of Metric (SI) orders of magnitude.
Inside Back Cover...shows how to build and interpret the station model.

Chapter Plan

Chapter Introductions...lay the groundwork for the chapter content.
Weather Folklore and Commentary ...appear in many chapters. Weather lores are homespun forecasts based on observations of nature. Weather commentaries are consumer reports on products dealing with the dissemination of weather information.
Focuses on Optics...expose you to the colorful wonders of the sky. These essays on optical phenomena are scattered throughout the text.

End of Chapter Exercises

Questions for Laboratory ...provide you with "hands-on" experience in meteorology.
Questions for Review...test your retention and understanding of basic concepts.

End of Text

Electronic Resources and Additional Print Resources...are identified by chapter. These offer suggestions for supplementary reading in print and on the Web.
Glossary ...serves as a ready reference for vocabulary terms.
Index...provides for quick page reference for important terms from the text.

NEW TO THIS EDITION

The avalanche of weather information on the World Wide Web continues to snowball. Most notably, the computer models of the atmosphere used in operational forecasting are now at our "beck-and-click." Never before have the tools of the trade been so accessible to the general public. But computer models, by virtue of their sophistication, have been off-limits to courses in basic meteorology. We believe, however, that the accessibility of forecasting tools on the Internet and the noble goal of life-long learning can be unified to give our students the opportunity to develop their skills in basic weather forecasting. To this end, we have written two new chapters on weather forecasting and numerical weather prediction that will allow students to assume the role of apprentice forecaster.
  Please be advised that we do not give these forecasting chapters the usual broad-brush found in most textbooks. Rather, we fully distill all the fundamental issues inherent to the short-range and medium-range computer models routinely used by professional forecasters. As instructors, we have seen how an apprenticeship in weather forecasting can spark interest and discussion among students in basic meteorology. Students are excited to be able to "eavesdrop" on the inner circle of operational weather forecasting. Their enthusiasm can carry over to forecasting contests that motivate students to actively learn more about atmospheric science. Such active learning involves group discussion and teamwork, which, in our opinion, gives students more ownership for their learning.
  We both believe that the chapters on weather forecasting are unique and revolutionary in the arena of general education in meteorology. Moreover, these chapters are congruent with our resolute philosophy that students should be challenged while learning science. We reject prevailing philosophies that cater to the dilution of science to make it more palatable for students. Education, in our opinion, does not advance by requiring less of our students. Indeed, we believe that the noble struggle with ideas relating to weather forecasting will enhance critical thinking in our students and prepare them to better solve problems in their professional lives. More importantly, the new chapters will provide an avenue for lifelong learning by removing the mystique of forecasting products on the World Wide Web. Many of the laboratory exercises at the end of the chapters simulate an informal forecasting environment that can serve as a springboard into rewarding, life-long learning.
  Also new to this edition is a revolutionary treatment of the cyclone model. In most elementary textbooks, the formation and intensification of a mid-latitude cyclone amounts to a fuzzy broad-brush of what really happens during cyclogenesis. In most textbooks, low-pressure systems are portrayed as helpless "sticks" carried along in a fast stream of high-altitude steering winds. Such inadequate presentations in basic meteorological textbooks plant seeds that grow into "scientific ragweed" that spread in the wind of bad meteorology.
  Educational ragweed has spread to television, where weather casters purport that "Rising air causes low pressure." This ragweed must be plucked from the garden of education and replaced with hearty plants of sound scientific understanding. For the first time in a basic text on meteorology, we present an understandable and coherent treatment of the cyclone model based on the HoratioAlger-like concept of self-development, a positive feedback between cold advection, the strength of the 500-mb trough and the deepening surface low.
  In a new chapter that discusses mid-latitude cyclones as a catalyst for severe weather, we use the infamous outbreak of tornadoes across central Oklahoma on May 3, 1999. In this chapter, we further point out that only a few tenths of one percent of a "watch box" issued by the Storm Prediction Center in Norman, OK, typically experience severe weather. When large-scale windstorms called derechos develop, however, as much as 20 percent of a watch box can be affected by damaging winds that qualify as severe weather. Derechos are a new and important topic that we've included in this edition.
  In order to pave the way for new forecasting chapters and the revolutionary treatment of the cyclone model, we felt the need for a major reorganization. To this end, more than half of this edition is brand new. Even in the largely unchanged Chapter 1, The Basics: Meteorological Analysis, we've added new and complementary material on isoplething, which we steadfastly hold is an important tool for students to learn. Though computers routinely draw isopleths for meteorological fields, we believe that students "getting their hands dirty" while working with real data provides an invaluable learning experience. Many new laboratory exercises in this new edition require students to work with isoplethed meteorological fields.
  In the final analysis, we both believe that this edition more completely helps students to become critical weather consumers while setting the stage for life-long learning.

QUESTIONS FOR LABORATORY

1. Everyone, not just meteorologists, should know some geography! With this in mind:
   (a) Match the state names with their corresponding numbers on the map in Figure 1.15.
   (b) Name the state(s) where the following places or geographic features are located (you may want to consult Color Plate 1.A): Sierra Nevada Mountains; Brooks Range; Cape Hatteras; Great Basin; Great Salt Lake; Cascade Range; Ozarks; Black Hills; Cape Cod.
2. Perform the following unit conversions. Round your answers to one decimal place:
   (a) 100 yards (to meters)
   (b) the speed of light, 186,000 miles per second (to mls)
   (c) 1 ton, or 2000 Ib (to kg)
   (d) your height in inches (to cm)
   (e) 65 mph, the speed limit on interstates in many states (to km/hr)
   (f) Michael Johnson's average speed during his world-record 19.32-second 200-meter run at the 1996 Summer Olympics (to mph).
3. What is the numerical value of absolute zero on the Fahrenheit scale?
4. Below you are given the daily maximum and minimum temperature (high and low) and daily precipitation observed at The Franklin Institute Science Museum in downtown Philadelphia, PA, in July 1999 (The Franklin Institute is the only cooperative observing station in Center City Philadelphia). Temperature is given in °F. Precipitation is given in inches. Using this data, perform the following calculations:
   (a) Compute the average high temperature during July 1999. Compute the average low temperature. Then average the two to arrive at the overall average monthly temperature. Round your answers to one decimal place. What was the highest temperature for the month? What was the lowest?
   (b) What was the total precipitation for the month? Make a conclusion about the relative wetness or dryness of this month in Philadelphia, given that the average precipitation for July in Philadelphia is about 4.25 inches.
   (c) Back to temperature: the official surface observing site for Philadelphia is located at the International Airport, several miles southwest of downtown. For July 1999, the average maximum temperature at the Airport observing site was 91.1°F, while the average minimum temperature was 71.2°F. Was the average maximum temperature at the Airport higher or lower than downtown? What about the average minimum temperature?
   
   
   
5. Use Color Plate 1.B to answer the following:
   (a) Assume that a weather map is dated 23Z, January 10, 2000. What is the local time in Greenwich, England? Albany, NY? Houston, TX? Seattle, WA? Would any of the times be different if the date was July 28, 2000? Why or why not?
   (b) You're studying late at night for final exams. At 2:30 a.m., you receive e-mail from a friend who is spending a semester abroad in Sydney, Australia. Assuming that the message was delivered to your computer account immediately upon being sent, what was the local time in Sydney when your friend sent the note?
6. In this exercise, we'll get some practice isoplething, with each step a little more difficult than the previous step.
   (a) Figure 1.16a shows regularly spaced data. Draw the 30, 40, 50, and 60 isopleths.
   (b) Figure 1.16b shows another set of regularly spaced data. This time some of the isopleths will close on themselves. Draw all isopleths that are multiples-of-ten.
   (c) In reality, weather data is not observed on a regularly spaced grid. Figure 1.16c shows temperature data observed across North America on a February day in 1994. Draw the o°F isotherm and all other isotherms at 20°F intervals from 0°F. Name three states that have relatively large north-to-south temperature gradients. Name three states that have very little north-to-south temperature gradient.
7. Assume that the lines on Figure 1.17 are isotherms. Note in Figure 1.17 that the 30°F isotherm branches while the 50°F and 60°F isotherms cross. Neither of these isotherm patterns is physically realistic.
   (a) Recall that a fundamental rule of isoplething is that an isopleth of value X must pass between points having value greater than X and points having value less than x. With this rule in mind, what values must the temperature assume in the region marked A? What about in the region marked B? Use your answers to these questions to arrive at an inconsistency regarding the value of temperatures in the regions marked C, thereby demonstrating that the branching structure of the 30°F isotherm is not physically realistic.
   (b) What inconsistency arises regarding the intersection of the 50°F and the 60°F isotherms that demonstrates that such an intersection is not physically realistic?
8. Figure 1.18 shows a group of regional weather observations of wind, temperature, cloud cover, visibility, and obstructions to visibility.
   (a) Decode the station models marked with the letters A through F.
   (b) Create station models of your own at the points on the map marked with the letters G and H using the following weather data (the cloud-cover circles have already been drawn for you):
   G: Cloudy, temperature 29°F, light snow, visibility 3 miles, wind northwest at 10 mph.
   H: Cloudy, temperature 59°F, light rain, visibility 2 miles, wind southeast at 6 mph.
   (c) Draw the 30°F, 40°F, 50°F, and 60°F isotherms.
9. Use Figure 1.19, a topographic map of the island of Hawaii, to answer the following questions:
   (a) Assume that during a vacation to the island, you want to take a long hike—let's assume 30 km in length. Three possible trails, each 30 km long, are marked by lines AB, CD, and EF. Compute the gradient of each of these three paths. To do so, estimate the elevation of each point from the contours, with one exception: use 13,795 feet as the elevation of point D, the summit of Mauna Kea (some closed contours near the peak of Mauna Kea were left out to avoid cluttering the diagram, so using the contours would give you an estimate that was too low). Which of the trails is least steep (and thus least exhausting), in terms of average steepness? Which would be the most challenging (that is, the steepest, on average)?
   (b) In words, describe the ups and downs of the terrain if you were to walk from point D to point F.
   (c) Clouds tend to form where air rises, and tend to dissipate (or not form at all) where air sinks. Air impinging upon mountains is forced to rise, and after cresting the peak, air sinks down the other side. Using only these simple guidelines, answer the following: On a day with a north wind, would the weather along path CD tend
   to be cloudy or sunny? On a day with a southeast wind, would the weather along path AB tend to be cloudy
   or sunny? Explain your answers. Given that the prevailing wind direction (that is, the wind direction that is most commonly observed) on the island is a northeast wind, which of the three paths do you think would tend to experience the least rain, on average? Explain your answer.
10. Figure 1.20 shows the total rainfall in Pennsylvania, in tenths of inches (so, for example, 102 = 10.2 inches), produced by the remains of Hurricane Agnes, in June 1972. Agnes was Pennsylvania's worst natural disaster, responsible for 40 deaths and billions of dollars in damage to personal property and the state's transportation
    infrastructure.
    (a) Draw isohyets (lines of equal rainfall) of 8 inches and 12 inches to identify the regions that experienced the heaviest rainfall and consequently some of the worst flooding.
    (b) Consult an atlas to determine what major river drains the area hit with the heaviest rains from Agnes. (Hint: this river drains into Chesapeake Bay, and passes through Pennsylvania's capital city.)
11. Figure 1.21 shows wind speeds, in knots, at about 10,000 m (33,000 ft) altitude. The data was derived from the launch of a radiosonde.
    (a) Draw the 60, 90, and 120-knot isotachs (lines of equal wind speed).
    (b) In doing so, you should be able to identify a channel of faster winds that generally stretches in a west-east direction, with some north-south undulations. This corridor of higher wind speeds is the so-called "jet stream," which will be discussed in more detail in Chapter 5. To help find the jet stream, shade all areas on the map with wind speeds greater than 60 knots. Then draw a heavy dark line within the shaded area to represent the narrow channel of fastest winds.
12. Figure 1.22 shows station models in Texas and surrounding states.
    (a) Draw isotherms at all multiples-of-ten.
    (b) You may have heard the word "front" used by meteorologists. In general, a front is a boundary between huge masses of air of contrasting temperature. Thus, fronts are zones of relatively large temperature gradients on a weather map, such a zone would be identified by tight packing of isotherms. To isolate that zone on Figure 1.22, shade in all areas where temperatures are above 60°F (representing relatively mild air). Also, shade in areas where temperatures are below 40°F (representing relatively cold air). What's left is the zone of largest temperature gradient.
    (c) Using other information in the station models, make a conclusion about the behavior of the wind (specifically, its direction) as you cross from one side of a front to the other.
13. In Figure 1.23 you are given isotherms, drawn every 5°F. Five locations on the map are marked with the letters A, B, C, D, and E.
    (a) From the isotherms, estimate the temperature at the five locations.
    (b) Estimate the north-south temperature gradient in the vicinity of each location (express your answer in °F per mile). To do this at a given location, center a north-south line, one inch long, on the location (one inch is an arbitrary choice). Compute the temperature difference as the temperature at the southernmost point on the line minus the temperature at the northernmost point on the line. Assume that one inch equals 300 miles on this map.
    (c) Generally, when the term "gradient" is used without any reference to direction, the gradient should be computed at right angles to (that is, perpendicular to) the isopleth passing through the point of interest.
    This direction gives the largest rate of change, and thus the largest value of the gradient. Repeat the gradient calculation in part (b), but at each point orient the line at a right angle to the isotherm passing through the point (if none of the isotherms shown pass right through the point, estimate what that isotherm would look like if it was drawn). Again, assume that one inch equals 300 miles.
14. In some places, characteristics such as the shape of the terrain or the type of underlying surface can create localized pockets where weather conditions differ significantly from those in nearby areas. One such microclimate is found in the Barrens, a low-lying area located a few miles west of State College, PA. The combination of sandy soil, low surface moisture availability, and a valley location means that overnight temperatures in the Barrens often drop well below temperatures just a few miles away.
    One night, a thermometer aboard a car driving into the Barrens registered a temperature drop from 2°F to -25°F in two minutes. Assume that the car was traveling at 10 mph. From this information, estimate the temperature gradient (in °F per mile) along the path traveled by the car.
15. Discuss the meaning of the cartoon in Figure 1.24. Specifically, what's the fallacy of the man's statement?
16. For one value of the temperature, you would not have to specify whether the temperature was in degrees Fahrenheit or degrees Celsius because the two temperature scales actually agree. What is this value of temperature?

WEATHER FOLKLORE

Making Dew

"When the dew is on the grass,
Rain will never come to pass.
When grass is dry at morning light,
Look for rain before the night. "

Dew and ground fog are close cousins. Dew forms when water vapor in a thin layer of air next to the ground condenses into beads of water on grass as the temperature of the ground falls just below the dew point. Fog forms when water vapor in a thicker layer of air next to the ground condenses onto airborne condensation nuclei. Both require a clear, cool night to form, but slightly different wind conditions. Dew forms on a truly calm night when the greatest chill is confined to grasshopper level. For fog to occur, a very light wind of a few kilometers pe! hour is needed to spread the chill from the ground through a deeper layer of air.
  The clear, cool and tranquil conditions needed for dew (as well as ground fog) to form at night are typically fostered by an area of high pressure. Thus, the first part of this folklore hasmeritsince an evening dew on the grass is consistent with the presence of a fair-weather high-pressure system. If the grass is dry at morning light, then the night was likely too windy, too cloudy, or just too warm. Since any of these conditions could occur ahead of an approaching low-pressure system that promises rain, the second part of the lore has some validity as well.

FOCUS ON ATMOSPHERIC OPTICS

Superior Mirage: The Phantom of the Arctic

Suppose, on an adventurous excursion to the Arctic, you came upon an eerie mountain of ice like the one shown in Color Plate OP9. What would you do? Perhaps you'd choose to turn around and go home. That's exactly what John Ross did in 1818, when, as a captain in the British Navy, he led a sailing expedition to Canada's Lancaster Sound in search of the "Northwest Passage" to the Pacific Ocean. After balking at such an Arctic spectacle, Ross returned to Britain, only to have his story and reputation ridiculed when his second-in-command, a fellow by the name of Perry, sailed through the Sound without a hitch a year later.
  What Ross observed was not a daunting mountain of ice—it was likely a mirage made fearsome by upwardly magnified images of tiny pieces of snow or ice resting on the Arctic tundra. Unlike the inferior mirage discussed in Chapter 4 (an example of which is the water on the road), this type of mirage, called a superior mirage because images of an object appear above the object, forms when cold, dense air at the ground is overlain by warmer, less dense air. More specifically, temperature first increases slowly with height above the frigid tundra. Above this layer of slow temperature increase, temperature starts to increase more rapidly with height. Completing the temperature-inversion sandwich, the temperature increase returns to a slower rate higher above the ground.
  As light rays from a point on the snowpack travel through the temperature-inversion sandwich, they can be refracted either strongly (while passing through regions of strong vertical temperature gradients) or not so strongly (while passing through regions where vertical temperature gradients are weaker). Given that light can bend in a variety of ways while traveling through these fluctuating gradients, it should come as no surprise that several distinct rays can reach an observer's eyes from a single point on the snowpack.
  The rays are concave down because light always bends so that denser, colder air lies on the inside of the curve. But now recall that our brains are programmed to interpret all the light waves that reach our eyes as having traveled in straight lines. Thus, according to Figure 7.9b, the observer can see three distinct images of the snow at point A above the snowpack—and probably many more that aren't even shown. Each of these images of point A is a superior mirage because it appears above point A.
  When superior mirages stack up on top of one another like this, a strange, vertical magnification results. Moreover, when there are small variations in the temperature-inversion sandwich, one point might be greatly magnified while its next-door neighbor might appear almost normal. The end result would be an image like the one in Color Plate OP9—like the one that John Ross probably saw.
  By the way, a similar spectacle can sometimes be seen over cool, mid-latitude lakes during summer (because the same type of temperature-inversion sandwich can occur there). So, if you're boating, be on the lookout for superior mirages suspended over the lake-they would likely appear as a series of towers or a wall with gaps.
  If you do see one, there's no need to do a John Ross impression and go home.