Weather

WX 2008Revised Edition 2012

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This manual includes copyrighted material from USA Today and International Marine/McGraw-Hill with permission all rights reserved. Some images courtesy NOAA and NASA including on the front cover NASA’s

“ Blue Marble” image with an overlaid GOES cloud view. Chapter opener photos courtesy of: Robert Sweet (1), Mila Zinkova (2), Richard Palmer (3), Arnold Paul (4), André Karwath (5), Fanny Schertzer (6)

The Marine Environment Committee gratefully acknowledges the contributions and support of the National Weather ServiceMarine and Coastal Weather Services Branch, and the NWS Jetstream Online Weather School.

Weather Course edited by R/C Robert E. Brandenstein, SN with contributions by members of the Marine EnvironmentCommittee: P/R/C John Bedrossian, SN; P/Lt/C Donald T. Hansen, SN; P/C Ruth A. Appel, SN; P/D/C John H. Little,Jr., JN; Lt/C Norman I. Condit, SN; Henry Warren Timm, SN; R/C Jean L. Hamilton, SN; Stephen E. Gareau; P/D/C

Leo Kohl, SN; and P/C Aulis Lind, JN

The Marine Environment Committee appreciates the contributions and the support of the Publishing Committee and its Chair, R/C S. Patricia Greer, SN.

Final text written or edited by R/C Ronald H. Kessel, SNGraphics created or edited by V/C Robert J. Sweet, SN

Book Design and Layout by Janet Robbins, North Wind Design & Production

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This publication is designed to provide general information onboating safety, piloting, and navigation. It was prepared bythe United States Power Squadrons and its members, and issold with the understanding that professional nautical serv-ices are not being provided. Do not use this publication as asubstitute for original sources of nautical information, includ-ing government publications, rules, regulations, laws, and

manufacturers’ recommendations, where appropriate. Thesubject matter of this publication is likely to become outdatedover time, and may have changed since our last publicationdate. Consequently, you must research original sources ofauthority to update this material and ensure accuracy whendealing with specific “on the water” conditions or boatingproblems.

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iii

Introduction v1. The Atmosphere:

Temperature and Heat 1NWS JetStream References 1The Atmosphere 2Composition of Air 4Temperature 4Heat 5

Conduction 6Convection 6Radiation 7

Specific Heat 7Insolation 8Air Masses 10Lapse Rates 12Weather and Climate 13

Climate 13Circulatory Systems 13

Station Model 14Summary 15Homework 16

2. Wind, Pressure, andWaves 19

NWS JetStream References 19Global Winds 19Global Highs and Lows 22Atmospheric Pressure 22Surface Pressure 24Barometers 25Factors Affecting the Wind 26

Pressure Gradient Force 26Friction 28Coriolis Effect 28

Surface Winds 30Winds Aloft 31Buys Ballot’s Law 34Wind Speed and Direction 34The Beaufort Scale 34Wind Shifts 36

Veering Wind 36Backing Wind 36

Boat Wind 36Local Winds 36

Sea Breeze 37Katabatic Wind 37Chinook (Foehn) Wind 37Drainage Winds 38

Waves 38Station Model 39Summary 40Homework 41

3. Water, Humidity, and Fog 45

NWS JetStream References 45Water 45Hydrological Cycle 46Phase Changes/Latent Heat 48Humidity 48

Mixing Ratio 49Relative Humidity 49Dew Point 50

Supersaturation 51Dew and Frost 51Fog 52

Radiation Fog 52Advection Fog 53Steam Fog 54Precipitation Fog 54Seamanship 55

Floods 55River Basin Flooding 56Flash Floods 56National Weather Service 56Boaters 57

Droughts 57Station Model 58Summary 58Homework 61

4. Stability, Clouds, and Precipitation 64

NWS JetStream References 64Stability 64

Lifting Processes 65Lapse Rates 66Types of Stability 68

Absolute Stability 68Absolute Instability 68Conditional Stability 68Cloud Base 69

Clouds 70Cloud Classification 70

Cloud Height 70Cloud Shape 71

Principal Cloud Types 71High 71Middle 73Low 73Vertically Developed 74

Precipitation 74Bergeron Process 75Collision-Coalescence 76

Forms of Precipitation 76Rain 77Freezing Rain 77Sleet 77Hail 77Snow 78

Winter Deposits 78Winter Marine Concerns 78Wind Chill Index 79Station Model 79Summary 79Homework 80

5. Thunderstorms, Tornadoes,Lightning, and OpticalEffects, etc. 84

NWS JetStream References 84Scale of Weather Events 85Thunderstorms 85Single Cell 86Multi Cell 88Supercell 91Downbursts, etc. 92Lightning 93

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Thunder 94Marine Considerations 94General Safety Concerns 95Tornadoes 95

Tornado Genesis 96The Enhanced F-Scale 96Watches and Warnings 97

Waterspouts 98Other Vortices 99

Gustnados 99Dust Devils 99

Skywarn 99Optical Effects 99

Twilight 99Colors of the Sky 100Halos, etc. 102Mirages 102Green Flash 103Coronas, etc. 103Rainbows 104

Summary 104Homework 106

6. Mid-latitude Weather and Storms 110

NWS JetStream References 110Mid-latitude Weather 110

Air Masses 111Highs and Lows 111Steering Winds 112Fronts 112

Frontal Weather 114Warm Fronts 114Cold Fronts 114Occluded Fronts 115Stationary Fronts 115North of the Low 116

Polar Front Theory 116Mid-latitude Storms 118

South of the Low 118North of the Low 119

Modern Cyclogenesis Theory 120Convergence and Divergence 120Polar Jet Stream 121

Conveyor Belt Models 124Storm Tracks 125Non-frontal Storms 126Map Symbols 126Summary 127Homework 128

7. Tropical Weather and Storms 132

NWS References 132Tropical/Subtropical Weather 133

ITCZ 133Subtropical Highs 134Storm Source Regions 135

Hurricanes 135Necessary Conditions 136Longer-term Factors 137

El Nino 137Other Factors 138

Tropical Storm Life Cycle 138Tropical Cyclone Structure 139

Mid-latitude Comparison 140Atlantic Tropical Storm Tracks 141Hurricane Damage 143

Wind and Flood 143Storm Surge 143Saffir-Simpson Scale 143

Katrina 144National Hurricane Center 144

Naming Tropical Storms 145Watches and Warnings 145

Maritime Considerations 145Warning Signs 145Taking Action 146

Summary 148Homework 149Exam Review:

Weather Scenarios 152

8. Forecasting 154NWS References 155Part 1: Professional Forecasting 155

Modern Forecasting 155Input Data 156Computer Processing 156Output Data and Reports 159Weather Websites 164NWS Weather Page 164Weather Radio 169Marine Considerations 170Skew-T Plot 171

Part 2: Weather Maps 171NWS Surface Weather Maps 171

Weather Elements 171Weather Features 172

Station Model Surface Map 172Station Model Review 173

Continental U.S Air 173Masses Review

Air Mass Analysis 174Isobars 175Surface Highs and Lows 176Surface Ridges and 177

TroughsFronts 177Prognostic Surface 178

ChartsPart 3: Weather Observations 179

Wx Seminar and 179QuickGuide

Background Information 179Observations 180Barometric Pressure 181Temperature and Dew Point 182Wind Speed & Direction 183Clouds 183Visibility 186Sea State 186Upper-Air Winds 187Folklore 187Weather Log 189Making a Forecast 189

Part 4: Map/Forecasting 191Exercises

I. Drawing a Simple Map 191II. Analyzing Weather Maps 191

1 October Map 1932 October Map 194

III.Drawing a Prognostic 195Chart

IV. Using Maps to Forecast 195V. Using Observations to 198

Forecast

AppendicesA—Weather Glossary 199B—Homework and Exercise 216

Answers

Contentsiv

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v

001 The Weather Course is a core part of theUSPS Elective Courses Curriculum for goodreasons. The safety and comfort of those whoventure out on the water have always beenweather dependent. In the days of sail theweather was the major determinant of coursesand speeds. For both sailors and power boaterstoday, weather still determines whether theyhead out or stay ashore. Just as they have overthe centuries, responsible mariners todaystudy the weather for practical reasons andpractical concerns.

002 Even the ancients, however, had a curios-ity beyond the practical as they mused andtheorized about what caused weather events.The subject captivated them. This course is anopportunity for students to experience thesame fascination but with the benefit of mod-ern science.

003 And then there is the beauty: the enchant-ing ever-changing cloud formations; the dra-matic spectacles of sunrises and sunsets; andawe inspiring clear starry nights. In thiscourse students will be looking up to learnwhat the sky has to say. But becoming keenerweather observers is not enough. Weatherobservations only have meaning in the contextof the basic principles of meteorology— thescience of the atmosphere.

Welcome to USPS Weather 004 USPS students certainly live in the right

place to study meteorology. The mid-latitudesare a war zone where enormous air masses

battle one another along fronts and powerfulstorm systems travel across the continent,lakes and oceans. Mid-latitude weatherincludes the blizzards of winter, the tornadoesof spring and summer, and the hurricanes offall. Seasonal weather cycles produce temper-ature ranges from hot and humid in thenineties to freezing in the subzero twenties.

005 For boaters, however, there are dangerseven in the relatively benign conditions of every-day weather. Wind, rain, fog and waves presentspecial challenges to them. While the courseobviously deals with such marine matters, it isbasically a general weather course designed tobenefit those sitting in their living rooms asmuch as those standing behind the helm.

Course Materials

006 The Weather Course (i) focuses on howweather systems form, behave, move, andinteract with one another, (ii) reflects the avail-ability of all sorts of weather reports and fore-casts on the internet, and (iii) provides guid-ance for anticipating weather conditionsthrough onboard and onshore observation.Each student receives:

00 • The Manual for the USPS ® WeatherCourse—an explanatory text with photo-graphs and drawings covering weather inthe United States and its coastal and inlandwaters;

00 • A set of three Daily Weather Maps—learn-ing aids with a complete explanation of mapsymbols designed to develop weather mapreading and analysis skills; and

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vi Introduction

00 • NOAA/NASA’s Sky Watcher Chart—a ref-erence to assist in identifying cloud types—helpful indicators of approaching weather.

007 The manual has two appendices: a Glos-sary (terms appearing in the manual for thefirst time are in italics) and Homework/ Exercise Answers.

NWS JetStream

008 The National Weather Service publishesonline educational materials for public useabout the Earth’s weather, water, and climate.The NWS JetStream Online School forWeather provides educators, emergency man-agers and the general public with a series ofmodules about specific weather related sub-jects. The modules are an additional learningresource. NWS JetStream website referencesare given at the beginning of each chapter inthe manual. These and all other websiteaddresses in the course materials were currentas of the time of publication but may change.

Course Procedure

009 The course has been designed to be a ten-session course. The manual has eight chaptersthat vary somewhat in length and difficulty. Ingeneral, except for the chapter on forecasting,each chapter can be covered in one two-hour

class session. Individual instructors will deter-mine the pace of the course.

010 The first seven chapters each contain a setof multiple choice homework questions. Thequestions reinforce the major points in thechapter and serve as a review at the end of thecourse. The forecasting chapter has a numberof map drawing and analysis exercises. Theinstructor will guide students through them.

011 At the end of the course there will be a typ-ical USPS multiple-choice closed book exami-nation on Chapters 1–7 but not Chapter 8. Ifthe student understands the concepts listed inthe short summary at the end of each chapterand reviews the homework questions, takingthe examination will be smooth sailing. In fact,most of the test questions will be takendirectly from the chapter homework.

012 While the formal part of the course endslooking down at test questions, the importantpart of the course is looking up. The USPSWeather Course provides a basis for “ read-ing” skyscapes that are as compelling andbeautiful as any seascape or landscape.

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