1 Also by Steve and Linda Dashew: Circumnavigator’s Handbook Bluewater Handbook Offshore Cruising Encyclopedia–First Edition Offshore Cruising Encyclopedia–Second Edition In Search of the Perfect Yacht Dashew Offshore Video Defensive Seamanship (available spring 1999) Dashew, Steve and Linda, 1941- Mariner’s Weather Handbook Copyright 1999, Beowulf, Inc. All rights reserved. No part of this CD-ROM may be reproduced or used in any form or by any means—graphic, electronic, or mechan- ical, including photocopying, taping, or information storage and retrieval systems without the written permission of the publishers. CD-ROM release history: Version 2.1 January 1999 Beowulf, Inc. 6140 East Finisterra Drive Tucson, Arizona 85750 USA www.SetSail.com CD-ROM designed and produced by Elyse Dashew, EDX Communications. 2 DEDICATION Voyaging around the world, it soon becomes evident one can call upon a number of unofficial sources of weather data. These are retired or amateur land- based forecasters, with access to the Internet, facsimile charts from gov- ernments, and sometimes various computer models. They broadcast weather data to sail- ors within their radio range on both SSB and Ham frequencies, answer questions, and try to keep us all in fair winds and smooth seas. Forecasters like Arnold Gibbons in the South Pacific and Herb Hilgen- berg in the Atlantic (along with many others) work long hours without pay and often without thanks from those they serve so selflessly. We, along with thousands of other sailors, have benefited from their ser- vices at various times all over the world. It is to these selfless men and women that we dedicate Mariner’s Weather Handbook. 3 4 TABLE OF CONTENTS DEDICATION . .3 TABLE OF CONTENTS . .5 INTRODUCTION . .12 PREFACE . .13 Having a Feel for the Weather . .14 Understanding the Basics . .14 Proximity—A Key Ingredient . .15 Using This Book . .16 Background Data . .18 Our Approach at Sea . .22 ACKNOWLEDGMENTS . .24 Weather Reports . .26 Stay In Touch . .27 Register Your Book . .27 Visit Us On the Internet . .27 WHAT CREATES THE WEATHER? . .29 Insolation . .29 Effect on The Atmosphere . .30 Global Weather Patterns . .31 Seasonal Variations . .32 Effects of Differences in Pressure . .32 Coriolis Effect . .33 Evaporation . .34 Condensation . .34 Weather on a Global Scale . .35 Local Weather Patterns . .37 Backing and Veering Winds . .38 Wind Flow in Highs and Lows . .38 FRONTS . .40 Birth of a Depression . .40 Warm Fronts . .43 Cold Fronts . .44 Frontal Speed . .45 Occluded Fronts . .46 Stationary Fronts . .46 Secondary Depressions . .46 ONBOARD PREDICTION . .48 Wind Strength . .48 Messages in the Clouds . .50 The Barometer . .50 Along the South African Coast . .57 Wind Direction . .59 Gust Factors . .60 Data from Other Yachts . .62 What the Waves Tell You . .63 Anomalies . .64 Finding Pressure Centers . .64 READING THE CLOUDS . .66 General Categories of Clouds . .66 Cirrus . .67 Cirrocumulus . .68 Cirrostratus . .69 5 TABLE OF CONTENTS Altocumulus . 70 Altostratus . 71 Stratus . 71 Nimbostratus . 71 Cumulonimbus . 72 Cumulus . 73 THE EXTRATROPICAL FORECASTING PROCESS . 76 National Weather Service’s Marine Prediction Center . 76 Upper Level Conditions . 78 Computer Modeling . 79 Forecast Cycle . 81 Budget Issues . 82 When You’re at Sea . 83 Who Are the Forecasters? . 84 SURFACE FORECASTS . 90 Common Terms . 90 RADIO FORECASTS . 94 Antenna Selection . 95 Onboard Interference . 95 Band Conditions . 95 Receiving Voice Forecasts . 96 Nets and Weather Relays . 96 THE WEATHER FAX . 98 Dedicated Fax or Computer? . 98 At the Beginning of a Passage . 99 Symbols . 100 Sample Fax Charts . 103 Interpreting Surface Fax Charts . 110 Compression Zones . 114 Tropical and Subtropical Lows . 115 Isobar Curvature and Wind Speed . 116 FORECASTING IN THREE DIMENSIONS . 118 3-D Low Structure . 119 Divergence . 122 3-D High Structure . 122 Convergence . 124 Upper-Level Westerlies . 124 Meandering Upper-Level Winds . 126 Ridges and Troughs . 127 UPPER-LEVEL CHARTS . 128 500mb Fax Charts . 128 The Professional’s View . 130 Surface Weather Review . ..