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Global Weather
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Table of Contents
Introduction
1.0 Equilibrium in the Atmosphere ...........................................1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
General
Stability and Instability—Definitions
What Causes Stability and Instability?
The Adiabatic Process
Relationship Between the Environment and Vertically Moving Air
Dry and Saturated Adiabatic Lapse Rates
The Chinook
Equilibrium and the Weather
How Medium and Upper Level Clouds Are Formed
2.0 General Circulation ..............................................................11
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
General
Terrestrial Circulation: Development of Hadley and Ferrel Cells
Primary High and Low Pressure Belts
Primary Global Wind Systems
El Niño and La Niña
Rossby Waves—The Great Eddies
Jet Streams
Secondary Wind Systems—Monsoons
2.8.1 Monsoons
2.8.2 Summer Monsoon
2.8.3 Winter Monsoon
2.9 Mid-latitude Storm Zones
2.10 Local Winds
3.0 Air Masses ...............................................................................19
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
General
Source Regions of Air Masses
Classification of Air Masses
Air Mass Characteristics
Modification of Air Masses
Winter Air Masses
Summer Air Masses
Weather Changes Caused by the Combined Effects of Seasonal and
Surface Modification
3.9 Lake Effect Snows
3.10 Permanent Fronts Between Major Air Masses
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4.0 Extratropical Cyclones ........................................................25
4.1
4.2
4.3
4.4
What Is a Cyclone?
Air Masses on the Move
Why Are Frontal Surfaces Sloping?
Development of a Frontal Depression
4.4.1 Frontogenesis—The Beginning
4.4.2 The Frontal Wave
4.4.3 The Low Pressure Centre Forms
4.4.4 The Frontal Pattern Appears
4.4.5 The Warm Front
4.4.6 The Warm Sector
4.4.7 The Cold Front
4.4.8 And Now the Final Stage of the Cycle, the Occlusion
4.4.9 The TROWAL, (Trough of Warm Air Aloft)
4.5 Weather in the Extratropical Regions of the Southern Hemisphere
5.0 Tropical Cyclones ..................................................................33
5.1 Difference Between Tropical and Extratropical Cyclones
5.2 The Stages of Development
5.2.1 Waves in the Easterlies
5.2.2 Tropical Disturbance
5.2.3 Tropical Depression
5.2.4 Tropical Storm
5.3 Finally, We Come to the Hurricane
5.4 Classification of Hurricanes
5.5 Formation and Structure of a Hurricane
5.5.1 Source and Initiation
5.5.2 Areas of Occurrence
5.5.3 Do Hurricanes Ever Form at the Equator?
5.5.4 The Effect of Wind Shear
5.5.5 Where Does All the Updraught Go?
5.6 The Eye of the Hurricane
5.6.1 Formation, Size and Wind Speed
5.6.2 The Calm Before the Storm! Leaving the Eye
5.7 Signs of the Approach and Advance of a Hurricane
5.7.1 Storm Swells
5.7.2 Pressure Changes
5.7.3 Winds—Finding the Storm Centre by Using Buys Ballot’s Law
5.7.4 Clouds—Sequence and Associated Precipitation
5.8 Sailor Beware!
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5.9 Dangerous and Navigable Semicircles
5.9.1 The Dangerous Semicircle
5.9.2 The Navigable Semicircle
5.9.3 Guidelines for Hurricane Navigation—Using a Storm Card
5.10 “Land Ho!” Effects of a Hurricane on Reaching Landfall
5.10.1 The Deadly Storm Surge
5.11 A Tip for the Navigator
5.12 The Hurricane Moves On
5.13 How Does a Tropical Cyclone Finally End?
5.14 Forecasting of Tropical Cyclones
5.15 No Two Hurricanes Are the Same
6.0 Forecasting..............................................................................45
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
Now We Put It All Together
The Professional Weather Map—Surface and Upper Air Information
How Do We Select Which Forecast to Use?
Offshore Sailing
6.4.1 Advance Planning
6.4.2 Weather Routing Service
6.4.3 Weather Facsimile Broadcast (Weather Fax)
6.4.4 The Internet
6.4.5 VHF Radio
Using the Forecast
Local Weather
Forecasting from Visual Observations—The Single Observer
6.7.1 Where Do We Start?
6.7.2 The Two Aspects of Observation
Your Observation Data in Detail
6.8.1 Pressure
6.8.2 Pressure Tendency
6.8.3 Air Temperature
6.8.4 Dew Point and Relative Humidity
6.8.5 Clouds
6.8.6 Wind Direction and Speed
6.8.7 Visibility
6.8.8 State of the Sea
6.8.9 Present Weather
6.8.10 Past Weather
The Moment of Truth! Now to Make a Good Forecast
In Conclusion
Global Weather
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Table of Contents
Glossary .................................................................................................55
Appendix A: The Saffir-Simpson Scale (Abridged) .....................63
Appendix B: Relative Humidity (%) ................................................65
Appendix C: Dew Point Temperature Tables................................67
Appendix D: Beaufort Wind Scale ...................................................69
Appendix E: Conversion Tables .......................................................71
Appendix F: Wind/Barometer Table ................................................73
Appendix G: Weather Log ..................................................................75
Bibliography .........................................................................................77
Index .......................................................................................................79