The Weather The Weather ofof

British ColumbiaBritish Columbia

Graphic Area Forecast 31Graphic Area Forecast 31

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The Weather The Weather ofof

British ColumbiaBritish Columbia

Graphic Area Forecast 31Graphic Area Forecast 31

by

Ross KlockJohn Mullock

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Copyright

Copyright © 2001 NAV CANADA. All rights reserved. No part of this documentmay be reproduced in any form, including photocopying or transmission electroni-cally to any computer, without prior written consent of NAV CANADA. The infor-mation contained in this document is confidential and proprietary to NAV CANA-DA and may not be used or disclosed except as expressly authorized in writing byNAV CANADA.

Trademarks

Product names mentioned in this document may be trademarks or registered trade-marks of their respective companies and are hereby acknowledged.

Relief Maps

Copyright © 2000. Government of Canada with permission from Natural ResourcesCanada

Design and illustration by Ideas in MotionKelowna, British Columbia ph: (250) 717-5937ideasinmotion@shaw.ca

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The Weather of British Columbia Graphic Area Forecast 31 Pacific Region

Preface

For NAV CANADA’s Flight Service Specialists (FSS), providing weather briefingsto help pilots navigate through the day-to-day fluctuations in the weather is a criticalrole. While available weather products are becoming increasingly more sophisticated,and at the same time more easily understood, an understanding of local and regionalclimatological patterns is essential to the effective performance of this role.

This British Columbia Local Area Knowledge Aviation Weather manual is one ofa series of six publications prepared by Meteorological Service of Canada (MSC) forNAV CANADA. Each of the six manuals corresponds to a specific graphic forecastarea (GFA) Domain, with the exception of the Nunavut – Arctic manual that coverstwo GFA Domains. These manuals form an important part of the training programon local aviation weather knowledge for FSS working in the area and a useful tool inthe day-to-day service delivery by FSS.

Within the GFA domains, the weather shows strong climatological patterns con-trolled either by season or topography. This manual describes the Domain of theGFACN31. This area offers beautiful open spaces for flying but can also provideharsh flying conditions. As most pilots flying the region can attest, these variations inweather can take place quiet abruptly. From the rocky coast to jagged mountain peaks,local topography plays a key role in determining both the general climatology andlocal flying conditions in a particular region.

This manual provides some insight on specific weather effects and patterns in thisarea. While a manual cannot replace intricate details and knowledge of BritishColumbia that FSS and experienced pilots of the area have acquired over the years, thismanual is a collection of that knowledge taken from interviews with local pilots, dis-patchers, Flight Service Specialists and MSC personnel.

By understanding the weather and hazards in this specific area, FSS will be moreable to assist pilots to plan their flights in a safe and efficient manner. While this isthe manual’s fundamental purpose, NAV CANADA recognizes the value of theinformation collected for pilots themselves. More and better information on weatherin the hands of pilots will always contribute to aviation safety. For that reason, themanuals are being made available to NAV CANADA customers.

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Acknowledgements

This manual was made possible through funding by NAV CANADA, FlightInformation Centre project office.

NAV CANADA would like to thank The Meteorological Service of Canada(MSC), both national and regional personnel, for working with us to compile theinformation for each Graphic Area Forecast (GFA) domain, and present it in a user-friendly, professional format. Special thanks also go to meteorologists Ross Klockand John Mullock, Mountain Weather Centre, Kelowna. Ross’s regional expertise hasbeen instrumental for the development of the Pacific GFA document while John’sexperience and efforts have ensured high quality and consistent material fromAtlantic to Pacific to Arctic.

This endeavour could not have been as successful without the contributions ofmany people within the aviation community. We would like to thank all the participants who provided information through interviews with MSC, includingflight service specialists, pilots, dispatchers, meteorologists and other aviation groups.Their willingness to share their experiences and knowledge contributed greatly to thesuccess of this document.

Roger M. BrownJanuary, 2002

Readers are invited to submit any comments to:

NAV CANADACustomer Service Centre77 Metcalfe St.Ottawa, OntarioK1P 5L6

Toll free phone line: 1-800-876-4693-4 (within North America disregard the last digit)Toll-free fax line: 1-877-663-6656 E-mail: service@navcanada.ca

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TABLE OF CONTENTSPREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ivINTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ixCHAPTER 1 BASICS OF METEOROLOGY . . . . . . . . . . . . . . . . . .1

Heat Transfer and Water Vapour . . . . . . . . . . . . . . . . . . .1Lifting Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2Subsidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3Temperature Structure of the Atmosphere . . . . . . . . . . . .4Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Air Masses and Fronts . . . . . . . . . . . . . . . . . . . . . . . . . . .6

CHAPTER 2 WEATHER HAZARDS TO AVIATION . . . . . . . . . . .9Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Icing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

The Freezing Process . . . . . . . . . . . . . . . . . . . . . . . . .10Types of Aircraft Ice . . . . . . . . . . . . . . . . . . . . . . . . .10Meteorological Factors Affecting Icing . . . . . . . . . . . .11Aerodynamic Factors Affecting Icing . . . . . . . . . . . . .14Other Forms of Icing . . . . . . . . . . . . . . . . . . . . . . . . .15

Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Types of Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . .16Causes of Reduced Visibility . . . . . . . . . . . . . . . . . . .17

Wind Shear and Turbulence . . . . . . . . . . . . . . . . . . . . . .19Stability and the Diurnal Variation in Wind . . . . . . . .19Wind Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20Relationship Between Wind, Shear and Turbulence . .21Low Levels Jets - Frontal . . . . . . . . . . . . . . . . . . . . . .21Low Levels Jets - Nocturnal . . . . . . . . . . . . . . . . . . . .22Topographical Effects on Wind . . . . . . . . . . . . . . . . .22

Lee Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28The Formation of Lee Waves . . . . . . . . . . . . . . . . . . .28Characteristics of Lee Waves . . . . . . . . . . . . . . . . . . .29Clouds Associated with Lee Waves . . . . . . . . . . . . . .30

Fronts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Frontal Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32Frontal Waves and Occlusions . . . . . . . . . . . . . . . . . .33

Thunderstorms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35The Life Cycle of a Thunderstorm . . . . . . . . . . . . . . .36Types of Thunderstorms . . . . . . . . . . . . . . . . . . . . . . .38Thunderstorm Hazards . . . . . . . . . . . . . . . . . . . . . . .40Severe Thunderstorms . . . . . . . . . . . . . . . . . . . . . . . .40

Cold Weather Operations . . . . . . . . . . . . . . . . . . . . . . . .43Volcanic Ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Deformation Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

CHAPTER 3 WEATHER PATTERNS OF BRITISH COLUMBIA 49Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49South Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50North Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52Thompson-Okanagan . . . . . . . . . . . . . . . . . . . . . . . . . .53Kootenays and Columbias . . . . . . . . . . . . . . . . . . . . . . .55Central and Northern Interior . . . . . . . . . . . . . . . . . . . .57

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Northeast British Columbia . . . . . . . . . . . . . . . . . . . . . .59The Mean Atmospheric Circulation System . . . . . . . . .60Upper Troughs and Upper Ridges . . . . . . . . . . . . . . . . .61Semi-Permanent Surface Features . . . . . . . . . . . . . . . . .62Migratory Surface Weather Systems . . . . . . . . . . . . . . .63Winter Storms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63Gulf of Alaska Lows . . . . . . . . . . . . . . . . . . . . . . . . . . .64Coastal Lows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65Winter Frontal Systems . . . . . . . . . . . . . . . . . . . . . . . . .66Winter High Pressure Systems . . . . . . . . . . . . . . . . . . .66Arctic Outbreaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67Outflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68Inflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68Summer Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69Summer Fronts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69Thermal Troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70Cold Lows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71

CHAPTER 4 SEASONAL WEATHER AND LOCAL EFFECTS .75Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75South Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80

East Coast of Vancouver Island – Victoria to Nanaimo .80Vancouver Area Including Pitt Meadows, Langley and Boundary Bay . . . . . . . . . . . . . . . . . . . . . . . . . . .82Abbotsford to Hope . . . . . . . . . . . . . . . . . . . . . . . . .83Vancouver to Pemberton along Howe Sound . . . . . . .85Strait of Georgia – Vancouver - Nanaimo -Powell River - Comox . . . . . . . . . . . . . . . . . . . . . . . .87Inner Straits from Powell River/Comox – Queen Charlotte Sound . . . . . . . . . . . . . . . . . . . . . .88West Vancouver Island Including Routes toPort Alberni and Tofino . . . . . . . . . . . . . . . . . . . . . . .91

North Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96

Northern Vancouver Island to McInnes Island . . . . . .96Bella Bella to Prince Rupert . . . . . . . . . . . . . . . . . . . .98Central Coast to the Interior Plateau . . . . . . . . . . . .100Bella Bella to Kitimat (Douglas Channel) . . . . . . . . .102Prince Rupert to Stewart . . . . . . . . . . . . . . . . . . . . .103Queen Charlotte Islands . . . . . . . . . . . . . . . . . . . . .104Prince Rupert to Terrace/Kitimat . . . . . . . . . . . . . .105Routes Inland from Terrace . . . . . . . . . . . . . . . . . . .106

Thompson - Okanagan . . . . . . . . . . . . . . . . . . . . . . . . .107Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110

Routes Inland from the Coast . . . . . . . . . . . . . . . . .110Cache Creek to Kamloops . . . . . . . . . . . . . . . . . . . .114Princeton to Penticton . . . . . . . . . . . . . . . . . . . . . . .115

Okanagan Valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116Kamloops to Salmon Arm . . . . . . . . . . . . . . . . . . . .116

Kootenays and Columbias . . . . . . . . . . . . . . . . . . . . . . .118Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120

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Southern Route – Osoyoos to Cranbrookand Eastwards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120

Osoyoos to Castlegar . . . . . . . . . . . . . . . . . . . . . . . .121Castlegar to Cranbrook . . . . . . . . . . . . . . . . . . . . . .123Cranbrook and eastwards through the Crowsnest Pass 124

Northern Route – Salmon Arm to Banff . . . . . . . . . . .125Salmon Arm to Revelstoke . . . . . . . . . . . . . . . . . . .125Revelstoke to Golden to Banff . . . . . . . . . . . . . . . . .126

Rocky Mountain Trench . . . . . . . . . . . . . . . . . . . . . . .128Cranbrook to Golden . . . . . . . . . . . . . . . . . . . . . . .129Blaeberry Pass, Golden to the North Saskatchewan Crossing . . . . . . . . . . . . . . . . .130Golden - the Mica Dam – Tete Juane - Jasper . . . . .132

Central and Northern Interior . . . . . . . . . . . . . . . . . . .134Local Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .136

Lillooet - Pavilion - Clinton . . . . . . . . . . . . . . . . . . .136Clinton to Williams Lake . . . . . . . . . . . . . . . . . . . . .137Williams Lake - Alexis Creek - Puntzi Mountain - Anahim Lake . . . . . . . . . . . . . .138Anahim Lake to the Coast . . . . . . . . . . . . . . . . . . .139Williams Lake to Quesnel . . . . . . . . . . . . . . . . . . . .139Quesnel to Prince George . . . . . . . . . . . . . . . . . . . .140Kamloops - Vavenby - Blue River - Tete Jaune . . . . .141Tete Jaune Cache - Mcbride - Prince George . . . . . .142Prince George to Mackenzie . . . . . . . . . . . . . . . . . .143Crossing the Rockies: Mackenzie to Dawson Creek/Fort St. John . . . . . . . . . . . . . . . . . .144Mackenzie to Watson Lake along the Rocky Mountain Trench . . . . . . . . . . . . . . . . . . .146Prince George to Smithers . . . . . . . . . . . . . . . . . . . .148Smithers to Terrace via Telkwa Pass . . . . . . . . . . . . .149Smithers to Terrace via the Bulkley / Skeena River Valleys and the Shorter Route via the Kitseguecla Valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .150North from Terrace to the Nass River Valley . . . . . . .151New Hazelton to Meziadin via the Kispiox River Valley or the Kitwanga River Valley . . . . . . . . . . . . .152Meziadin to Stewart . . . . . . . . . . . . . . . . . . . . . . . . .153Meziadin to Bob Quinn Lake . . . . . . . . . . . . . . . . .154Bob Quinn Lake to Dease Lake . . . . . . . . . . . . . . . .155Dease Lake to Watson Lake . . . . . . . . . . . . . . . . . . .156Dease Lake to Teslin . . . . . . . . . . . . . . . . . . . . . . . .157Atlin north to the Yukon Border . . . . . . . . . . . . . . .158

Northeast British Columbia . . . . . . . . . . . . . . . . . . . . .159Local effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .162Dawson Creek to Fort Nelson . . . . . . . . . . . . . . . . .162Fort Nelson to Watson Lake . . . . . . . . . . . . . . . . . .164

CHAPTER 5 Airport Climatology . . . . . . . . . . . . . . . . . . . . . . . . . . .167GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201TABLE OF SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .207APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .208MAP INDEX Chapter 4 Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .209

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Introduction

Meteorology is the science of the atmosphere, a sea of air that is in a constant stateof flux. Within it storms are born, grow in intensity as they sweep across sections ofthe globe, then dissipate. No one is immune to the day-to-day fluctuations in theweather, especially the aviator who must operate within the atmosphere.

Traditionally, weather information for the aviation community has largely beenprovided in textual format. One such product, the area forecast (FA), was designed toprovide the forecast weather for the next twelve hours over a specific geographicalarea. This information consisted of a description of the expected motion of significantweather systems, the associated clouds, weather and visibility.

In April 2000, the Graphical Area Forecast (GFA) came into being, supercedingthe area forecast. A number of MSC Forecast Centres now work together, usinggraphical software packages, to produce a single national graphical depiction of theforecast weather systems and the associated weather. This single national map is thenpartitioned into a number of GFA Domains for use by Flight Service Specialists,flight dispatchers and pilots.

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This Pacific Local Area Knowledge Aviation Weather Manual is one of a series ofsix similar publications. All are produced by NAV CANADA in partnership with theMSC. These manuals are designed to provide a resource for Flight Service Specialistsand pilots to help with the understanding of local aviation weather. Each of the sixmanuals corresponds to a Graphical Area Forecast (GFA) Domain, with the excep-tion of the Nunavut - Arctic manual which covers two GFA Domains. MSC aviationmeteorologists provide most of the broader scale information on meteorology andweather systems affecting the various domains. Experienced pilots who work in oraround it on a daily basis, however, best understand the local weather. Interviews withlocal pilots, dispatchers and Flight Service Specialists form the basis for the informa-tion presented in Chapter 4.

Within the domains, the weather shows strong climatological patterns that arecontrolled either by season or topography. For example, in British Columbia there isa distinctive difference between the moist coastal areas and the dry interior becauseof the mountains. The weather in the Arctic varies strongly seasonally between thefrozen landscape of winter and the open water of summer. These changes are impor-tant in understanding how the weather works and each book will be laid out so as torecognize these climatological differences.

This manual describes the weather of the GFACN31 Pacific. This area often hasbeautiful flying weather but can also have some of the toughest flying conditions inthe world. Shifting dramatically from the dripping, fog-shrouded rain forests of theWest Coast, through the parched valley bottoms of the Interior, to the majestic snow-capped peaks and glaciers of the Rocky Mountains, few places in the world offer morevisual splendors for a pilot and passengers. At the same time, mountains also evokerapidly changing weather conditions that all too often have contributed to a tragedy.Between 1976 and 1994, there were 419 flying accidents in British Columbia whereweather has been identified as one of the contributing factors. In these accidents, 319people were killed and 89 people injured seriously. Mountain flying itself is not inher-ently dangerous, rather it is the weather associated with these areas that tends to beunforgiving of the rash, the negligent and the unlucky.

This manual is “instant knowledge” about how the weather behaves in this area butit is not “experience”. The information presented in this manual is by no meansexhaustive. The variability of local aviation weather in British Columbia could resultin a publication several times the size of this one. However, by understanding someof the weather and hazards in these areas, pilots may be able to relate the hazards totopography and weather systems in areas not specifically mentioned.

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Chapter 1

Basics of Meteorology

To properly understand weather, it is essential to understand some of the basicprinciples that drive the weather machine. There are numerous books on the marketthat describe these principles in great detail with varying degrees of success. This sec-tion is not intended to replace these books, but rather to serve as a review.

Heat Transfer and Water Vapour

The atmosphere is a “heat engine” that runs on one of the fundamental rules ofphysics: excess heat in one area (the tropics) must flow to colder areas (the poles).There are a number of different methods of heat transfer but a particularly efficientmethod is through the use of water.

Within our atmosphere, water can exist in three states depending on its energylevel. Changes from one state to another are called phase changes and are readilyaccomplished at ordinary atmospheric pressures and temperatures. The heat taken inor released during a phase change is called latent heat.

How much water the air contains in the form of vapour is directly related to itstemperature. The warmer the air, the more water vapour it can contain. Air that con-tains its maximum amount of water vapour, at that given temperature, is said to besaturated. A quick measure of the moisture content of the atmosphere can be made

Fig. 1-1 - Heat transfer and water vapour

MELTIN

G

FREE

ZING

SUBLIMATION

DEPOSITION

CONDENSATIONEVAPORATION

LATENT HEAT ABSORBED

LATENT HEAT RELEASED

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by looking at the dew point temperature. The higher (warmer) the dew point tem-perature, the greater the amount of water vapour.

The planetary heat engine consists of water being evaporated by the sun into watervapour at the equator (storing heat) and transporting it towards the poles on thewinds where it is condensed back into a solid or liquid state (releasing heat). Most ofwhat we refer to as “weather,” such as wind, cloud, fog and precipitation is related tothis conversion activity. The severity of the weather is often a measure of how muchlatent heat is released during these activities.

Lifting Processes

The simplest and most common way water vapour is converted back to a liquid orsolid state is by lifting. When air is lifted, it cools until it becomes saturated. Anyadditional lift will result in further cooling which reduces the amount of water vapourthe air can hold. The excess water vapour is condensed out in the form of clouddroplets or ice crystals which then can go on to form precipitation. There are severalmethods of lifting an air mass. The most common are convection, orographic lift(upslope flow), frontal lift, and convergence into an area of low pressure.

Fig. 1-2 - Convection as a result of � daytime heating

Fig.1-3 - Orographic (upslope) lift

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Subsidence

Subsidence, in meteorology, refers to the downward motion of air. This subsidingmotion occurs within an area of high pressure, as well as on the downward side ofa range of hills or mountains. As the air descends, it is subjected to increasingatmospheric pressure and, therefore, begins to compress. This compression causesthe air’s temperature to increase which will consequently lower its relative humidity.As a result, areas in which subsidence occurs will not only receive less precipitationthan surrounding areas (referred to as a “rain shadow”) but will often see the cloudlayers thin and break up.

Surfacrface DivergenceceeronvergenceonveConve

Fig. 1-5 - Divergence and convergence at the surface and aloft in a high low couplet

Fig.1-4 - Warm air overrunning cold air along a warm front

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Temperature Structure of the Atmosphere

The temperature lapse rate of the atmosphere refers to the change of temperaturewith a change in height. In the normal case, temperature decreases with heightthrough the troposphere to the tropopause and then becomes relatively constant in thestratosphere.

Two other conditions are possible: an inversion, in which the temperature increas-es with height, or an isothermal layer, in which the temperature remains constant withheight.

Fig. 1-7 - Different lapse rates of the atmosphere

-9°C

3°C

3°C

-2°C

NORMAL

ISOTHERMALLAYER

INVERSION

ALTITUDE

Fig.1-6 - Moist air moving over mountains where it loses its moisture and sinks � into a dry subsidence area

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The temperature lapse rate of the atmosphere is a direct measurement of the sta-bility of the atmosphere.

Stability

It would be impossible to examine weather without taking into account the stabil-ity of the air. Stability refers to the ability of a parcel of air to resist vertical motion.If a parcel of air is displaced upwards and then released it is said to be unstable if itcontinues to ascend (since the parcel is warmer than the surrounding air), stable if itreturns to the level from which it originated (since the parcel is cooler than the sur-rounding air), and neutral if the parcel remains at the level it was released (since theparcel’s temperature is that of the surrounding air).

The type of cloud and precipitation produced varies with stability. Unstable air,when lifted, has a tendency to develop convective clouds and showery precipitation.Stable air is inclined to produce deep layer cloud and widespread steady precipitation.Neutral air will produce stable type weather which will change to unstable typeweather if the lifting continues.

The stability of an air mass has the ability to be changed. One way to destabilizethe air is to heat it from below, in much the same manner as you would heat water ina kettle. In the natural environment this can be accomplished when the sun heats theground which, in turn, heats the air in contact with it, or when cold air moves over awarmer surface such as open water in the fall or winter. The reverse case, cooling theair from below, will stabilize the air. Both processes occur readily.

Consider a typical summer day where the air mass is destabilized by the sun, result-ing in the development of large convective cloud and accompanying showers or thun-dershowers during the afternoon and evening. After sunset, the surface cools and theair mass stabilizes slowly, causing the convective activity to die off and the clouds todissipate.

Fig. 1-8 - Stability in the atmosphere - (a) Stable (b) Unstable (c) Neutral

(a) (b)

(c)

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On any given day there may be several processes acting simultaneously that caneither destabilize or stabilize the air mass. To further complicate the issue, these com-peting effects can occur over areas as large as an entire GFA domain to as small as a football field. To determine which one will dominate remains in the realm of a mete-orologist and is beyond the scope of this manual.

Wind

Horizontal differences in temperature result in horizontal differences in pressure. Itis these horizontal changes in pressure that cause the wind to blow as the atmosphereattempts to equalize pressure by moving air from an area of high pressure to an areaof low pressure. The larger the pressure difference, the stronger the wind and, as aresult, the day-to-day wind can range from the gentlest breeze around an inland air-field to storm force winds over the water.

Wind has both speed and direction, so for aviation purposes several conventionshave been adopted. Wind direction is always reported as the direction from which thewind is blowing while wind speed is the average steady state value over a certainlength of time. Short-term variations in speed are reported as either gusts or squallsdepending on how long they last.

Above the surface, the wind tends to be relatively smooth and changes directionand speed only in response to changes in pressure. At the surface, however, the windis affected by friction and topography. Friction has a tendency to slow the wind overrough surfaces whereas topography, most commonly, induces localized changes indirection and speed.

Air Masses and Fronts

Air Masses

When a section of the troposphere, hundreds of miles across, remains stationary ormoves slowly across an area having fairly uniform temperature and moisture, then the

Pressure difference from A to B is 4 hPa in 70 miles

Pressure difference from C to D is 4 hPa in 200 miles

Fig. 1-9 - The greater pressure changes with horizontal difference, the stronger the wind

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air takes on the characteristics of this surface and becomes known as an air mass. Thearea where air masses are created are called “source regions” and are either ice or snowcovered polar regions, cold northern oceans, tropical oceans or large desert areas.

Although the moisture and temperature characteristics of an air mass are relativelyuniform, the horizontal weather may vary due to different processes acting on it. It isquite possible for one area to be reporting clear skies while another area is reportingwidespread thunderstorms.

Fronts

When air masses move out of their source regions they come into contact withother air masses. The transition zone between two different air masses is referred toas a frontal zone, or front. Across this transition zone temperature, moisture content,pressure, and wind can change rapidly over a short distance.

The principal types of fronts are:

More will be said about frontal weather later in this manual.

Cold Front - The cold air is advancing and undercutting the warm air. The leading edge of the cold air is the cold front.

Warm front - The cold air is retreating and being replaced by warm air. The trailing edge of the cold air is the warm front.

Stationary front - The cold air is neither advancing nor retreating. These fronts are frequently referred to quasi-stationary fronts although there usually is some small-scale localized motion occurring.

Trowal - Trough of warm air aloft.

Table 1-1

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