Intro to basic navigation lrg

Grunt Productions 2007

INTRODUCTION TO BASIC NAVIGATION

A Brief By Lance GrindleyA Brief By Lance Grindley


Table of ContentsTable of Contents

Section 1Section 1 Types of NavigationTypes of Navigation Section 2Section 2 Terrestrial CoordinatesTerrestrial Coordinates Section 3Section 3 ChartsCharts Section 4 Section 4 CompassCompass Section 5Section 5 Navigational AidsNavigational Aids


Table of ContentsTable of Contents

Section 6 Section 6 Position Lines and FixesPosition Lines and Fixes Section 7Section 7 TidesTides Section 8 CurrentsSection 8 Currents Section 9 WeatherSection 9 Weather


Types of NavigationTypes of Navigation


Navigation DefinedNavigation Defined

Navigation The process of safely and efficiently directing the movements of a vessel from one place to another.



1. Piloting (Coastal) Navigation 2. Dead Reckoning3. Celestial Navigation 4. Electronic Navigation



1. Piloting (Coastal) Navigation This is the process by which the ship’s position is found usually at a set interval, by taking 3 compass bearings of fixed, prominent and identifiable charted objects. These bearings, when corrected for deviation and variation are plotted on the chart, and the vessel’s position at that time is found.A sextant can be used on coastal navigation as well.



1. Piloting (Coastal) Navigation



2. Dead ReckoningThis type of navigation is used, working from a last known position fix.The vessel’s steady course and speed over a known period of time is used to calculate the True Course and Distance traveled over that period of time. This True Course and Distance is plotted from the last known position fix, and a Dead Reckoning Position obtained.



2. Dead Reckoning



3. Celestial Navigation This form of navigation is using a sextant to measure the vertical angle of sun, moon, planets or stars above the horizon, combined with exact GMT time taken from a chronometer.A calculation based on a dead reckoning position, will yield the distance towards or away the celestial object from that position, and a single position line is found.



3. Celestial Navigation (Continued)If a number of stars altitudes are taken at around the same time, normally at twilight, a fix can be made. Similarly if a planet and the sun are about 60 degrees or more in azimuth, can be measured at about the same time an reasonably accurate fix can be obtained.



3. Celestial Navigation (Continued)Otherwise the most common method is to use a running fix with two sights of the sun taken over about three hours, of which one may be when the sun is due north or south.



4. Electronic NavigationThis form of navigation is any navigation undertaken using electronic navigational aids.These include:

LORAN CRadarTransit Satellite NavigatorGlobal Positioning System



5. Electronic Navigation (Continued)It is important that the navigator understands the limitations and error that these systems are prone to.Only then can a true appreciation of the fix accuracy be made, and the accuracy of the position of the vessel be made.


Section 2 Terrestrial Coordinate Section 2 Terrestrial Coordinate SystemSystem

Grunt Productions 2007For navigational purposes, it’s considered a “true” sphere with a circumference of 21,600 NM

Earth: A “not-so-perfect” SphereEarth: A “not-so-perfect” Sphere


Terrestrial Coordinate SystemTerrestrial Coordinate System Great CircleGreat Circle: The intersection of a plane passing : The intersection of a plane passing

through two points on the surface of the earth and through two points on the surface of the earth and the center of the earth. the center of the earth.


Terrestrial Coordinate SystemTerrestrial Coordinate System Examples are: The Equator, Meridians of Examples are: The Equator, Meridians of

Longitude, the Prime Meridian being through Longitude, the Prime Meridian being through Greenwich, near London, United Kingdom.Greenwich, near London, United Kingdom.


EquatorEquator The great circle formed by passing a plane The great circle formed by passing a plane

perpendicular to the earth’s axis halfway between perpendicular to the earth’s axis halfway between its poles. its poles.


EquatorEquator The equator divides the earth into northern and The equator divides the earth into northern and

southern hemispheres.southern hemispheres. One of the two great circles from which all One of the two great circles from which all

locations on the earth’s surface are referenced.locations on the earth’s surface are referenced.


Terrestrial Coordinate SystemTerrestrial Coordinate System

Small Circle: A circle formed from the intersection of Small Circle: A circle formed from the intersection of a plane not passing through the center of the earth. a plane not passing through the center of the earth.

Examples are Parallels of LatitudeExamples are Parallels of Latitude


Measurement of ArcMeasurement of Arc

Positions in relationship to Earth’s Coordinates system are expressed in:Positions in relationship to Earth’s Coordinates system are expressed in: PRONOUNCEDPRONOUNCED SYMBOLSYMBOL

DegreesDegrees (°) (°)

MinutesMinutes (´)(´)

SecondsSeconds (´´) (´´)


LatitudeLatitude

LatitudeLatitude - angular distance north or south between the - angular distance north or south between the equator and the parallel of a point. Latitude is measured in equator and the parallel of a point. Latitude is measured in degrees of arc from 0degrees of arc from 0 either north or south of the equator. either north or south of the equator.

Latitude is measured along a meridian.Latitude is measured along a meridian.


LatitudeLatitude

Latitude is always expressed using 2 digits, e.g 49ºLatitude is always expressed using 2 digits, e.g 49º

Always given first when giving a position Always given first when giving a position

The length of 1 degree of latitude is always 60NMThe length of 1 degree of latitude is always 60NM


Parallels of LatitudeParallels of Latitude


Prime MeridianPrime Meridian The meridian that passes through the original position of the The meridian that passes through the original position of the

Royal Greenwich Observatory near London, England. Royal Greenwich Observatory near London, England.

Constitutes the second reference line for the terrestrial coordinate Constitutes the second reference line for the terrestrial coordinate system. system.


Prime MeridianPrime Meridian All other meridians are referenced to the prime meridian; All other meridians are referenced to the prime meridian;

it divides the earth into the eastern and western it divides the earth into the eastern and western hemispheres.hemispheres.


LongitudeLongitude Longitude - angular distance E/W between the Longitude - angular distance E/W between the

prime meridian and the meridian of a point. prime meridian and the meridian of a point.

Longitude is measured in degrees of arc from 0 to Longitude is measured in degrees of arc from 0 to 180 degrees east or west of the prime meridian.180 degrees east or west of the prime meridian.

Longitude is measured along parallels of latitudeLongitude is measured along parallels of latitude


LongitudeLongitude Longitude is always expressed using 3 digits, e.g Longitude is always expressed using 3 digits, e.g

123º.123º.

One degree of long does not equal 60 NM unless One degree of long does not equal 60 NM unless measured along the equator.measured along the equator.

Always given after Latitude when giving a Always given after Latitude when giving a position.position.


Lines of Longitude


Section 3: ChartsSection 3: Charts


Desirable qualities of a chart projection:Desirable qualities of a chart projection:

1. Maintain 1. Maintain true shapetrue shape of physical features. of physical features.

2. Maintain 2. Maintain correct proportionscorrect proportions of features relative to of features relative to one another.one another.

3. 3. True scaleTrue scale, permitting accurate measurement of , permitting accurate measurement of distance.distance.

4. 4. Rhumb linesRhumb lines plot as straight lines. They are lines on plot as straight lines. They are lines on the earth’s surface that cross all meridians at the the earth’s surface that cross all meridians at the same anglesame angle

Chart ProjectionsChart Projections


Mercator ProjectionMercator Projection


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ADVANTAGESADVANTAGES Position, distance, and direction can be accurately Position, distance, and direction can be accurately

measuredmeasured True shape of features is maintained over small True shape of features is maintained over small

areasareas


Chart ScaleChart Scale

The relationship between two measurements. The relationship between two measurements. Expressed as a ratio.Expressed as a ratio.

The scale to which a chart is drawn appears The scale to which a chart is drawn appears directly under its title.directly under its title.


Scale Conversion and ReferenceScale Conversion and Reference



Large scale chart covers a small area and are Large scale chart covers a small area and are used for piloting and inshore navigation.used for piloting and inshore navigation.



Small scale charts are less detailed than large scale charts and cover a large area.


Types of ChartsTypes of Charts

Coastal charts:Coastal charts:

Large Scale ChartsLarge Scale Charts 1:50,000 - 1:150,0001:50,000 - 1:150,000

For approaching bays and harbors, and used for For approaching bays and harbors, and used for coastal navigation showing outlying reefs and coastal navigation showing outlying reefs and shoals.shoals.


Plotting a Position 1Plotting a Position 1

1. Determine the parallels 1. Determine the parallels on the chart that on the chart that bracket the latitude.bracket the latitude.

2. Place the pivot point of 2. Place the pivot point of the compass on the the compass on the closest line.closest line.



3. Spread the compass 3. Spread the compass until the lead rests on until the lead rests on the given latitude.the given latitude.

4. Move to the 4. Move to the approximate longitude approximate longitude and swing an arc.and swing an arc.



5. The same process is repeated 5. The same process is repeated using the longitude scale and using the longitude scale and the given longitude.the given longitude.

6. The desired position is the 6. The desired position is the intersection of these two arcs.intersection of these two arcs.



7. If plotted correctly, the 7. If plotted correctly, the intersection should occur intersection should occur at the crest of both arcs.at the crest of both arcs.


Measuring DistanceMeasuring Distance The latitude scale can be used to measure distances, The latitude scale can be used to measure distances,

since one degree of latitude equals 60 nautical miles, since one degree of latitude equals 60 nautical miles, everywhere on the earth.everywhere on the earth.


Measuring DistanceMeasuring Distance

• NEVER use the longitude

scale to determine distances on a chart.


Measuring DirectionMeasuring Direction All rhumb lines on a Mercator projection represent All rhumb lines on a Mercator projection represent

truetrue directions. directions.

Measurement of direction Measurement of direction on a Mercator chart is on a Mercator chart is accomplished by using a accomplished by using a parallel ruler to transfer the parallel ruler to transfer the direction of a rhumb line to direction of a rhumb line to a nearby compass rose.a nearby compass rose.


Measuring DirectionMeasuring Direction

• A

• B

045ºTrue

060ºMagnetic


Correction of ChartsCorrection of Charts

The Hydrographer issues weekly Notices to The Hydrographer issues weekly Notices to Mariners, which include corrections to be made to Mariners, which include corrections to be made to UK charts.UK charts.

When charts are bought, they are generally When charts are bought, they are generally corrected up to date.corrected up to date.

Once in use Notices to Mariners should be Once in use Notices to Mariners should be checked and corrections to charts made as checked and corrections to charts made as necessary.necessary.


Correction of ChartsCorrection of Charts

When a correction has been made, a note of the When a correction has been made, a note of the year and Notice to Mariner number should be year and Notice to Mariner number should be made in the bottom left hand corner of the chart, made in the bottom left hand corner of the chart, having checked that the previous correction has having checked that the previous correction has been made.been made.

2008 2008 - 41- 74 - 86 - 127- 41- 74 - 86 - 127


Section 4 CompassSection 4 Compass


Types of CompassesTypes of Compasses

Magnetic CompassMagnetic Compass

A compass that senses direction by interaction A compass that senses direction by interaction between its own permanent magnets and the between its own permanent magnets and the earth’s magnetic field.earth’s magnetic field.

Gyroscopic CompassGyroscopic Compass

A electrical gyroscopic that is designed to seek true A electrical gyroscopic that is designed to seek true northnorth


Compass RoseCompass Rose


Directional Reference SystemsDirectional Reference Systems

Directional ReferencesDirectional References Relative BearingsRelative Bearings ( (R) = bearings measured R) = bearings measured

with reference to the ship’s longitudinal axiswith reference to the ship’s longitudinal axis Magnetic BearingsMagnetic Bearings ( (M) = bearings measured M) = bearings measured

with respect to magnetic north.with respect to magnetic north. True BearingsTrue Bearings ( (T) = bearings measured with T) = bearings measured with

respect to true of geographic north. respect to true of geographic north.


Directional Reference SystemsDirectional Reference Systems

Ship’s Head (or heading)Ship’s Head (or heading) a special bearing denoting the direction in a special bearing denoting the direction in

which the ship is pointing.which the ship is pointing.


270ºT

000ºT

090ºT

180º T

True Bearings


Magnetic CompassMagnetic Compass


270ºM

000ºM

090ºM

180º M

Magnetic Bearings

Variation Easterly


000ºR

090º R

270ºR

180ºR

Relative Bearings


Dead Ahead

Starboard Beam

Port Beam

Right Astern

Relative Bearings


Magnetic Compass Error: VariationMagnetic Compass Error: Variation Variation is Variation is the angle between a magnetic line of the angle between a magnetic line of

force and a geographic (true) meridian at any location force and a geographic (true) meridian at any location on the earth.on the earth.

Variation exists because the earth’s magnetic and Variation exists because the earth’s magnetic and geographic poles are not in the same location.geographic poles are not in the same location.


Magnetic Compass Error: VariationMagnetic Compass Error: Variation Magnetic anomalies in the earth’s crust also Magnetic anomalies in the earth’s crust also

contribute to variation.contribute to variation.


True North PoleMagnetic North Pole

Notice that the two poles aren’t together. The magnetic compass points to the magnetic pole, and this gives us VARIATION.


Magnetic Compass Error: Magnetic Compass Error: VariationVariation

Variation also changes from year to year as the Variation also changes from year to year as the earth’s magnetic poles tend to wander.earth’s magnetic poles tend to wander.

Variation is printed inside compass roses on all Variation is printed inside compass roses on all navigation charts. navigation charts.

Always use the compass rose nearest your current Always use the compass rose nearest your current Dead Reckoning position.Dead Reckoning position.



• Variation changes as an observer moves along Variation changes as an observer moves along the globe. the globe.

• However, if a ship moves in such a way that the However, if a ship moves in such a way that the meridians remained constant, it would be meridians remained constant, it would be moving along an moving along an isogonic lineisogonic line - a line along - a line along which variation remains constant.which variation remains constant.



• The amount that the Variation changes annually The amount that the Variation changes annually is called the Annual Change.is called the Annual Change.

• The amount of Annual Change is to be found The amount of Annual Change is to be found on every compass rose on the chart, next to the on every compass rose on the chart, next to the Variation, and is normally expressed as 004°W Variation, and is normally expressed as 004°W 1995 (8’E)1995 (8’E)..

• To calculate change in 2008, multiply 8’E by To calculate change in 2008, multiply 8’E by 13 (years from 1995) 104’ or 1.75°E 13 (years from 1995) 104’ or 1.75°E

• Apply 1.75°E to Variation of 004°W = 2.25°WApply 1.75°E to Variation of 004°W = 2.25°W


Magnetic Compass Error: Magnetic Compass Error: DeviationDeviation

This isThis is the angle between the magnetic meridian and the angle between the magnetic meridian and the north line on the compass card.the north line on the compass card.

Deviation is caused by the interaction of the ship’s Deviation is caused by the interaction of the ship’s metallic structure, electrical systems, metallic objects metallic structure, electrical systems, metallic objects (such as a cell phone left close to the compass) with (such as a cell phone left close to the compass) with the earth’s magnetic field.the earth’s magnetic field.


Deviation

A ship’s compass also must deal with magnetic forces from the ship itself, e.g.magnets, electrical cabling. The sum total of these forces pulls the compass slightly away from magnetic north, producing DEVIATION.


Deviation

Deviation will change in size, dependant upon the course of the vessel.

Swinging of the ship and proper correction using soft and/or permanent magnets , deviation can be minimized.


Compass ConversionsCompass Conversions

Compass to TrueCompass to True

1. 1. C D M V T (AE)C D M V T (AE)

Can Dead Men Vote Twice (at elections)?Can Dead Men Vote Twice (at elections)?

2. C A D E T2. C A D E T

Compass Add East TrueCompass Add East True



Convert Compass Courses to True Courses - this also Convert Compass Courses to True Courses - this also applies to bearings, using the deviation for the vessel’s applies to bearings, using the deviation for the vessel’s head.head.

Compass Course 145°CCompass Course 145°C

Deviation 2°WDeviation 2°W

Magnetic Course 143°MMagnetic Course 143°M

Variation 22°EVariation 22°E

True Course 165°TTrue Course 165°T



Converting True to CompassConverting True to Compass

T V M D C (AW)T V M D C (AW)

True Virgins Make Dull Companions (At Weddings)True Virgins Make Dull Companions (At Weddings)



Convert True Course to Compass Course - this also Convert True Course to Compass Course - this also applies to bearings, using the deviation for the vessel’s applies to bearings, using the deviation for the vessel’s head.head.

True Course 165°T True Course 165°T

Variation 22°E Variation 22°E

Magnetic Course 143°M Magnetic Course 143°M

Deviation 2°W Deviation 2°W

Compass Course 145°CCompass Course 145°C


Section 5Navigational Aids


Navigational Aid: Any device external to a vessel or Navigational Aid: Any device external to a vessel or aircraft intended to assist in determining position and aircraft intended to assist in determining position and safe course, or to warn of dangers or obstructions.safe course, or to warn of dangers or obstructions.

Significance of Navigational AidsSignificance of Navigational Aids


Navigational aids will include:Navigational aids will include:

LighthousesLighthouses

Transit MarksTransit Marks

Leading LinesLeading Lines

BuoyageBuoyage

Beacons & Day MarksBeacons & Day Marks

Identifiable charted objectIdentifiable charted object

Navigational AidsNavigational Aids


Criteria: Criteria: DAYTIMEDAYTIME

• LocationLocation

• ShapeShape

• Color SchemeColor Scheme

• Auxiliary featuresAuxiliary features

• Special MarkingsSpecial Markings

Positive Identification of Navigation Positive Identification of Navigation AidsAids

NIGHTNIGHT

• Phase characteristicPhase characteristic

• Period & ColorPeriod & Color


Phase Characteristics Phase Characteristics Chart SymbolChart Symbol MeaningMeaning

FixedFixed FF Steady, unblinkingSteady, unblinking

FlashingFlashing FlFl Flashes at Flashes at regular regular intervalsintervals

Quick Flash Quick Flash Qk. Fl.Qk. Fl. Flash at least 60 Flash at least 60 times/min.times/min.

Group FlashGroup Flash Gp. Fl.Gp. Fl. Group of two or Group of two or more flashesmore flashes

Positive Identification of Navigation Positive Identification of Navigation Aids (at night)Aids (at night)


Phase Characteristics Phase Characteristics Chart SymbolChart Symbol MeaningMeaning

Morse Code Morse Code Mo. [A]Mo. [A] Morse alpha Morse alpha (short/long)(short/long)

Occulting Occulting Occ.Occ. On longer than it’s On longer than it’s offoff

PeriodPeriod Length in seconds of repetitionLength in seconds of repetition Color (red, green, yellow, or white)Color (red, green, yellow, or white)

Positive Identification of Navigation Positive Identification of Navigation Aids (at night)Aids (at night)


CHARACTERISTICS OF LIGHTS

Flashing pattern and period (|-----|) Type AbbreviationDescription

Fixed A light showing continuously and steadily F

Fixed and flashing A light in which a fixed light is combined witha flashing light of higher luminous intensity

F Fl

Flashing A flashing light in which a flash is regularlyrepeated (frequency not exceeding 30 flashesper minute)

Fl

Group flashing A flashing light in which a group of flashes, specified in number, is regularly repeated.

Fl (2)

Composite group flashing A light similar to a group flashing light exceptthat successive groups in the period have dif-ferent numbers of flashes

Fl (2+1)

Isophase A light in which all durations of light anddarkness are equal

Iso

Single occulting An occulting light in which an eclipse, orshorter duration than the light, is regularlyrepeated.

Oc

Group occulting An occulting light in which a group ofeclipses, specified in number, is regularlyrepeated.

Oc (2)

Quick A quick light in which a flash is regularlyrepeated at a rate of 60 flashes per minute

Q

Interrupted quick A quick light in which the sequence of flashesis interrupted by regularly repeated eclipses of constant and long duration

lQ

Group quick A group of 2 or more quick flashes, specifiedin number, which are regularly repeated. (Notused in the waters of the United States.)

Q(3)

Morse code A light in which lights of two clearly different durations (dots and dashes) are grouped to represent a character or characters in theMorse code.

Mo (A)

Alternating A light showing different colours alternately Al RW

Long flashing A flashing light in which the flash is 2 seconds or longer

LFl

Composite group occulting A light, similar to a group occulting light,except that successive groups in a period have different numbers of eclipses

Oc (2+1)


Special Purpose LightsSpecial Purpose Lights

Sector LightsSector Lights red light used in red light used in

dangerous sectorsdangerous sectors sector limits are sector limits are

expressed in degrees expressed in degrees true true as observed from a as observed from a vesselvessel, not from the , not from the light!light!


Special Purpose LightsSpecial Purpose Lights

Sector LightsSector Lights


Other Navigational aids, providing they are Other Navigational aids, providing they are charted, will include:charted, will include:

Other Navigational AidsOther Navigational Aids


Navigation Marks and BuoyageNavigation Marks and Buoyage


Determining the Computed Determining the Computed Visibility of a NavAidVisibility of a NavAid


Horizon distanceHorizon distance = the line of sight from a position = the line of sight from a position above the earth’s surface to the visual horizon.above the earth’s surface to the visual horizon.

Geographic rangeGeographic range = the maximum distance that a = the maximum distance that a light may be seen in perfect visibility by an light may be seen in perfect visibility by an observer’s eye who is at sea level.observer’s eye who is at sea level.



Computed rangeComputed range = the distance at which a light = the distance at which a light could be seen in perfect visibility (taking into could be seen in perfect visibility (taking into account elevation, observer’s height of eye, and the account elevation, observer’s height of eye, and the curvature of the earth). Computed Range = curvature of the earth). Computed Range = Horizon Distance + Geographic DistanceHorizon Distance + Geographic Distance




Computed visibilityComputed visibility = The maximum distance at = The maximum distance at which a light can be seen in the current which a light can be seen in the current meteorological conditions.meteorological conditions.



Luminous rangeLuminous range = the maximum distance at which a = the maximum distance at which a light may be seen under under the current light may be seen under under the current meteorological conditions.meteorological conditions.

Nominal rangeNominal range = a special case of the luminous = a special case of the luminous range. It is the distance a light could be seen in range. It is the distance a light could be seen in “clear” weather. Also called the charted range.“clear” weather. Also called the charted range.


Section 6 Position Lines and FixesSection 6 Position Lines and Fixes


Position LinesPosition Lines •Position Lines (P/L) - A single observation that does not establish a fix, but does mean that ship’s position is somewhere along that line.

•Label - After the position line is drawn from a charted object, a four digit time must be written above and parallel to the position line.


Position LinesPosition Lines

•All Compass bearings that are to be plotted on the chart, must be corrected to True Bearings, allowing for any compass error, including deviation and variation, before plotting.

•All True bearings/ courses taken from the chart, must be corrected for any compass error to obtain Compass Bearings/compass before use on radar or vessel’s magnetic compass.


Sources of Position LinesSources of Position Lines

A visual position line can be taken, using charted A visual position line can be taken, using charted fixed navigational aids such as tanks, water towers, fixed navigational aids such as tanks, water towers, church steeples, spires, radio and TV towers, day church steeples, spires, radio and TV towers, day marks, fixed navigation lights, flagpoles, or tangents marks, fixed navigation lights, flagpoles, or tangents

to points of land.to points of land.

In general fixing off floating objects, especially In general fixing off floating objects, especially buoys, should be avoided, if there are fixed charted buoys, should be avoided, if there are fixed charted objects available.objects available.


Visual Visual Position Position

LineLine

1000


Radar Radar Range Range

Position Position LineLine


Position Line MeasurementPosition Line Measurement Visual Bearings can be measured in:Visual Bearings can be measured in:

1. Degrees Relative ( # # # 1. Degrees Relative ( # # # 00R )R )

2. Degrees per Gyro Compass ( # # # ºG )2. Degrees per Gyro Compass ( # # # ºG )

3. Degrees Magnetic ( # # # 3. Degrees Magnetic ( # # # 00M ) M ) The navigator must convert any of these types of The navigator must convert any of these types of

bearings to True before they can be plotted on the bearings to True before they can be plotted on the chart.chart.

Degrees True ( # # # Degrees True ( # # # 00T)T)


Plotting and Labeling a FixPlotting and Labeling a Fix

•Fix - The point where two or more position lines, taken at the same time, cross. This indicates the ship’s position on the chart.

•Label - Use the four digit time next to the fix,it should be parallel to the bottom of the chart. The times of the individual position lines are not written.



Fix 1Fix 1

Compass bearing of Abode Island bearing 009°Compass, deviation 1ºW, variation 23ºE, gives 030 º True Bearing



Fix 2Fix 2

Compass bearing of Grebe Island Light bearing 058 º Compass, deviation 1ºW, variation 23ºE, gives 080 º True Bearing



Fix 3Fix 3

Compass bearing of Pt. Atkinson Light bearing 098ºCompass, deviation 1ºW, variation 23º E, gives True Bearing of 120 º T



Fix 4Fix 4

1230Insert fix circle on intersection of position lines, and time of fix


Cocked HatsCocked Hats

•In a perfect world, with due allowance made for compass error, the three position lines will cross at one point.•However depending on the speed of the vessel, the proximity of the object from which a vessel is being fixed, and the accuracy of the bearing when taken, and other factors, it is far more likely that a cocked hat will be obtained. •The larger the cocked hat, the larger an error on one, two or all of the position lines is likely to be.


Cocked Cocked HatHat

1230

In this example there is an error of 3ºE on the compass bearing of Point Atkinson Light and a cocked hat is formed.


Cocked HatsCocked Hats

• Where a plotted position is a cocked hat, and there is no obvious error (such as in calculation), it should be generally assumed the position of the vessel is the point in the cocked hat closest to the nearest danger.

•Another position should be taken a soon as convenient to check on the position.An


Reducing ErrorsReducing Errors

• When taking distances or ranges, always take the ranges ahead or astern first, to minimize errors (as these ranges will change quickest with the speed of the vessel) before taking ranges on the beam.•When taking compass bearings, always take the bearings on the beam first, to minimize errors (as these bearings will change quickest with the speed of the vessel) before taking bearings ahead or astern.


Radar FixesRadar Fixes

• Radar bearings are subject to compass error.

• Therefore the best way to obtain a fix by radar, is to take three radar distances off charted and identified objects.


Radar Radar Position 1Position 1

Using radar:

Grebe Is

Electronic Bearing Marker showing 058 º M

Variable Range Marker showing 0.82’



From radar, plot position circle:

Grebe Is

Distance 0.49 nm



Grebe Is Range 0.82’

A second range of 0.93’ off Eagle Is. would give fix

Mark fix position and time. Best fix would be have third range.

1000



Radar bearing of Grebe Is. is 058 º compass

Deviation 1ºW

Variation 23ºE

True Bearing 080 ºT which confirms ranges

1000


Electronic PositionElectronic Position

• The GPS can give an accurate electronic position.

•First check that the GPS information is live, and not on Dead Reckoning (which GPS reverts to with certain faults).

•Also check that the HDOP figure is low - 1 is best.


Electronic Electronic Position 1Position 1

Note down Latitude and Longitude

49º 20.38’N

123º 17.23’W



Plot Latitude and Longitude

49º 20.38’N

123º 17.23’W



1000

Insert fix symbol, and time


TransitsTransits


TransitsTransits Transits are the most accurate type of position line, Transits are the most accurate type of position line,

when two charted objects line up.when two charted objects line up. Transits are one of the most valuable tools when Transits are one of the most valuable tools when

close to dangers or the land.close to dangers or the land. Some transits are man made (intentional) and others Some transits are man made (intentional) and others

are natural (coincidental).are natural (coincidental).


TransitsTransits The main benefits of transits are:The main benefits of transits are:

1. There is no compass deviation or variation. 1. There is no compass deviation or variation.

2. They can be used when the vessel's motion interferes 2. They can be used when the vessel's motion interferes with the use of a compass. with the use of a compass.

3. They are instantaneous and can be monitored 3. They are instantaneous and can be monitored continuously. continuously.

4.They occur frequently when in confined waters.4.They occur frequently when in confined waters.


TransitsTransits

•Good transit - Beacon in line with lighthouse


TransitsTransits

•Poor transit - Buoy in line with end of land. This may be inaccurate due to land changing due to tidal height and the buoy being set by tidal stream or current.


TransitsTransits

0945A transit can give either a position line, or as shown, a heading to steer on from the northwest, before altering to about 045°T into Fisherman's Cove


Symbol Type Meaning

Labeling Fixes

Fix

Fix

DR

EP

Accurate Visual Fix

Accurate Fix obtained by electronic means

Dead reckon position, advanced from previous fix.

Estimated position. Most probable position of ship.


Dead ReckoningDead Reckoning

• Dead Reckoning is the process of determining a ship’s approximate position by applying, from its last known position, a vector or a series of consecutive vectors representing the true courses steered and the distances run as determined by the ship’s speed and time, without considering the effects of wind and current.• From a known ship’s position, predicted future positions are plotted.


Dead Dead ReckoningReckoning

1230

DR 1245

From ship’s known position at 1230, a future position is plotted for 1245, knowing vessel’s course and speed.


Dead ReckoningDead Reckoning

• Dead Reckoning is derived from DEDUCED, or DED, reckoning which was the process by which a vessel’s position was computed trigonometrically in relation to a known point of departure.


Estimated Estimated PositionPosition

1230

EP 1245

From ship’s known position at 1230, a future position is plotted for 1245, knowing vessel’s course and speed, and allowing for set and drift of tide.


Parallel IndexingParallel Indexing


Parallel IndexingParallel Indexing

• Parallel indexing is using the radar to monitor the track of a vessel along a preplanned course, maintaining a distance off a known charted object.• Where using a magnetic compass input to a radar, the true bearing will have to be corrected for variation and deviation before setting the Electronic Bearing Marker.


Parallel Parallel IndexingIndexing

CIR 0.32’

015ºT

Find a radar conspicuous object on the chart. Draw a line parallel to the required course touching the object. Measure the distance between the course line and the parallel index line. That is the Cross Index range.



Offset and set up the Variable Range Marker to the distance off a conspicuous point of land that is required, and set the Electronic Bearing Marker to the required compass course.

Course 017°C

VRM 0.18nm

EBL 017°C



The VRM should run up the EBL if the vessel is staying on track.

Course 017°C

VRM 0.18nm

EBL 017°C


Time-Speed-Distance CalculationsTime-Speed-Distance Calculations


Time-Speed-Distance CalculationsTime-Speed-Distance Calculations

• These calculations can be made using a nautical slide rule, electronic calculator, set of pre-computed tables, or the speed nomogram. D = S x T where: D = distance traveled

note: ( 1 nm = 2000 yds) S = speed in knots(nautical miles per hour)

T = time in hours


3 Minute Rule3 Minute Rule

Distance traveled in 3 minutes (yards) =Distance traveled in 3 minutes (yards) =

Ship’s speed (knots) X 100Ship’s speed (knots) X 100

6 Minute Rule6 Minute Rule

Distance traveled in 6 minutes (nm) =Distance traveled in 6 minutes (nm) =

Ship’s Speed (knots) divided by 10.Ship’s Speed (knots) divided by 10.

Simple RulesSimple Rules


Section 7: TidesSection 7: Tides


Tides DefinedTides Defined

Tides are the Tides are the verticalvertical rise and fall of the ocean level due rise and fall of the ocean level due to the gravitational and centrifugal forces between the to the gravitational and centrifugal forces between the

earth and the moon, and to a lesser extent, the sun.earth and the moon, and to a lesser extent, the sun.


Spring TidesSpring Tides

When the tidal effects of the sun and the moon When the tidal effects of the sun and the moon act in concert.act in concert.


Neap TidesNeap Tides

When the tidal effects of the sun and the moon are in When the tidal effects of the sun and the moon are in opposition to one another.opposition to one another.


Tidal Reference PlanesTidal Reference Planes

Mean high-water springs (MHWS)Mean high-water springs (MHWS) average height of all spring tide high-water levelsaverage height of all spring tide high-water levels

Mean higher high water (MHHW)Mean higher high water (MHHW) average of the higher of the high-water levels each tidal day, average of the higher of the high-water levels each tidal day,

19-year period19-year period

Mean high water (MHW)Mean high water (MHW) average of all high-tide water levels, 19-year periodaverage of all high-tide water levels, 19-year period

Mean high-water neaps (MHWN)Mean high-water neaps (MHWN) average recorded height of all neap tide high-water levelsaverage recorded height of all neap tide high-water levels



Mean low-water neaps (MLWN)Mean low-water neaps (MLWN) average recorded height of all neap tide high-water levelsaverage recorded height of all neap tide high-water levels

Mean low water (MLW)Mean low water (MLW) average of all low-tide water levels, 19-year periodaverage of all low-tide water levels, 19-year period

Mean lower low water (MLLW)Mean lower low water (MLLW) average of the lower of the low-water levels each tidal day, average of the lower of the low-water levels each tidal day,

19-year period19-year period

Mean low water springs (MLWS)Mean low water springs (MLWS) average of all spring tide low-water levelsaverage of all spring tide low-water levels



Height marked on chart

Depth marked on chart


Tidal PatternsTidal Patterns

In general in most of the world, the tides go up and In general in most of the world, the tides go up and down on a semi diurnal curvedown on a semi diurnal curve


Tidal Patterns - SemidiurnalTidal Patterns - Semidiurnal


Tidal Patterns - DiurnalTidal Patterns - Diurnal


Calculating Rise of TideCalculating Rise of Tide

Q. If a low water was at 0600, with a height of 0.2 Q. If a low water was at 0600, with a height of 0.2 meters, and the next high water was at 1200 , with a meters, and the next high water was at 1200 , with a height of 5.6 meters, what would be the approximate height of 5.6 meters, what would be the approximate rise of tide and therefore approximate height of tide if rise of tide and therefore approximate height of tide if

your vessel was setting out at 0900.your vessel was setting out at 0900.


Calculating Rise and Height of TideCalculating Rise and Height of Tide

A. A. 1200 LT High water 5.6 m1200 LT High water 5.6 m

0600 LT Low water 0.2 m0600 LT Low water 0.2 m

6.00hrs Range 5.4 m6.00hrs Range 5.4 m

0900 LT 0900 LT

0600 LT Low water 0.2 m0600 LT Low water 0.2 m

3.00 hrs3.00 hrs

Approximate rise of tide is (3hrs/6hrs) x 5.4m = 2.7 m Approximate rise of tide is (3hrs/6hrs) x 5.4m = 2.7 m

Approximate height of tide above chart datum, if your vessel was Approximate height of tide above chart datum, if your vessel was setting out at 0900 would be : Ht of LW (0.2m) + setting out at 0900 would be : Ht of LW (0.2m) +

rise of tide (2.7m) = 2.9 m. rise of tide (2.7m) = 2.9 m.


Calculating Rise of TideCalculating Rise of Tide

In this case allow only 2.5 meters. Always allow less In this case allow only 2.5 meters. Always allow less rise of tide close to low water due to the rate of rise of tide close to low water due to the rate of change of height being least close to time of low change of height being least close to time of low water (and high water).water (and high water).


Section 8 Ocean CurrentsSection 8 Ocean Currents


Ocean CurrentsOcean Currents

Giant patterns of rotation “gyres” in each of the Giant patterns of rotation “gyres” in each of the major ocean basins.major ocean basins.

Caused by natural effects: salinity, Caused by natural effects: salinity, temperature, the Coriolis Effect, etc.temperature, the Coriolis Effect, etc.

Described in the Sailing DirectionsDescribed in the Sailing Directions Examples are the Gulf Stream, the Kuro Shio Examples are the Gulf Stream, the Kuro Shio

and the Owa Shioand the Owa Shio


Ocean CurrentsOcean Currents


Tidal CurrentsTidal Currents



Caused by the rise and fall of tides in coastal Caused by the rise and fall of tides in coastal waters.waters.

Speed and timing is dependent upon whether it Speed and timing is dependent upon whether it is spring or neap tides, and the shape of the is spring or neap tides, and the shape of the coast and the sea floor.coast and the sea floor.


Relationship of TermsRelationship of Terms

Flood CurrentFlood Current A tidal current that flows towards shore A tidal current that flows towards shore

(follows a low tide).(follows a low tide).



Ebb CurrentEbb Current A tidal current that flows away from shore A tidal current that flows away from shore

(follows a high tide).(follows a high tide).



Slack WaterSlack Water A period where there is no horizontal A period where there is no horizontal

movement of water. Corresponds to the movement of water. Corresponds to the “stand” of the tide.“stand” of the tide.


Set and DriftSet and Drift

Set: the direction of the current pushing; Set: the direction of the current pushing; normally expressed in normally expressed in ooT.T.

Drift: the speed of the water, normally Drift: the speed of the water, normally expressed in knots.expressed in knots.

Set and drift combined describe the current.Set and drift combined describe the current.


WavesWaves


WavesWaves

• If the wind is blowing from the water onto the If the wind is blowing from the water onto the land they are onshore winds. This causes land they are onshore winds. This causes waves to break a little earlier, thus pushing waves to break a little earlier, thus pushing them over. them over.


WavesWaves

• If the wind blows from the land out to sea, If the wind blows from the land out to sea, they are offshore winds. They blow against they are offshore winds. They blow against the incoming swell and sustain the waves the incoming swell and sustain the waves from breaking while they jack up a little higher from breaking while they jack up a little higher and steeper before they break. and steeper before they break.


SwellSwell


SwellSwell

Most of the swells on the British coast are Most of the swells on the British coast are generated by storms that start in the Atlantic and generated by storms that start in the Atlantic and spin up the coast of Europe all year. spin up the coast of Europe all year.


SwellSwell


SwellSwell

This forces the water up and sort of trips the This forces the water up and sort of trips the wave and it breaks, the top of the wave falls wave and it breaks, the top of the wave falls down in front of itself. down in front of itself.


Rip CurrentsRip Currents



A Rip Current is a current of water flowing out to A Rip Current is a current of water flowing out to sea. sea.

Rips form when waves push large volumes of Rips form when waves push large volumes of water onto the shore and the water returns water onto the shore and the water returns seaward thorough channels between sand bars, seaward thorough channels between sand bars, behind islands and around rocky headlands.behind islands and around rocky headlands.



On a sea coast, they can be identified by a line On a sea coast, they can be identified by a line of discolored water, foam and debris floating of discolored water, foam and debris floating seaward or an area of choppy or confused water seaward or an area of choppy or confused water in the swell.in the swell.


Local KnowledgeLocal Knowledge

On all voyages, observe local currents and On all voyages, observe local currents and waves, what direction they flow at what times, waves, what direction they flow at what times, where the areas.where the areas.

This will assist in:This will assist in:

1. Plotting the best course in certain weathers.1. Plotting the best course in certain weathers.


Section 9 WeatherSection 9 Weather


Atmospheric PressureAtmospheric Pressure



•The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being precisely equal to 101.325 kilopascals, 1013.25 millibars, or 29.92 inches of mercury.

•The pressure gradient between a high pressure area and a low pressure area governs the strength of the wind, the wind blowing from high pressure to low pressure.

•The greater the gradient the stronger the wind.



•An extreme example is the centre of a hurricane which can go as low as 94.8 kilopascals. The pressure gradient is huge, causing the winds to blow at 100 to 150 knots (nautical miles per hour).


Mean Sea Level PressureMean Sea Level Pressure

15 year average Mean Sea Level Pressure for June July August

15 year average Mean Sea Level Pressure for December January February


Global Global CirculationCirculation

The Earth rotates at a constant rate, and the The Earth rotates at a constant rate, and the winds blow, the transfer of momentum between winds blow, the transfer of momentum between Earth/atmosphere /Earth must be in balance; Earth/atmosphere /Earth must be in balance; and the angular velocity of the system and the angular velocity of the system maintained. maintained.

The atmosphere is rotating in the same direction The atmosphere is rotating in the same direction as the Earth but westerly winds move faster and as the Earth but westerly winds move faster and easterly winds move slower than the Earth's easterly winds move slower than the Earth's surface. surface.


Global CirculationGlobal Circulation

Remember winds are identified by the direction Remember winds are identified by the direction they are coming from, not heading to!they are coming from, not heading to!


Weather FrontsWeather Fronts

•Where air masses meet, there are well-marked boundary zones called fronts. This is where most cloud and precipitation occurs.

•In the northern hemisphere the circulation is anticlockwise around low pressure and clockwise around high pressure. The air flows almost parallel to the isobars but actually 10-15 degrees inwards towards the low pressure.


Weather FrontsWeather Fronts

• There are three types of front:

1. Warm front

2. Cold front

3. Occlusions and Occluded Fronts


Warm FrontsWarm Fronts

•When a warm moist air mass rises above a cold air mass, a warm front forms. The gradient of the front is very shallow. Warm fronts occur at the forward edge of a depression (a low-pressure system).



Weather Phenomenon

Prior to the Passing of the Front

Contact with the Front

After the Passing of the Front

Temperature Cool Warming suddenly Warmer then leveling off

Atmospheric Pressure

Decreasing steadily Levelling off Slight rise followed by a decrease

Winds S to SE Variable S to SW

Precipitation Showers, snow, sleet or drizzle

Light Drizzle None

Clouds Cirrus, cirrostratus, altostratus, nimbostratus, and then stratus

Stratus, sometimes cumulonimbus

Clearing with scattered stratus, sometimes scattered cumulonimbus


Cold FrontsCold Fronts



Weather Phenomenon

Prior to the Passing of the Front

Contact with the Front

After the Passing of the Front

Temperature Warm Cooling suddenly Cold and getting colder

Atmospheric Pressure

Decreasing Steadily Levelling off then increasing

Increasing steadily

Winds S to SE Variable and Gusty W to NW

Precipitation Showers Heavy rain or snow, sometimes hail

Showers then clearing

Clouds Cirrus and cirrostratus, changing later to cumulus and cumulonimbus

Cumulus and cumulonimbus

Cumulus



A cold front marks the advance of colder air undercutting warm air. The gradient of the cold front is steeper than that of a warm front, and the rainfall is usually heavier. Thunderstorms sometimes form along a cold front.


Occluded FrontsOccluded Fronts

•Depressions and other frontal systems have a three-dimensional structure.

•Most depressions weaken when the cold front catches up with the warm front and cuts it off from the ground.

•If the cold front rises over the warm front, this is a warm occlusion.

•If the cold front undercuts the warm front this is a cold occlusion.


Occluded FrontsOccluded Fronts

• Weather systems grow mature and decay and as they do, new ones are created. This creates families of weather systems.


Wind


WindWind

Wind is primarily the result of uneven heating of the earth’s surface, which causes large hotter areas and large cooler areas.


Wind ForceWind Force

FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots0 0-1 0-1 Calm Sea like a mirror



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots1 1-3 1-3 Light air Ripples with the appearance of scales are formed, but without foam crests.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots2 4-7 4-6 Light Breeze Small wavelets, still short, but more pronounced. Crests have a glassy appearance and do not break.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots3 8-12 7-10 Gentle Breeze Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots4 13-18 11-16 Moderate Breeze Small waves, becoming larger; fairly frequent white horses.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots5 19-24 17-21 Fresh Breeze Moderate waves, taking a more pronounced long form; many white horses are formed. Chance of some spray.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots6 25-31 22-27 Strong Breeze Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots7 32-38 28-33 Near Gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots8 39-46 34-40 Gale Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots9 47-54 41-47 Severe Gale High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots10 55-63 48-55 Storm Very high waves with long over- hanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of the wind. On the whole the surface of the sea takes on a white appearance. The 'tumbling' of the sea becomes heavy and shock-like. Visibility affected.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots11 64-72 56-63 Violent Storm Exceptionally high waves (small and medium-size ships might be for a time lost to view behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected.



FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE AT SEA 10 m above ground miles/hour knots12 73-83 64-71 Hurricane The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected.


Sea BreezeSea Breeze

•A sea-breeze (or onshore breeze) is a wind from the sea that develops over land near coasts.

•It is formed by increasing temperature differences between the land (which heats up faster) and water (which warms slower) which create a pressure minimum over the land due to its relative warmth and forces higher pressure, cooler air from the sea to move inland.


Sea BreezeSea Breeze

It generally occurs in the afternoon.


Land BreezeLand Breeze

•A land-breeze (or offshore breeze) is a wind to the sea that develops over land near coasts.

• It is formed by increasing temperature differences between the land (which cools faster) and water (which cools slower) which create a pressure minimum over the sea due to its relative warmth and forces higher pressure, cooler air from the land to move offshore.


Land BreezeLand Breeze

It generally occurs in the very early morning.


Katabatic WindsKatabatic Winds

•A katabatic wind, from the Greek word katabatikos meaning "going downhill", is a wind that blows down a topographic incline such as a hill, mountain, or glacier.

•The cold form of katabatic wind originates in a cooling, either radiatively or through vertical motion, of air at the top of the mountain, glacier, or hill.


Katabatic WindsKatabatic Winds

•Since the density of air increases with lower temperature, the air will flow downwards, warming adiabatically as it descends, but still remaining relatively cold.


Wind Force & Sea StateWind Force & Sea State

•The visible effects of the wind on the sea will be modified by the relative directions of wind and tide.

•If the wind and tide are in opposite directions, then a larger chop will be created, giving the impression of the wind being stronger.

•If wind and tide are in the same direction, the amount of sea will be reduced, giving the impression of the wind being less.


Sea and SwellSea and Swell

•Sea is the effect of wind passing over the water locally.

•Swell is only found in the open ocean and will be effects of weather systems, hundreds of miles away.


Fog


FogFog


FogFog

• Fog is a cloud in contact with the ground.

• Fog differs from other clouds only in that fog touches the surface of the Earth.

• The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges.

• Fog is distinct from mist only in its density.


FogFog

• Fog is defined as cloud which reduces visibility to less than 1 nautical mile, where as mist is that which reduces visibility to more than 1 nautical mile.


FogFog

•Fog forms when water vapor in the air at the surface begins to condense into liquid water.

•Fog normally occurs at a relative humidity of 100%. This can be achieved by either adding moisture to the air or dropping the ambient air temperature.

•Fog can form at lower humidities, and fog can sometimes not form with relative humidity at 100%.


FogFog

•Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area significantly cooler. It's most common at sea when tropical air encounters cooler waters, or in areas of upwelling.


Upslope FogUpslope Fog


Other Types of FogOther Types of Fog


FogFog “Slight Sea, Low Swell, Cloudy, Fine and Clear”“Slight Sea, Low Swell, Cloudy, Fine and Clear”


Precipitation


Orographic RainOrographic Rain

•Orographic rain (or relief rain) is caused when the warm moisture-laden wind blowing in to the land from the sea encounters a natural barrier such as mountains. This forces the wind to rise.

•With gain in altitude, the air expands dynamically due to a decrease in air pressure.

•Due to this the wind experiences a decrease in temperature, which results in the increase of the relative humidity.


Orographic RainOrographic Rain

•This causes condensation of the water vapor into water droplets to form clouds.

•The relative humidity continues to increase until the dewpoint reaches the level of condensation, causing air to be saturated.

•This height where the condensation occurs is called the level of condensation.

•When the cloud droplets become too heavy to be suspended, rain falls.


Orographic Rain Orographic Rain

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