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 4Section 4 CompassCompass
 Section 5Section 5 Navigational AidsNavigational Aids

Table of ContentsTable of Contents
 Section 6Section 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 Reckoning
3. Celestial Navigation
4. Electronic 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.

2. Dead Reckoning
This 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 Navigation
This form of navigation is any navigation
undertaken using electronic navigational aids.
These include:
LORAN C
Radar
Transit Satellite Navigator
Global 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 CoordinateSection 2 Terrestrial Coordinate
SystemSystem

For 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 andthrough two points on the surface of the earth and
the center of the earth.the center of the earth.

 Examples are: The Equator, Meridians ofExamples are: The Equator, Meridians of
Longitude, the Prime Meridian being throughLongitude, the Prime Meridian being through
Greenwich, near London, United Kingdom.Greenwich, near London, United Kingdom.

EquatorEquator
 The great circle formed by passing a planeThe great circle formed by passing a plane
perpendicular to the earth’s axis halfway betweenperpendicular to the earth’s axis halfway between
its poles.its poles.

EquatorEquator
 The equator divides the earth into northern andThe equator divides the earth into northern and
southern hemispheres.southern hemispheres.
 One of the two great circles from which allOne of the two great circles from which all
locations on the earth’s surface are referenced.locations on the earth’s surface are referenced.

 Small Circle: A circle formed from the intersection ofSmall 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 arePositions in relationship to Earth’s Coordinates system are
expressed in:expressed in:
PRONOUNCEDPRONOUNCED SYMBOLSYMBOL
DegreesDegrees (°)(°)
MinutesMinutes (´)(´)
SecondsSeconds (´´)(´´)

LatitudeLatitude
 LatitudeLatitude - angular distance north or south between the equator and the- angular distance north or south between the equator and the
parallel of a point. Latitude is measured in degrees of arc from 0parallel of a point. Latitude is measured in degrees of arc from 0°−90°°−90°
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 positionAlways 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 ofThe meridian that passes through the original position of
the Royal Greenwich Observatory near London,the Royal Greenwich Observatory near London,
England.England.
 Constitutes the second reference line for the terrestrialConstitutes the second reference line for the terrestrial
coordinate system.coordinate 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 westernit divides the earth into the eastern and western
hemispheres.hemispheres.

LongitudeLongitude
 Longitude - angular distance E/W between theLongitude - 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 toLongitude 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.gLongitude is always expressed using 3 digits, e.g
123º.123º.
 One degree of long does not equal 60 NM unlessOne degree of long does not equal 60 NM unless
measured along the equator.measured along the equator.
 Always given after Latitude when giving aAlways 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. Maintain1. Maintain true shapetrue shape of physical features.of physical features.
2. Maintain2. Maintain correct proportionscorrect proportions of features relative toof 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 onplot as straight lines. They are lines on
the earth’s surface that cross all meridians at thethe earth’s surface that cross all meridians at the
same anglesame angle
Chart ProjectionsChart Projections

Mercator ProjectionMercator Projection

ADVANTAGESADVANTAGES
 Position, distance, and direction can be accuratelyPosition, distance, and direction can be accurately
measuredmeasured
 True shape of features is maintained over smallTrue 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 appearsThe 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 areLarge 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 forFor approaching bays and harbors, and used for
coastal navigation showing outlying reefs andcoastal navigation showing outlying reefs and
shoals.shoals.

Plotting a Position 1Plotting a Position 1
1. Determine the1. Determine the
parallels on theparallels on the
chart that bracketchart that bracket
the latitude.the latitude.
2. Place the pivot2. Place the pivot
point of thepoint of the
compass on thecompass on the
closest line.closest line.

3. Spread the3. Spread the
compass until thecompass until the
lead rests on thelead rests on the
given latitude.given latitude.
4. Move to the4. Move to the
approximateapproximate
longitude andlongitude and
swing an arc.swing an arc.

5. The same process is5. The same process is
repeated using therepeated using the
longitude scale and thelongitude scale and the
given longitude.given longitude.
6. The desired position is6. The desired position is
the intersection of thesethe intersection of these
two arcs.two arcs.

7. If plotted correctly, the7. If plotted correctly, the
intersection should occurintersection 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 representAll rhumb lines on a Mercator projection represent
truetrue directions.directions.
 Measurement of directionMeasurement of direction
on a Mercator chart ison a Mercator chart is
accomplished by using aaccomplished by using a
parallel ruler to transfer theparallel ruler to transfer the
direction of a rhumb line todirection 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 toThe Hydrographer issues weekly Notices to
Mariners, which include corrections to be made toMariners, which include corrections to be made to
UK charts.UK charts.
 When charts are bought, they are generallyWhen charts are bought, they are generally
corrected up to date.corrected up to date.
 Once in use Notices to Mariners should beOnce in use Notices to Mariners should be
checked and corrections to charts made aschecked and corrections to charts made as
necessary.necessary.

Correction of ChartsCorrection of Charts
 When a correction has been made, a note of theWhen a correction has been made, a note of the
year and Notice to Mariner number should beyear 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 hashaving checked that the previous correction has
been made.been made.
20082008 - 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 interactionA compass that senses direction by interaction
between its own permanent magnets and thebetween 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 trueA 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 measuredR) = bearings measured
with reference to the ship’s longitudinal axiswith reference to the ship’s longitudinal axis

Magnetic BearingsMagnetic Bearings ((°°M) = bearings measuredM) = bearings measured
with respect to magnetic north.with respect to magnetic north.

True BearingsTrue Bearings ((°°T) = bearings measured withT) = 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 ina 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 isVariation is the angle between a magnetic line ofthe angle between a magnetic line of
force and a geographic (true) meridian at any locationforce and a geographic (true) meridian at any location
on the earth.on the earth.
 Variation exists because the earth’s magnetic andVariation 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 alsoMagnetic 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 theVariation 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 allVariation is printed inside compass roses on all
navigation charts.navigation charts.
 Always use the compass rose nearest your currentAlways use the compass rose nearest your current
Dead Reckoning position.Dead Reckoning position.

VariationVariation
•
Variation changes as an observer moves alongVariation changes as an observer moves along
the globe.the globe.
•
However, if a ship moves in such a way that theHowever, if a ship moves in such a way that the
meridians remained constant, it would bemeridians remained constant, it would be
moving along anmoving along an isogonic lineisogonic line - a line along- a line along
which variation remains constant.which variation remains constant.

VariationVariation
•
The amount that the Variation changes annuallyThe amount that the Variation changes annually
is called the Annual Change.is called the Annual Change.
•
The amount of Annual Change is to be foundThe amount of Annual Change is to be found
on every compass rose on the chart, next to theon every compass rose on the chart, next to the
Variation, and is normally expressed as 004°WVariation, and is normally expressed as 004°W
1995 (8’E)1995 (8’E)..
•
To calculate change in 2008, multiply 8’E byTo calculate change in 2008, multiply 8’E by
13 (years from 1995) 104’ or 1.75°E13 (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

VariationVariation

DeviationDeviation
 This isThis is the angle between the magnetic meridian andthe 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’sDeviation is caused by the interaction of the ship’s
metallic structure, electrical systems, metallic objectsmetallic 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 - thisConvert Compass Courses to True Courses - this
also applies to bearings, using the deviation for thealso applies to bearings, using the deviation for the
vessel’s head.vessel’s 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 alsoConvert True Course to Compass Course - this also
applies to bearings, using the deviation for theapplies to bearings, using the deviation for the
vessel’s head.vessel’s head.
True Course 165°TTrue Course 165°T
Variation 22°EVariation 22°E
Magnetic Course 143°MMagnetic Course 143°M
Deviation 2°WDeviation 2°W
Compass Course 145°CCompass Course 145°C

Section 5
Navigational Aids

 Navigational Aid: Any device external to a vessel orNavigational Aid: Any device external to a vessel or
aircraft intended to assist in determining position andaircraft 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 NavigationPositive Identification of Navigation
AidsAids

NIGHTNIGHT
• Phase characteristicPhase characteristic
• Period & ColorPeriod & Color

 Phase CharacteristicsPhase Characteristics
Chart SymbolChart Symbol MeaningMeaning
FixedFixed FF Steady, unblinkingSteady, unblinking
FlashingFlashing FlFl Flashes at regularFlashes at regular
intervalsintervals
Quick FlashQuick Flash Qk. Fl.Qk. Fl. Flash at least 60Flash at least 60
times/min.times/min.
Group FlashGroup Flash Gp. Fl.Gp. Fl. Group of two orGroup of two or
more flashesmore flashes
Aids (at night)Aids (at night)

 Phase CharacteristicsPhase Characteristics
Chart SymbolChart Symbol MeaningMeaning
Morse CodeMorse Code Mo. [A]Mo. [A] Morse alphaMorse alpha
(short/long)(short/long)
OccultingOcculting Occ.Occ. On longer than it’sOn 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)
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 with
a flashing light of higher luminous intensity
F Fl
Flashing A flashing light in which a flash is regularly
repeated (frequency not exceeding 30 flashes
per 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 except
that successive groups in the period have dif-
ferent numbers of flashes
Fl (2+1)
Isophase A light in which all durations of light and
darkness are equal
Iso
Single occulting An occulting light in which an eclipse, or
shorter duration than the light, is regularly
repeated.
Oc
Group occulting An occulting light in which a group of
eclipses, specified in number, is regularly
repeated.
Oc (2)
Quick A quick light in which a flash is regularly
repeated at a rate of 60 flashes per minute
Q
Interrupted quick A quick light in which the sequence of flashes
is interrupted by regularly repeated eclipses of
constant and long duration
lQ
Group quick A group of 2 or more quick flashes, specified
in number, which are regularly repeated. (Not
used 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 the
Morse 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 inred light used in
dangerous sectorsdangerous sectors

sector limits aresector limits are
expressed in degreesexpressed in degrees
truetrue as observed from aas 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 areOther 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 ComputedDetermining 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 anlight 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 intocould be seen in perfect visibility (taking into
account elevation, observer’s height of eye, and theaccount 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 currentwhich 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 currentlight 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 inrange. 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 chartedA 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, daychurch steeples, spires, radio and TV towers, day
marks, fixed navigation lights, flagpoles, or tangentsmarks, fixed navigation lights, flagpoles, or tangents
to points of land.to points of land.
 In general fixing off floating objects, especiallyIn general fixing off floating objects, especially
buoys, should be avoided, if there are fixed chartedbuoys, should be avoided, if there are fixed charted
objects available.objects available.

VisualVisual
PositionPosition
LineLine
1000

RadarRadar
RangeRange
PositionPosition
LineLine

Position Line MeasurementPosition Line Measurement
 Visual Bearings can be measured in:Visual Bearings can be measured in:
1. Degrees Relative ( # # #1. Degrees Relative ( # # # 00
R )R )
2. Degrees per Gyro Compass ( # # # ºG )2. Degrees per Gyro Compass ( # # # ºG )
3. Degrees Magnetic ( # # #3. Degrees Magnetic ( # # # 00
M )M )
 The navigator must convert any of these types ofThe navigator must convert any of these types of
bearings to True before they can be plotted on thebearings to True before they can be plotted on the
chart.chart.
Degrees True ( # # #Degrees True ( # # # 00
T)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.

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

VisualVisual
PositionPosition
Fix 2Fix 2
Compass bearing
of Grebe Island
Light bearing 058
º Compass,
deviation 1ºW,
variation 23ºE,
gives 080 º True
Bearing

VisualVisual
PositionPosition
Fix 3Fix 3
Compass bearing
of Pt. Atkinson
Light bearing
098ºCompass,
deviation 1ºW,
variation 23º E,
gives True
Bearing of 120 º T

VisualVisual
PositionPosition
Fix 4Fix 4
1230
Insert 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.

CockedCocked
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.

RadarRadar
Position 1Position 1
Using radar:
Grebe Is
Electronic
Bearing Marker
showing 058 º M
Variable Range
Marker showing
0.82’

RadarRadar
From radar, plot
position circle:
Grebe Is
Distance 0.49 nm

RadarRadar
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

RadarRadar
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.

ElectronicElectronic
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 whenTransits 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 othersSome 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 interferes2. 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 monitored3. 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.

DeadDead
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.

EstimatedEstimated
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.

ParallelParallel
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.

ParallelParallel
IndexingIndexing
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

ParallelParallel
IndexingIndexing
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 theTides are the verticalvertical rise and fall of the ocean level duerise and fall of the ocean level due
to the gravitational and centrifugal forces between theto 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 moonWhen 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 inWhen 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 andIn 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 0.2Q. If a low water was at 0600, with a height of 0.2
meters, and the next high water was at 1200 , with ameters, and the next high water was at 1200 , with a
height of 5.6 meters, what would be the approximateheight of 5.6 meters, what would be the approximate
rise of tide and therefore approximate height of tide ifrise 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 LT0900 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 mApproximate rise of tide is (3hrs/6hrs) x 5.4m = 2.7 m
Approximate height of tide above chart datum, if your vessel wasApproximate 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 lessIn this case allow only 2.5 meters. Always allow less
rise of tide close to low water due to the rate ofrise of tide close to low water due to the rate of
change of height being least close to time of lowchange 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 theGiant 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 ShioExamples are the Gulf Stream, the Kuro Shio
and the Owa Shioand the Owa Shio

Ocean CurrentsOcean Currents

Tidal CurrentsTidal Currents

Tidal CurrentsTidal Currents
 Caused by the rise and fall of tides in coastalCaused by the rise and fall of tides in coastal
waters.waters.
 Speed and timing is dependent upon whether itSpeed and timing is dependent upon whether it
is spring or neap tides, and the shape of theis 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 shoreA tidal current that flows towards shore
(follows a low tide).(follows a low tide).

 Ebb CurrentEbb Current

A tidal current that flows away from shoreA tidal current that flows away from shore
(follows a high tide).(follows a high tide).

 Slack WaterSlack Water

A period where there is no horizontalA period where there is no horizontal
movement of water. Corresponds to themovement 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 innormally expressed in oo
T.T.
 Drift: the speed of the water, normallyDrift: 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 theIf the wind is blowing from the water onto the
land they are onshore winds. This causesland they are onshore winds. This causes
waves to break a little earlier, thus pushingwaves 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 againstthey are offshore winds. They blow against
the incoming swell and sustain the wavesthe incoming swell and sustain the waves
from breaking while they jack up a little higherfrom 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 areMost of the swells on the British coast are
generated by storms that start in the Atlantic andgenerated 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
 This forces the water up and sort of trips theThis forces the water up and sort of trips the
wave and it breaks, the top of the wave fallswave 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 toA Rip Current is a current of water flowing out to
sea.sea.
 Rips form when waves push large volumes ofRips form when waves push large volumes of
water onto the shore and the water returnswater 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 lineOn a sea coast, they can be identified by a line
of discolored water, foam and debris floatingof discolored water, foam and debris floating
seaward or an area of choppy or confused waterseaward or an area of choppy or confused water
in the swell.in the swell.

Local KnowledgeLocal Knowledge
 On all voyages, observe local currents andOn 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

GlobalGlobal CirculationCirculation
 The Earth rotates at a constant rate, and theThe Earth rotates at a constant rate, and the
winds blow, the transfer of momentum betweenwinds 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 systemand the angular velocity of the system
maintained.maintained.
 The atmosphere is rotating in the same directionThe atmosphere is rotating in the same direction
as the Earth but westerly winds move faster andas the Earth but westerly winds move faster and
easterly winds move slower than the Earth'seasterly winds move slower than the Earth's
surface.surface.

Global CirculationGlobal Circulation
Remember winds are identified by the directionRemember 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.

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 knots
0 0-1 0-1 Calm Sea like a mirror

10 m above ground
miles/hour knots
1 1-3 1-3 Light air Ripples with the appearance of
scales are formed, but without
foam crests.

10 m above ground
miles/hour knots
2 4-7 4-6 Light Breeze Small wavelets, still short,
but more pronounced. Crests
have a glassy appearance and
do not break.

10 m above ground
miles/hour knots
3 8-12 7-10 Gentle Breeze Large wavelets. Crests begin
to break. Foam of glassy
appearance. Perhaps scattered
white horses.

10 m above ground
miles/hour knots
4 13-18 11-16 Moderate Breeze Small waves, becoming larger;
fairly frequent white horses.

10 m above ground
miles/hour knots
5 19-24 17-21 Fresh Breeze Moderate waves, taking a more
pronounced long form; many
white horses are formed.
Chance of some spray.

10 m above ground
miles/hour knots
6 25-31 22-27 Strong Breeze Large waves begin to form; the
white foam crests are more
extensive everywhere.
Probably some spray.

10 m above ground
miles/hour knots
7 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.

10 m above ground
miles/hour knots
8 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.

10 m above ground
miles/hour knots
9 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.

10 m above ground
miles/hour knots
10 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.

10 m above ground
miles/hour knots
11 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.

10 m above ground
miles/hour knots
12 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.

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.

•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.

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