Changing Nav / Met Flashcards
Define Moonrise / Moonset
True - When the moons centre appears practically on the Visible Horizon
Visible - When the Upper Limb appears above or disappears below the observers Visible Horizon
Define sunrise / sunset
The time of true rising and setting occurs when the centre of a heavenly body is on the observers Celestial Horizon
True - When the suns centre lies on the celestial horizon, the suns lower limbs appears one semi-diameter above the Visible Horizon
Visible - When the upper limb of a heavenly body is just appearing above or disappearing below the observers Visible Horizon
True and Visible must not be confused.
Define Celestial Horizon
The great circle on the celestial sphere midway between the observer’s zenith and nadir
Define Visible Horizon
The line at which the sky and Earth appear to meet
List the seven surface currents around Australia
- South Equatorial current
- West Australian current
- Leeuwin current
- East Australian current
- Tasman front
- Zeehan current
- Antarctic Circumpolar current
List the factors determining the growth of sea and swell
- Wind strength
- Wind duration
- Fetch
- Distance (for swell)
Depending on the factors determining the growth of sea and swell, the sea is categorised by what three names
- Duration limited
- Fetch limited
- Fully developed
Sea states according to the Beaufort Scale
0 = calm - 0m 1 = rippled - 0.1m 2 = smooth - 0.1-0.5m 3 = gentle breeze/slight - 0.5-1.25m 4 = moderate - 1.25-2.5m 5 = rough - 2.5-4m 6 = very rough - 4-6m 7 = high - 6-9m 8 = very high - 9-14m 9 = phenomenal - >14m
Describe the basic weather conditions associated with Ridges
- An extension from an anticyclone / High
- Marked by ‘U’ shaped isobars
- Light winds
- Winds move anticlockwise
Define Meridian
A meridian is a semi-Great circle on the Earth’s surface which passes through both poles
Define civil twilight
Nautical Almanac calculated as the moment when the sun’s centre is 6 degrees below the celestial horizon.
- Morning: Roughly when the horizon becomes clear (Dawn)
- Evening: Roughly when the horizon becomes in distinct (Dusk)
Define Nautical and Astronomical Twilight
Nautical - The suns centre is 12 degrees below the celestial horizon
Astronomical - The suns centre is 18 degrees below the celestial horizon
Define total darkness
After Astronomical Twilight, with no moon present (more than 18 degrees below the celestial horizon)
Define tides
Tides are the Periodic Vertical Reversing movements of the water on the Earth’s surface, caused by the Tides Raising forces of the moon and sun.
Define Perigee and its relation to tides
When the moon is closer to the Earth. There are greater tidal ranges.
Define Apogee and its relation to tides
The moon is further away from the Earth. There are lesser tidal ranges
Define Perihelion and its relation to tides
The sun is closer to the Earth. There are greater tidal forces
Define Aphelion and its relation to tides
The sun is further away from the Earth. There is less effect on tidal forces
Procedure for weather observing and reporting at sea IAW AFGO’s
Rough log:
- compiled by the QM
- Observations every woman’s
Ships log:
- compiled by the OOW
- entered every 6 hours
- entered at the end of every watch
Observations are reported as:
- routine reports
- non-routine reports
Parts of a wave: Define height
The difference between a crest and trough
Parts of a wave: Define amplitude
The difference between the still water line and the crest/trough
Parts of a wave: Define Period
The time difference between successive points for a stationary observer.
- For ocean waves, a 100m wavelength = 10sec period
Parts of a wave: Define wave length
The difference between successive points on a wave. Eg. crest to crest, or trough to trough
Describe tropical cyclones and the hazards associated with them, as well as duration of Australian cyclone season
- Synoptic scale, intense, low pressure systems
- Behind tornadoes, one of the most dangerous weather phenomena
- Only occurs in tropical latitudes
Duration: 01 Nov to 01 May
Define Solar Day
Interval that elapses between two successive transits of a heavenly body across the same meridian
State the difference between rain and showers
Rain:
- Falls from Stratiform clouds, Widespread and continuous rain fall
Showers:
- Falls from Cumuliform clouds, Isolated and intermittent rain fall
Definition of atmosphere
The atmosphere is the gaseous envelope surrounding the earth.
No recognised upper limit to the atmosphere.
The atmosphere is retained around the earth by gravitational forces.
What is the seasonal movement of the general circulation?
Winter solstice - Sun over the Tropic of Cancer Circulation shifts North
Summer solstice - Sun over the Tropic of Capricorn Circulation shifts South
List the 5 formation requirements for a tropical cyclone
- A pre existing area of low pressure
- Sea Surface Temp (SST) at least 26 degrees
- Latitude minimum of 5 degrees N/S
- Upper level divergence
- Little or no Vertical Wind Shear
How does the balance of Pressure Gradient Forces (PGF) and Coriolis cause geostrophic wind to flow parallel to isobars?
PGF and Coriolis forces act in opposite directions to produce geostrophic wind to flow parallel to isobars.
Describe the basic weather conditions associated with Depressions/Lows
- Wind circulates clockwise
- Winds strongest near the centre
- Winds generally strong
- Air converges at low levels and diverges at high levels
State the composition of the atmosphere and the names of the two main categories
Atmosphere id composed of a mixture of gases but also contains solid and liquid particles. Two main categories:
- Non-variable constituents - nitrogen, oxygen, inert gases
- Variable constituents - carbon dioxide, water, dust, salt, smoke, volcanic ash
State the vertical layers and boundaries throughout the atmosphere
Thermosphere - layer, closest to sun Mesopause - boundary Mesosphere - layer Stratopause - boundary Stratosphere - layer Tropopause - boundary Troposphere - layer, closest to earth
Describe the basic weather conditions associated with Troughs
- An extension from a depression/low
- ‘U’ shaped isobars (the stronger the U the worse the weather)
- Strong winds
- Wind moves clockwise
- Marked by - - - - - - - -
State the four trigger mechanisms associated with the formation of a cloud
- Convection: occurs when air heated at the earths surface rises in the form of a thermal current or bubbles
- Turbulence: when airflow moves over a trough surface, causing eddies
- Orographic ascent: air is forced upwards by a barrier of maintains or hills
- Widespread ascent: movement of cold air mass forcing warm air to rise before it
List the 3 requirements for a thunderstorm
- Instability
- Moisture
- Trigger: convection, orographic uplift, mass (frontal) ascent
Definition of convection
The movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, while colder, denser material to sink under the influence of gravity which consequently results in the transfer of heat.
Describe thunderstorms and the hazards associated with them
- Most severe type of small-scale weather feature
- Consists of a cumulonimbus cloud (reaches up to tropopause, produces heavy rain, hail, lightning and thunder
- Lightning produced
- Reduced visibility
- Squalls - strong gusty winds
- Turbulence
- Icing
- Hail
List the types of logs available
- Electromagnetic speed log
- Doppler speed log
- Sonar speed log
- Sonar correlation speed log
- Dutchman’s log
What are speed logs used for?
Record ships speed and distance run
What is LRG ‘Lock in’ error
At low rotation rates the LRG contra rotating beams can lock in
Define Orthomorphism or Conformal
A chart projection of the earth is correctly presented including shape and scale, and bearing. At the expense of some area properties.
What are the equipment requirements of having and using AIS
Mandatory to be carried on all vessels
AIS station consists of:
- One VHF transmitter
- Two VHF receivers
- One VHF DSC receiver
- A standard marine electronic comms link and sensor system
What are the 3 different modes of AIS
Static mode - MMSI, Identity, Size, Type, NAVaid location
Dynamic mode - Position, time stamp, COG, SMG, Direction, Turn
Voyage Related mode - Draught, cargo, NAVplan, short text messages
