Weather

Question 1

Weather

Sources of Met information:
ALRDS Volume 3 & 4

Answer 1

1. VHF
2. Naxtex - 518 kHz - MF - 200 mile range
3. • 490 kHz - used for local language
     - A,B,D,L are all pre set and cannot be deselected
     - A = Nav Warnings B= Met Warnings D= SAR and Piracy L= Additional Nav Warning
4. Inmarsat C - Safety Net
5. Weather Routing
6. Weather Fax
7. Internet
8. Radio Stations
9. Coast Radio Stations

Question 2

Weather

What is radiation fog?

Answer 2

Radiation = Land Fog (imagine someone hugging a radiator) = Landlubber

• Warm moist air over cooling land mass
• Very clear sky at night time
• Low wind
• Can drift out over the sea
• Worse before dawn / sunrise
• Air is a poor conductor of heat - land a good conductor (whilst the warm moist air is sitting on the cold ground and it condenses out into fog)
• 3 - 5 mph of wind the fog is lifted off the ground
• >5 mph of wind it become a low cloud

Question 3

How would you predict the likely onset of fog?

Answer 3

1. New a wet and dry bulp
2. Compare the wet and dry bulb
3. Enter the dew point table
4. Plot dew point against sea temperature to give indication of the likelyhood of fog using the graph.

Question 4

Weather
Ballots Law

Answer 4

Northern hemisphere back to wind left hand extended out points towards the centre of the low. (Southern hemisphere you face the wind)

Veering is clockwise

Backing is anti clockwise

Question 5

Weather
Synoptic Chart - Be able to point out:

Surface Pressure Chart for the UK

Answer 5

• Surface pressure chat.
• High Pressure - Low Pressure - Warm Front - Cold Front - Occluded
• Front - Wind speed - measured by distance between isobars
  (Off Geostrophic wind scale on synoptic chart will give wind speed at 600m - 70% of that will be at sea level)
• How to read a synoptic chart
• Warm sector between a warm and cold front, cloudy variable weather.
• Black lines mark troughs - rain as a result of warm air trying to get above cold air.
• High pressures air moves around clockwise
• Low pressures air moves around anti clockwise

Question 6

Weather

Met Instruments

What is a Hygrometer?

What is a Barometer?

Answer 6

Hygrometer placed inside a Stevenson Screen:

• Stevenson Screen protects the hygrometer from sun, wind etc.
• Measures Relative humidity,
• Allows you to calculate dew point
• Wet (Muslin with wick into distilled water) and Dry bulb
• Dry bulb temp vs Wet bulb temp gives you the depression (Difference between the two thermometers)
• Dew point is found in the Mariner Handbook NP100 P.88
• Need to plot the the dew point on a graph to see if the dew point and the dry bulb temp lines converge, if they converge there will be dew.
• If you plot dew point against seawater temp and where the two temps come together there will be advection fog
• Hang it to windward as we want to see the temp of the air
• The bigger the difference between the two thermometers = the dryer the air is. (More evaporation, cooling the “wet thermometer”. If they were the same / close means more humidity.

Barometer:

• Corrected for HoE against temp, Noted on card on reverse of box Lind
• Measures atmospheric pressure

Question 7

Weather
What are the Common types of clouds in the troposhere?

Answer 7

Question 8

Weather

Met Instruments:

What is a Barograph?
What is a Anemometer?

Answer 8

Barograph / Barometer / Aneroidbarometer:

• Measures a pressure trend on a mechanized chart drum over a seven day period.
• Enables you to try and predict weather.
• 1030 mb is a high pressure, fine clear weather? Need a trend one reading is not sufficiently. Need a tendancy. The steeper the change the windier it will have been.
• Diurnal variation of the barometer: Atmostpheric pressure wave that goes around the world recorded daily, must compare day by day.

Anemometer:

• Measure wind speed (apparent / relative)
• If GPS input available you will get a true wind speed also

Question 9

Weather

Low pressure systems:

Answer 9

• As the warm front approaches
  
  • The pressure would fall
  • The wind direction would be steady
  • Wind force increasing
  • Temperature steady
  • Cloud would be high cirrus and cumulus, then cirro stratus and finally nimbo stratus
  • Light rain increasing to moderate
  • Visibility deteriorating
• In the warm sector:
  
  • The pressure would stop falling
  • Wind evens out
  • Temperature begins to rise
  • Cloud would be nimbo stratus, then stratus (Thick layering)
  • Rain would stop or change to drizzle
  • Visibility could be poor
• As the cold front begins to pass:
  
  • The pressure starts rising
  • The wind veers
  • The wind force increases
  • Temperature falls (Due to wind veering blowing from north)
  • Clould - cumulonimbus
  • Heavy Rain, possibly hail and thunderstorms
  • Visibility Poor
• In the rear of the cold front:
  
  • Wind direction steady
  • Wind force decreasing
  • temperature steady
  • Cloud cumulus
  • Possible showers, clearing
  • Visibility clearing

Question 10

Weather

What are your actions in the different sectors of a TRS?

Answer 10

• Wind Veering = Dangerous semicircle
  
  • Wind on STBD bow and steam away at full speed,
  • Whilst altering course to STBD keeping the wind on the STBD bow (Outwards and away from the center of the storm)
• Wind Backing = Navigable Semicircle
  
  • wind on STBD quarter If the wind is a steady direction then you are in the path of the storm and would do the same as if you were in the navigable semicircle

Northern Hemisphere

• Veering = Dangerous - STBD Bow -
• Backing = Navigable - STBD Quarter

Southern Hemisphere:

• Veering = Navigable - PT Quarter
• Backing = Dangerous - PT BOW

Question 11

Weather
Winds
Katabatic
Land Breeze
Sea Breeze

Answer 11

• Land Breeze sets in late in the evening dies down by sunrise
• Sea Breeze sets in by full noon (Land at its hottest, creating a low pressure, filled in by the sea)
• Katabatic, cold air falling down steep land mass creates a very strong land breeze, can set on rapidly.

Question 12

Weather
What are the requirements for a TRS to form?

Answer 12

• Latitude of 5 - 15 degrees N or S of the equator
• Sea temp above 26 degrees
• A wind sheer causing an uprising of the wind
• Drop in pressure of more than 3mb for seasonal average corrected for diurnal variation (Average can be obtained from routing charts or sailing directions)
• 3mb or more = TRS in the area
• 5mb or more below season average
• Must take avoiding action
• Long lowly swell coming from a different direction to the wind
• Lack of wildlife
• Strange and colourful cloud formations (usually cirrus followed by broken cumulus)

Question 13

Weather

What is frontal fog?

Answer 13

Frontal Fog = Warm Front and Cold Front Meeting or Occlusion

• Fog is due to the mixing of the warm and cold air of the two mixing fronts.
• High wind speed with an occluded front (30 - 35 kts)
• Artic Sea Smoke - Occurs in high latitudes when extremely cold air blows over a relatively warmer sea surface.

Question 14

Weather

What is advection fog?

Answer 14

Advection = Sea fog

• Warm Moist air flows over a cold sea
• Need between 5-25 kts of wind
• >25 kts it become low cloud
• Local Srping and early summer when sea temp is at its lowest