Weather and the deck log: oral questions and answers.

Weather watch: a depression passing

Spot the falling glass, read the gale warning, find the low by Buys Ballot, read the charts, judge fog risk and prepare for heavy weather.

1. State Buys Ballot's law, and show how you would use it to find the centre of a low.Often asked

   A good answer covers:

   • In the Northern Hemisphere, stand with your back to the true wind and the low pressure is on your left
   • In the Southern Hemisphere it is on your right
   • Surface wind blows in across the isobars towards the low, so the centre lies a little forward of the beam; for a tropical storm, facing the wind, it is about 8 to 12 points to the right in the Northern Hemisphere
   • Use the true wind, not the relative wind, and watch how the wind changes to see how the centre is moving
   • For a ship stopped or moving slowly in the Northern Hemisphere, a steady veering wind puts you on the right of the track, a backing wind on the left

   Show me on the bridgeRead the true wind on the anemometer display, then turn your back to it.

2. A depression passes to the north of you. Describe the weather as it goes by.Often asked

   A good answer covers:

   • Ahead of the warm front: the glass falls, cloud thickens and lowers, rain becomes continuous, the wind backs and freshens
   • At the warm front: the wind veers, the rain eases to drizzle, the temperature and dew point rise
   • In the warm sector: the glass is fairly steady, drizzle, poor visibility, mist or fog over a cooler sea
   • At the cold front: heavy rain and squalls, a sharp veer, the temperature falls and the glass starts to rise
   • Behind the cold front: the glass rises, visibility is good, showers, and often a strong north-westerly

   Show me on the bridgeLook at the barograph trace and the surface analysis chart pinned beside it.

3. How do you use the wet and dry bulb hygrometer to judge whether fog is likely?Often asked

   A good answer covers:

   • Read the dry bulb and the wet bulb; the difference is the depression of the wet bulb
   • Enter the hygrometric table with the dry bulb and the depression to find the dew point and the humidity
   • Compare the dew point with the sea temperature
   • Sea temperature at or below the dew point: advection fog is likely as the air is cooled from below
   • A small depression of the wet bulb means the air is close to saturation; keep watching it

   Show me on the bridgeThe thermometer, the hygrometer and the sea temperature are on the after bulkhead.

4. What does the barometer tell you on watch, and when do you call the master about the weather?

   A good answer covers:

   • The reading itself matters less than the tendency over the last three hours, read off the barograph
   • A fall of more than about 3.5 hectopascals in three hours is falling quickly, and more than 6 is very rapid: a depression or gale is likely close
   • Log the reading and the tendency every watch, and more often when it is falling fast
   • Call the master when the weather worsens or is forecast to, on a gale or storm warning, on a falling glass, or when in any doubt
   • Prepare the ship for heavy weather early, while it is still easy to work on deck
5. Where do you get weather information at sea?

   A good answer covers:

   • NAVTEX: forecasts and gale warnings; meteorological warnings cannot be switched off
   • SafetyNET by satellite for the high seas forecasts and warnings
   • Weather charts by radio facsimile or by internet: surface analysis and forecast charts
   • Coast radio and coastguard broadcasts on VHF and MF
   • Your own observations: barometer, wind, sea, cloud, visibility and the hygrometer