Meteorology

Question 1

Name some sources met information?

Answer 1

• World Meteorological Organization through 21 METAREAS
• UK offshore shipping forecast
• UK gale warnings
• UK coastal Inshore Forecast
• UK or other sources website forecast (Met office)

Question 2

Why do officers take weather observations?

Answer 2

1 - It will indicate the current weather the ship is experiencing and comparing this to forecasts allows the bridge team to predict what weather is to come.

2 - The ship may be involved with the Global Observation System

Question 3

What weather elements should the OOW observe?

Answer 3

1 - Atmospheric Pressure
2 - Humidity (Dewpoint)
3 - Wind speed and direction

Question 4

How will an OOW take weather observations?

Answer 4

They will use the ships onboard instruments:

• Barometer
• Barograph
• Hygrometer
• Whirling Psychrometer
• Anemometer

Question 5

What is a barometer?

Answer 5

A barometer is an instrument which will indicate the current atmospheric pressure and if you take readings over a period of time you are able to see the pressure trend.

Question 6

What weather can you expect with a high pressure reading on a barometer?

Answer 6

You can expect to see calm, clear, sunny but also sometimes trapped cloud or fog.

Question 7

What weather can you expect with a low pressure reading on a barometer?

Answer 7

You can expect to see clouds and precipitation with deep lows potentially leading to storm conditions.

Question 8

Are there any errors with the barometer?

Answer 8

There are 3 errors:
- Instrument error = this will be found on the manufactures calibration card, the barometer should be calibrated at regular intervals

• Reduction to sea level = a table for reducing to sea level will be supplied when the barometer is set to its station height.
• Temperature = on installation most barometers will be adequately compensated for temperature, if not, then a temperature correction chart will be provided.

Question 9

What is a barograph?

Answer 9

A marine barograph is also used to measure the atmospheric pressure. As it has 4 or 5 aneroid chambers, this will drive an arm with an ink stylus on to graph paper. to show the pressure tendencies.

To get an indication of whether the ship is transiting through an area of high or low pressure for a particular location the OOW should refer to the ‘Mean Barometer Pressure’ graph in the relevant Admiralty Sailing Directions.

Question 10

What is a hygrometer?

Answer 10

A hygrometer is used to measure the amount of humidity (water vapour) in the air. The hygrometer will have a ‘wet bulb’ and a ‘dry bulb’ this is how you will measure the current humidity. When comparing the two bulbs this is called the ‘depression of the wet bulb’ using this information you can find out the dew point of the current air from the dewpoint table in the Mariners Handbook (NP100). These hygrometers are usually kept in a box to protect them against the elements and are called ‘Stevenson Screens’.

Question 11

What is a dewpoint in relation to fog?

Answer 11

Fog is caused when air cools to the dewpoint. When sea temperature is relatively cold and encounters warm moist air flowing over the surface, the cooling effect can result in the water vapour above the sea reaching its dewpoint and forming a shallow layer of fog.

The Mariners Handbook states that a warning of onset fog can be obtained when the dewpoint is within 5’ of sea surface temperature.

Question 12

What places are know to have fog?

Answer 12

• Polar regions in the summer
• Grand Banks of Newfoundland
• NW Pacific Ocean
• The UK
• The western seaboards of Cali, Chile, Peru, SW Africa and Morocco

Question 13

What is a whirling psychrometer?

Answer 13

This is a handheld instrument used to measure the relative humidity of the air. It has a ‘wet’ and ‘dry’ bulb so you are able to find the dewpoint after calculating the depression of the wet bulb.

These are normally used in cargo holds to give an indication of when to ventilate a cargo. For example, too much moisture in the air may cause damage to a cargo.

Question 14

What is an anemometer?

Answer 14

A anemometer is an instrument used to measure the wind speed and direction. Using this information can assist the bridge team in ensuring that the ship remains on its charted track.

Question 15

What is a synoptic chart?

Answer 15

A synoptic chart is a visual representation of a weather forecast.

Question 16

What are some feature on a synoptic chart?

Answer 16

Features you can find are:

• Isobars = this indicates how strong the wind is and what direction is it coming from
• Air moves around an area of high pressure in a clockwise direction
• Air moves around an low pressure in an anti-clockwise direction
• Frontal systems = When air moves from high to low pressure
• Fronts are used to indicate the boundary between two air masses.
• Warm front = these are shown with a red line and semi-circles and warm air will follow this front but along the front an increased amount of cloud and rain will be experienced
• Cold front = these are shown with a blue line with triangles and cold air will follow this front but along the front an increase in the amount of rain and cloud will be experienced
• Occluded fronts = these are shown as a purple line with semi-circles and triangles, this is formed when a cold front catches up with a warm front.
• Troughs = these are shown as a thick black line and means usually unstable air and showers should be expected.

Question 17

Do you have to record weather observations?

Answer 17

It is a requirement under SOLAS ch V regulation 28 that every ship engaged on an international voyage must keep a record of navigational activities and incidents important to the safety of navigation, usually in the form of Bridge deck log book. This log book will keep details of weather and sea conditions.

Question 18

If a ship encounters unexpected weather, does she have to report it?

Answer 18

Yes, under SOLAS ch V regulation 31 and 32 ships must record to relevant NAVAREA coordination centre if they encounter:

• Dangerous ice
• Dangerous derelict
• Any direct danger to navigation
• Tropical storm
• Subfreezing air temperatures associated with gale force winds causing sever ice accretion on super structures
• Winds of force 10 or above non the Beaufort which no storm warning had been received.

Question 19

What is a TRS?

Answer 19

A Tropical Revolving Storm is a weather system created in either 5’-20’N or 5’-20’S. For it to be created you must have at least a temperature in the ocean of 27’C in at least a depth of 50m.

A TRS won’t form in very low latitudes or over land, they develop over open seas where the temperature and humidity are high and there is some form of trigger action available to cause a violent convection.

TRS’s can also be called - hurricanes, typhoons and cyclones

Question 20

What are the signs of a TRS?

Answer 20

Some signs are:

• Barometer pressure is 5mb or more lower than the mean average for that time of year
• A appreciable change in direction or strength of the wind
• A long low swell from the general direction of the expected storm
• Extensive cirrus clouds forming, followed by altostratus and then broken cumulus