Meteorology

In 1857, Buys Ballot formulated a law identifying the relationship between wind and pressure distribution. The law states that:-

In Northern Hemisphere, if observer faces the true wind, the center of low pressure will be on his right and somewhat behind him and the center of high pressure will be on his  and somewhat in front of him.

In Southern Hemisphere, if observer faces the true wind, the center of low pressure will be on his left and somewhat behind him and the center of high pressure will be on his right and somewhat in front of him.

This is because Coriolis force is negligible at or within few degrees of equator and therefore wind blows directly across the isobars from high pressure to low pressure areas.

Coriolis Force is the deflection of wind due to earth’s rotation.

When the wind encounters a solid object, a whirl of air (known as eddy) forms off the leeward side of the object.

NORWESTER:- Norwester is the thunderstorm that generally blows over Bangladesh usually in month of April-May from a North-westerly direction. 

ELEPHANTAS:- Elephantas is the South-westerly wind which blows over Malabar Coast of India in month of September & October.

• An Isobars is a line drawn on a weather map connecting all the places having same atmospheric pressure at the time for which that weather map has drawn.
• Isobars are used to depict atmospheric pressure over an area.
• For standarisation, isobars are drawn at 4 mb intervals and pressure denoted by isobars must be divisible by 4 like 996, 1000, 1004 etc & not like 997, 1002,1005 etc.
• Where consecutive isobars are very far apart, 2 mb interval may be inserted like 996,998,1000 etc
• Also for standarisation, all barometer readings are according to UTC (not ship’s time) & at sea level.

 Few more extra points (if you want to add):-

• Isobars are smooth lines which curves gently without any sudden change of direction except at “Fronts”.
• Isobars cannot cross or met because atmospheric values can not be different at one place at same time.

It is a low pressure area formed at the boundary between two different air mass.

The formation of frontal depression is assisted by a large difference between warm and cold air mass.

• Low pressure
• Heavy & Long swell
• Cirrus cloud will be at sky
• At sunset time, cloud color will be red or copper
• Cloud will be dense
• Frequent lightning.
• Possibility of rain with violent character
• Storm warning from local authority

Weather analysis chart: These charts gives the weather situation at the preceding synoptic hours, over the entire area. They show the weather experienced at each land and ship station, pressure systems, isobars, wind direction, fronts, frontal depression etc. With good knowledge of meteorology, Master can size up the overall situation and take any steps that are necessary to ensure safety of ship.

Weather prognosis chart: These chart contain same type of information as in weather analysis chart but predicted for specified future time. They may be made 12 hr, 18 hr, 1 day, 3 day or 5 days before the specified time, so Master can route the vessel accordingly.                      A weather prognosis chart is made by professional meteorologist after taking various factor into account. The construction of prognosis charts need deep knowledge in weather prediction and expertise gained over several years.

a) What are the principles & workings of the Aneroid Barometer?

Principle: 

• Aneroid (name came from Greece) which means no liquid.
• Like a mercury barometer, an aneroid barometer does not need any liquid for reading.
• A sealed chamber made of a very thin corrugated metal plate, having a partial vacuum inside it.
• This vacuum chamber is connected to a pointer by a system of levers and springs. The pointer is fitted over a circular graduated scale so that we can take reading directly.
• The thin metal has an elastic effect.

Working:

• If the atmospheric pressure increases, the chamber gets compressed. The inward movement of its wall is transmitted to the pointer by the levers and springs. The pointer then shows a higher reading on the scale.
• If the atmospheric pressure decreases, the opposite happens & the pointer then shows a higher reading on the scale.

b) What are the errors of the Aneroid Barometer?

   (i) Index error

• It is caused by the imperfect elasticity of the vacuum chamber. It changes with time & hence required to obtain at least once in a three month.
• The index error is obtained by comparing it with another aneroid barometer, whose index error is known (or by mercury barometer).
• The obtained index error should be entered in a card hung near the equipment.
• In case, the index error is quite high, it can be adjusted from the adjustment knob on the back of the instrument with the help of a screwdriver.

   (ii) Height error

• The reading that we get is at the bridge level. But for the sake of uniformity of climatic records, it is necessary to convert the reading to that of sea level.
• Since atmospheric pressure over sea level decreases by 1 mb for every 10 mtr increase of height.
• So, correction is calculated by   “Correction (in mb)= Height above sea level ÷ 10
• We will add the correction in our reading to get atmospheric pressure at sea level.

c) How to correct height correction in an aneroid barometer?

• Since atmospheric pressure over sea level decreases by 1 mb for every 10 mtr increase of height.
• So, correction is calculated by   “Correction (in mb)= Height above sea level ÷ 10
• We will add the correction in our reading to get atmospheric pressure at sea level.

• A barograph is a recording-type instrument that provides a continuous record of pressure on a paper chart. That paper chart having a continuous barograph trace on it is known as a Barogram.
• A barograph uses an aneroid barometer mechanism for the movement of a pen on a suitable chart. The pen is attached to the drum which moves by a clockwork.
• The barograph is adjusted to allow index error and height correction. The reading must be compared once a week.
• For the sake of uniformity of climatic records, the barograph should always be set to GMT.

A wet bulb thermometer is used in conjunction with a dry bulb thermometer to measure the relative humidity of the air.

• The term anemometer is derived from the Greek word “Anemo” which means wind.
• It is an instrument used on-board ships to measure wind speed and direction.
• It is generally fitted on the top of a mast or a place where there is no obstruction to the wind.
• The signals from this led to the wheelhouse repeaters by cables.
• When a ship is moving, the anemometer shows apparent wind which is a combination of true wind & ship’s speed.
• However, the modern anemometer can also give true wind as they are interfaced with a gyro and speed log.

Working principle:

• A vane anemometer consists of two elements: a wind vane and an anemometer.
• The anemometer portion of the vane anemometer is usually of the propeller type. It is calibrated in such a way that it measures wind speed by measuring the speed at which the wind causes the propeller to spin. It may use mechanical and electronic means to record and calculate this data.
• The wind vane element of the vane anemometer is used to determine wind direction. It is a simple device that always faces into any wind. Due to the aerodynamic principle and the design of the vane, it always turns to face any wind, regardless of wind strength or direction.    It is like an airplane with no wings, mounted on a swivel base which allows it to spin freely in any direction.

• Aneroid barometer
• Precesion aneroid barometer
• Barograph 
• Hygrometer in stevension screen
• Whirling pshycrometer
• Anemometer

=> If the vessel is in a dangerous quadrant

• Proceed with full speed with the wind on 1 to 4 points on the starboard bow in the Northern Hemisphere and port bow in the Southern Hemisphere.
• Alter course as the wind veers in the Northern Hemisphere and backs in the Southern Hemisphere.
• Continue until pressure rises back to normal.

=> If the vessel is in the path of the storm or if in the navigable semi-circle

• Proceed with full speed with the wind on 4 points on the starboard quarter in the Northern Hemisphere and port quarter in the Southern Hemisphere.
• Alter course as the wind backs in the Northern Hemisphere and veers in the Southern Hemisphere.
• Continue until pressure rises back to normal.