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Storm and Squall
Storm
A disturbed state of atmosphere, especially affecting its surface and strongly implying severe weather. It may be marked by strong wind, thunder and lightning, (thunderstorm), heavy precipitation such as ice (ice storm) or wind transporting some substance through the atmosphere (is in duststorm, snowstorm, hailstorm etc).
Squall
A sudden increase in wind force by at least three stages of the Beaufort scle (means increase of at least 16 knots wind speed reaching upto at lest force 6 (22 knots) and lasting for at least one minute. Squall is different from gust of wind by its longer duration.
Thunderstorm and normal Rains
- The difference between an ordinary shower and a thunderstorm is this type of storm is that it is always associated with thunder and lightning.
- Thunderstorms in the tropics are more frequent and more potent because of higher extent of troposphere which allows higher growth of Cb clouds.
Dangers of Thunderstorm to a modern ship
From Lightning at sea
- Lightning poses little danger to modern steel-hull ship are provides direct passage to any electrical discharge to the seawater which itself is a good conductor of electricity.
- However ships working inflammable cargos should suspend cargo handling during a thunderstorm because of the inherent danger of sparking which may be caused by a lightning.
- Thunderstorm squalls do not sink modern ships, but may cause heavy drift and grounding if ships ae in narrow channels.
- Wooden ships and ships with inflammable cargo on deck can suffer from fire.
- Lightning and thunder can cause short term disturbance to radio communications.
From lightning ashore
- When lightning strikes tall objects on the ground, the enormous heat can destroy the object physically and sets fire to the remaining portion when there are no lightning conductors.
- Lightning conductor is metal rod of sufficient height to protrude above the buildings which is connected to the ground with a thick a thick metal strip or a cable.
- The lightning conductor conducts the electric charge and the heat directly to the ground protecting the other parts of building from direct damage.
From Storm
- Modern ships are too big to be directly affected by the squall or a storm occurring during a thunderstorm but if on a passage through narrow channels, the sudden onset of strong winds may cause the ship to drift into shallow waters.
- In harbor, a ship may snap her moorings when tied ashore or drag her anchor dangerously and all necessary precautions to prevent mishap have to be taken.
- Sudden squalls and rough seas can sink smaller ships, though bigger ships are relatively safe.
From Rains
- Rains during thunderstorm are torrential. Though they may not pose a direct threat to a modern large ship, there is a strong threat due sudden loss of visibility of sea during such rains.
- In such case even a radar is of little use because heavy rains reflect back the radar energy entirely blanking out the radar.
- In such case, the solution lies in memorizing the radar situation before start of rains and proceed with caution bringing the ship down to safe speed.
- The AIS is not affected by rains and continue providing the surface picture, but here it needs to be remembered that ships and crafts not fitted with AIS will not be marked on the AIS display.
- Sudden loss of visibility and blanking off of Radars due to very heavy rains.
Essential conditions
- Warm Unstable condition of atmosphere.
- High relative Humidity – as much as 75% – are essential.
- Such conditions occur,
- On a hot day over land adjacent to or surrounded by large body of water. Typical examples are the equatorial and tropical coastal regions and islands.
- When a cold airmass wedges into a warm airmass causing it to rise rapidly.
- Thunderstorms in the tropics are more frequent and more potent because of higher extent of troposphere which allows higher growth of Cb clouds.
How do they occur?
- Key to understanding the formation of TS is to understand the formation of Cb clouds. Like all clouds, Cb clouds form due to rising moist air and its adiabatic cooling.
- But in case of Cb clouds, the rate of the rise of air is especially rapid. The Average rate of ascent of air during TS is about 6m/sec, but at times can be as much as 30m/sec. (6o knots)
- This rapid rise of air has 3 consequences
- The great heights of the Cb clouds which reach till the tropopause (8-16Km).
- Rapid cooling or water into ice crystals
- Built up of static electricity which give rise to lightning and thunder.
Buildup of Cb Clouds
- Thunderstorms occur because rise of air and associated adiabatic cooling of the vapor in the rising air which condenses, forms a cumulous cloud which finally leads to precipitation.
- If the atmosphere is unstable further, the cumulus cloud grows vertically to form cumulonimbus cloud and subsequently a thunderstorm,
- The difference between an ordinary shower and a thunderstorm is this type of storm is that it is always associated with thunder and lightning.
- The energy for the the thunderstorm is provided by the latent heat of condensation rapidly converting in to the kinetic energy of ascending air currents.
- Average rate of ascent of air during TS is about 6m/sec, but at times can be as much as 30m/sec. (6o knots)
The rise of PD
- Lower part of Cb consists of drops of water, the middle part of super-cooled water and upper part of ice-crystals.
- The drops of water grow in size during upward journey of air.
- When the drops in lower part of the cloud reach 6mm size, they break up into smaller drops.
- These smaller drops get pushed up to the top of the Cb cloud.
- These upwardly moving drops have positive charge as compared to the larger drops of water that remain below.
Lightning
- When a large PD develops between various layers of cloud or between various clouds themselves, a discharge occurs which forms the lightning.
- A lightning can occur either within the cloud, between two clouds or between the cloud and the earth.
- Lightning can be seen as far away as100 nm.
- Thunderstorms last from half an hour to 2hours.
Thunder
- Thunder happens because of enormous heating of air through which the lightning spark passes.
- The rumbling happens because numerous other lightnings which are happening in the innards of a cloud at all times.
- Thunders can be heard up to a distance of 10 miles.
- Thunder is always heard after the seeing of a lightning.
Weather associated with a TS
- Dark anvil shaped Cb.
- General reduction of daylight.
- Sudden calm and extraordinary visibility.
- Flashes of lightning.
- Rumble and loud noise of thunder or a series of thunders.
- Violent squalls.
- Torrential rains which are called thunder showers.
- Almost nil visibility during rains.
- Sometimes hail
- Unsteady rise of humidity to nearly 100%
- Sudden fall of temperature up to a max of 10°C
Typical Thunderstorms
Norwesters (Kal-Baishakhi)
- Norwesters are Thunderstorms, meso-scale disturbance, which occur frequently in the northeastern Indian subcontinent, and cause devastative damages in this region every year.
- They are called Kal-baishakhi in Bengal and Bangla Desh.
- They are so termed because they come from NW and occasionally from N
- They are experienced during hot seasons of March to May when land gets heated greatly.
- Norwesters stop as the SW monsoon sets in.
- They may happen during latter half of February. Details are
available in Sailing Directions.
They invariably happen in the after noon after a humid and hot day. - Air that becomes buoyant rises and is cooled by adiabatic expansion until it eventually reaches saturation point and causes a cumulus cloud
- First sign is a low bank of dark clouds in the NW the upper side of which with a form of an arch.
- These thunder storms can be violent and wind speed up to 100 knots have been noted.
- They pose danger to ships at anchor, and especially to ships alongside where mooring lines snap.
- Because of very steep temperature lapse rate, high water content of clouds and the cumulous updrafts, hail is common to a nor’wester.
- They are generally followed by cool and clear weather.