AUSTRALIAN WEATHER FOR SEAFARERS
(Extracts courtesy of BoM and ANTA publications, Ranger Hope © 2022 www.splashmaritime.com.au)
Tropical revolving storms (TRS)
In fair weather it is not hard to drive a boat, but when it turns really foul it can be suicidal to go out for even the most experienced. The force exerted by wind and wave do not increase linearly but by the square. A wind of 40 knots is not double the force of 20 knots, but four times as powerful. A full understanding of weather and its prediction is therefore the most important of all the lessons for the small craft mariner.
The atmosphere
The earth is covered in an ocean of air called the atmosphere, thinning out into the solar wind at a depth of 10,000 km from earth's surface. The lowest layer of the atmosphere is called the Troposphere and its upper boundary is called the Tropopause. All weather processes occur in this lowest 10-15 km of air that consists of approximately 21% oxygen, 78% nitrogen with significant amount of carbon dioxide, traces of ozone, helium and other inert gasses. The Ozone Layer, above the Sratosphere, creates a barrier from ultra-violet radiation reaching the Earth's surface.
Pressure falls with height from about 1,000 hPa (Hectopascals) at the surface to about 200 hPa at the Tropopause. Temperature also generally falls with height throughout the Troposphere, at an average rate of about 6°C/km.
Weather
Within the Troposphere the processes of heating and cooling of air and water occurs, the results of which we call weather. The study of weather is called Meteorology.
The sun's radiation is the source
of energy that drives the weather systems. Air and water spread the solar
energy from the equator towards the poles. Note that the earth's mean temperature
does not change significantly, so there must be a balance between the energy
absorbed from the sun and the energy re-radiated to space. This solar energy
is then distributed around the world in a variety of ways.
Heat acting upon air while modifying its moisture content are the prime causes of all weather conditions. Except for localized factors such as bushfires, volcanoes or nuclear explosions, the heat source for the earth's weather is the sun.
The land masses, which heat up quickly and cool equally quickly, produce heat radiation moved into specific areas by airflows. The sea, which retains its heat for much longer periods, results in constant temperature over vast areas with consequent heating of air from cooler regions or the cooling of warmer air from lower latitudes or land masses.
The sea can carry heat into specific areas, such as the Gulf Stream, or cold into others, such as the Labrador Current. This brings very cold Polar air down the East Coast of North America. Where this meets the warm, moist air above the Gulf Stream constant fogs occur, demonstrating cause and effect. The South East Trade winds blow balmy moisture laden air over the Queensland Coast. The East Australian Current washes warmer tropical water from the Great Barrier Reef down along the East Australia coastline.
Climate change
The climate is always changing. Over millions of years ice has covered the Earth many times, leaving its permanent record in the geography and in the extinction/birth of species, only to retreat again. The record also shows that past increases in certain gasses have created a heat trapping blanket on the Earth's surface. These gasses are collectively termed Greenhouse gasses due to their action and include carbon dioxide, methane and organic derivitives. Preventing heat from radiating away to space caused overheating in the past leading to desertification in some regions, ice melt in others and a significant rise in sea level. Our last recorded ice age was 10,000 years ago, but since then the geological record indicates an extended period of relative stability. That is until the last 200 years, and the coal burning started by the industrial revolution. Carbon dioxide pollution caused by man made activities has accelerated expotentially in the last 50 years.
In the 1980s it was noticed that the Ozone Layer was breaking down as holes over the Poles, risking the Earth's exposure to solar radiation. The culprit was identified as aerosol gasses used for spray cans and fridges. In 1987 all 198 United Nation Members agreed the Montreal Protocol to phase out the use of these gasses and the Ozone Hole now shows signs of repair.
No such international consensus has been actioned to date for carbon dioxide emmisions and consequent climate change. Few countries have set realistic limits to their share of emissions that would reduce the global warming to 2° Celsius by 2050, many have not. There are concerns that irreversibe tipping points are being reached. A consequence of partial loss of the ice sheets will be that heat reflective action and thawing methane release will hasten complete ice loss.
Bearing this in mind, it is likely that low lying and marginal agricutural lands are at risk. Significant weather events in the near future will be more regular, more severe and less predictable.
Meteorological instruments
To aid local observation ot temperature, pressure, wind speed, humidity and rain, a number of instruments are commonly used.
Barometers
Aneroid barometers
It is smaller and cheaper than a mercury barometer, responds quicker to small changes in pressure, but is not so accurate. An aneroid should be compensated for temperature changes by the manufacturer. It does not require correcting for changing latitude.
An aneroid should be checked occasionally against a standard barometer, which might be a mercury barometer or a precision aneroid by contacting the Bureau of Meteorology. An aneroid should be located where it is not liable to shocks or temperature changes, and out of direct sunlight. Tap gently before reading.
Barograph
The Barograph is simply an aneroid barometer with a pen arm recording on a clockwork drum with a paper chart on it. It provides a very useful record of barometric pressure.
Anemometer
Various types of anemometers are available from hand held blow tubes to blades and turbines. They must be used or sited in clear air flow.
Wet and dry bulb hygrometer
The Hygrometer compares the air temperature (dry bulb) with another wrapped in a water soaked rag (wet bulb). The wet bulb's temperature is depressed by the latent heat process of evaporation from the wet rag. The more humid the air is, then the less evaporation takes place, and the less the wet bulb temperature is depressed.
The difference between the two readings indicates the humidity which is found through consulting hygrometer tables.
Cloud
Beyond sun, air and water, a fourth weather element of clouds, is both cause and result. They result from the upper movement of moist air which condenses on reaching the cooler upper atmosphere, this air is condensed into droplets of water which form around dust or salt particles to form visible clouds. The moisture source is primarily from the sea, lakes and flood plains, the combination of the sun's heat and the resultant evaporation of the sea surface produces the moisture content in the air.
Clouds are named by their type and height. Cirrus clouds are the wispy high level clouds, below them are the fluffy fair weather Cumulus clouds, and below them the flat Stratus clouds. From the lowest levels to the top of the stack are the threatening Nimbus storm clouds. To qualify any cloud type the prefix Alto can be added to indicate the higher of that type, for instance Altocumulus. Additionally when clouds have features of two types they can be given a mixed name, for instance Stratocumulus.
In the photo above the highest level jet streamers indicate fast moving winds in the upper Troposphere. Below them are the ice crystal Cirrus clouds, often seen as wind-blown wisps.
In the photo above the high cumulus are called alto cumulus and are thickening into altostratus.
In the photo above the mid to low level cumulus are the fair weather clouds.
In the photo above the low and base level stratus clouds present a gloomy prospect.
In the photo above the Nimbus storm clouds reach from base level to the top of the Troposphere and are associated with thunderstorms, lightning, downpours and hail.
The effect of clouds on the weather is twofold
1. The degree of cloud cover will restrict both the heating effect of the sun and the earth's ability to radiate heat outwards. As such they can be the cause of keeping an area cool or warm.
2. The effect of the latent heat released into the atmosphere during the condensation process and the subsequent precipitation in the form of rain.
The amount of cloud cover is described by the number of eights (octas) that cover the sky-for example, half cloud cover is 4 octas.
Fog
Fog is a suspension of very small water droplets in the air that reduce visibility at ground level to less than one kilometre. It forms when the air temperature cools to the dew-point temperature, (the temperature at which condensation occurs), and the invisible water vapour condenses into the tiny water droplets of fog.
A critical factor in fog formation is the amount of moisture in the atmosphere. This is measured by the dew-point temperature, (the humidity of the air). The key requirement is that the air temperature and dew-point temperature are very close.
Radiation fog is formed as the air cools overnight and the ground radiates heat. The greatest rate of radiation occurs with the clear skies and light winds of high pressure weather. A classic pattern for fog formation is to have rain falling late in the day, followed by the arrival of a high-pressure system overnight.
Advection fogs are formed by warm moist air moving over a cool surface, such as sea fog.
Evaporation fog or sea smoke is caused by cold air passing over warmer water. When the warmed seawater evaporates into low air layers, it rises and mix with the cooler air where condensation and fog to occur.
Upslope fogs form when moist air is forced uphill and cools to saturation.
Predicting fog at sea by using the hygrometer as listed below.
Wind speed
Wind speed is estimated by observing the state of the sea. See the Beaufort scale
1 .There may be a lag between the wind getting up and the corresponding sea state.
2. The Beaufort scale assumes open-ocean, with plenty of fetch.
3. Off a weather shore, the sea state will be less than expected.
4. Don't confuse sea and swell.
5 .A weather tide will cause more 'lop'. A lee tide will reduce the sea.
6 .Heavy rains smooth the sea surface.
The Beaufort Wind Scale
| Beaufort Number |
Description |
Knots |
Sea Criterion |
| 0 |
Calm |
1 |
Sea like a mirror.
|
| 1 |
Light air |
1-3 |
Ripples with the appearance of scales, no foam crests. |
| 2 |
Light breeze |
4-6 |
Small wavelets. Crests do not break.
|
| 3 |
Gentle breeze |
7-10 |
Large wavelets. Crests begin to break. Scattered horses. |
| 4 |
Moderate breeze |
11-16 |
Small waves becoming longer. Fairly frequent white horses. |
| 5 |
Fresh breeze |
17-21 |
Moderate waves. Many white horses. Some spray. |
| 6 |
Strong breeze |
22-27 |
Large waves. Extensive white foam crests. Spray. |
| 7 |
Near gale |
28-33 |
Sea heaps up. White foam begins to be blown in streaks. |
| 8 |
Gale |
34-40 |
Moderately high waves. Crests begin to break. |
| 9 |
Strong gale |
41-47 |
High waves. Dense streaks of foam. Crests begin to roll over. Spray may affect visibility. |
| 10 |
Storm |
48-55 |
Very high waves. Tumbling sea. Surface mostly white. Visibility affected. |
| 11 |
Violent storm |
56-63 |
Exceptionally high waves. Small and medium vessels sometimes lost to view. |
| 12 |
Hurricane |
>64 |
Air filled with foam and spray. Sea completely white. Visibility very seriously affected. |
Wind direction is estimated by wave direction. If you use the relative wind (eg. funnel smoke) you must make allowance for the vessel's speed. Visibility is difficult to estimate at sea, unless there are echoes on your radar. Fog is visibility less than 1 km. Mist is more than 1 km.
Literally tens of thousands of weather reporting stations on land, sea, air and satellite contribute information that goes into a weather forecast. At the same time their are as many factors affecting a weather systems outcome. The accuracy and detail of weather reports decrease with the duration of the prediction. Over a few days they are extremely accurate, but over weeks and months the records of past seasonal events become more relied upon. In this time of climate change such historical records must be used judiciously. As mariners we are blessed with the best reporting in history, but should be aware of the limitations for our purposes.
To get this information we can access several sources. One way is by weather reports or forecasts broadcast by local radio stations, or published in newspapers.
However, as most vessels are equipped with either or both, VHF and HF radios, weather information can be readily obtained via Coastal Radio Stations through the regular weather and traffic schedules. These are generally twice a day for coastal weather forecasts and twice daily for high seas (offshore) forecasts. Weather information is also available on demand from coast stations. Mariners should make themselves familiar with the frequencies used by these stations, schedule times and procedures required to access them correctly.
Sources of Weather Information:
Current forecasts and advisories are available at the Bureau of Meteorology, and:
Commercial radio
Television
Newspapers
Weather fax
Telephone – recorded boating weather and weather warning services or by directly phoning the Duty Forecaster, Bureau of Meteorology
Coast radio stations - VHF and HF services
Volunteer Sea Rescue
Other vessels
Own observations
Internet
Forecasts for small craft
Forecasts issued in the mornings cover the remainder of the day, until midnight. Afternoon forecasts cover 24 hours from 1800. Forecasts issued at other times are usually for 24 hours from the time of issue.
Information given:
Time of issue
Forecast area
Expected wind direction and speed
Expected sea and swell condition
Visibility (if below 2 miles)
Any other significant weather
Any major changes expected during the forecast period
Warnings
Strong Wind Warning: (winds expected over 25 knots)
Gale Warning: (winds expected over 35 knots)
Storm Warning: (winds expected over 48 knots)
Cyclone Warning: issued as appropriate, with information about the location and expected movement of the centre of the storm, as well as the likely wind strength
Information given:
Time of issue
Area affected
Wind direction and strength
Sea state
Outlook
Interpretation of weather reports
Air Pressure
The pressure, or weight, of the atmosphere is measured by a barometer. This pressure is expressed as HectoPascals (hPa) and visually presented on a weather map by Isobars, which are simply lines joining places of equal pressure.
Temperature
The heat content of both the air and sea measured by a thermometer and expressed in degrees Centigrade or Celsius, can be visually presented on weather maps by lines called Isotherms. Isotherms are generally only included on upper atmosphere charts and not on MSL charts.
Moisture (humidity)
The quantity of water vapour in the air, measured by a Hygrometer, is called relative humidity and is expressed as a percentage of the maximum amount of water that the air could hold (saturation).
Air Movement (wind)
The surface movement of air is called wind, and is measured in knots over the water and in kilometres/hour over the land. Its direction is determined by the direction from which it is blowing. Strength is gauged visually or from an anemometer and can be visually presented on a weather map by symbols or in knots.
Correct Interpretation Of Weather Reports
Everybody will be familiar with the simplified weather map which appears in the daily newspapers around the nation and is commented upon during the evening television news sessions. These maps are more correctly called Synoptic Surface Weather Charts (the word synoptic meaning a summary or outline.
To achieve this a large number of observations need to be made simultaneously over a wide area.
These observations come from weather ships, meteorological stations, automatic weather recording stations, port meteorological officers, airport meteorology offices, co-operating marine observing vessels and aircraft. They are then coordinated into an area chart.
When air is warmed it becomes lighter and rises so drawing more air in behind. This lighter air registers as a low pressure on a barometer, which measures atmospheric pressure or the weight of the air. Cold upper atmosphere air sinks because it is heavy and registers as high pressure on the barometer. This air falls and spreads out on the surface away from the centre of the high. High pressure systems are generally less energised than low pressure systems, and are more often associated with fair weather than storms. Large areas of high and low pressure are represented on a weather chart as successive high and low pressure systems. But they can be very localised too.
Local winds
The heating or cooling of air and the consequent wind created can be very localised and violent. The Willy Willy and Tornado are examples generated by superheated land surface, and the Cyclone generated by superheated sea surface. More localised examples are an Anabatic wind (caused by rising air over a super heated bare mountain) or a Katabatic wind (caused by falling air over a super cooled snow slope). Such Katabatic conditions occur in winter along the eastern edge of the NSW Southern Highlands, creating challenging sailing conditions under the high cliffs.
Sea breeze
Much of Australia's coastlines experience the Sea and Land breeze effects. The sea breeze is a persistent summer event.
As the sun heats the land, air in contact with the land is heated and rises. Air over the adjacent water remains a comparatively cooler temperature and flows onto the land to replace the air that has risen. That wind that flows in from the sea is called the sea breeze, it is temperature driven and will cut in as early as late morning, attain maximum strength about mid-afternoon. As the heat goes out of the day the sea breeze will become gentle towards evening. If there is very little pressure gradient the sea breeze will generally average 15-18 knots and will not extend more than 20 miles from the coast. The sea breeze can be complemented by the trade winds.
Coastal waters forecasts will often mention trade winds being lighter inshore at night and in the early morning. Also late at night in the moonlight look for low level clouds moving much faster than the air speed at ground level would suggest as evidence of this effect.
Land breeze
At night the opposite effect occurs. The land breeze is an evening event and usually of lower intensity. As the land cools down the cool air flows downhill off the land and out to sea to displace the comparatively warmer air over the water. This land breeze (flowing off the land) is usually fairly gentle (4-6 knots) and does not extend far out to sea (5-10 miles). On the bushfire prone East Australia Coast it is wise for coasting sailors to beware that clear visibility of the day's seabreeze may quickly deteriorate overnight and next morning from the land breeze blown smoke haze.
Sometimes as the air closest to the ground cools it settles in layers like a thick blanket that may only be a couple of hundred metres thick. Along the coast and over the land the warmer gradient wind flows over the top of this layer. This layer is known as an ‘inversion layer’ and will exist until the sun begins to heat the land and restores the normal vertical temperature profile in the atmosphere.
Pressure system driven winds
Air will always flow from high to low pressure in this air movement that we call wind.
The difference in pressure over a distance is the pressure gradient and directly affects wind strength. If the difference in pressure over a particular distance increases the pressure gradient is said to become steeper and the wind will strengthen. Pressure gradient is represented on a weather chart as the distance between the isobars.
From the many land, ship and satellite reporting stations all over the world It is possible to chart the weather in what are called synoptic (summary) charts as shown above. The High and Low centres represent the focus of air outflow and inflow respectively. The extension of the higher pressure is termed a ridge and the extension of the lower pressure is termed a trough. Along the meeting boundary of high and lows there is a large pressure gradient and the maximum wind.
However, due to the centrifugal effect of the spinning earth, air as it flows from high to low pressure is deflected from a direct path. This deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As shown above for the Southern Hemisphere this causes the low pressure system to revolve clockwise, and the high pressure system to revolve anticlockwise. The opposite direction occurs in the Northern Hemisphere lows and highs.
A local observer in the Northern Hemisphere who stands with his/her back to the wind, will find his/her left arm pointing in the direction of the lower pressure.
A local observer in the Southern Hemisphere who stands facing the wind, will find his/her left arm pointing in the direction of the lower pressure.
Prevailing winds
The Doldrums, Trade winds, Horse Latitudes and Roaring Forties
Solar heating is distributed across the globe from the Equator towards the Poles. In the Southern Hemisphere the intense heat at the equator causes the warm air to rise so creating a belt of low pressure, the Equatorial Low Zone, or the windless Doldrums. That sucks in ever more air creating the prevailing South-East Trade Winds. As the air rises it moves poleward by virtue of the centrifugal and coriolis effects, then cools and falls back to the surface around the 30° South Latitude. This Hadley Cell cycle creates the predominantly high zone called the Sub-Tropical High Zone or the fickle Horse Latitudes. The prevailing winds generated below this ring of highs are Westerlies in 40° South Latitude called the Roaring Forties. They blow vigorously and mostly clear of landfalls all around the bottom of the World.
Poleward from the Roaring Forties is second but less intense cycle, the Ferrell Cell of rising air creating a less intense belt of low pressure and the Polar Easterly winds. These belts of Poleward movement of air are duplicated in the Northern Hemisphere in mirror image.
Polar Lows
Between the relatively warm and moist air of the temperate Westerlies and the cold and dry air of the Polar Easterlies a front of differing qualities of air meet and curl around each other from a central low pressure hub. So are born the Polar Lows Systems, often in a succession of groups of three or more. These Polar Lows with circling warm and a cold sectors are thrown up into the global westerly flow.
As it is named, the Warm Sector consists of warmer lower pressure air typified by Stratus cloud. As the system advances it wedges over the air ahead creating a long overcast front, becoming drizzly then raining as the front meets the sea surface.
In the Warm Sector conditions are typically dull and even muggy. While the Warm and Cold fronts are resistant to mingling, the cold air is more vigorous than the warm air, and as the Polar Low moves westward it eventually catches up and undercuts the Warm Sector. This Cold Front is a dramatic event with pressure rise, temperature drop, wind squals and shifts, lightning and torrential rain under glowering Cumulonimbus or Nimbostratus storm clouds.
Eventually the Cold Front drives right under the Warm Sector forcing its Warm Front interface up above the surface. This is then called an Occluded Front. In more northerly NSW only the Cold Front remains of these Occluded Fronts.
Australia is both an island and a continent. It experiences the full range of weather conditions from Tropical to Polar. The weather a locality experiences is directly related to the air quality it receives from the air masses that surround it, and that are generated in its hot centre. These air masses are maritime or continentally derived and are tropical, temperate and polar, being warm. cold, wet or dry.
Typically the annual summer roasting of the central desert promotes continental dry, low pressure conditions, whereas the equatorial monsoon drags down tropical wet warm air to the northern coasts. This may extend as a trough East/West down to NSW particularly if the Tasman Sea is beset by persistent low pressure due to a stalled blocking high east of Tasmania. Cyclones being birthed by the high sea surface temperature of the Coral and Arafura Seas contribute to these Troughs.
Sea breezes become significant and reliable in tropical and sub-tropical regions, such as the Fremantle Doctor (energised by the Westerlies) and the Queensland coast, where they are energised by the Trade winds.
The Sub-tropical highs shift southward during summer, bringing settled dry westerlies into NSW in autumn. In winter the track of the Polar Lows move further North, exposing the Bight, Southern Australia and Tasmania to the unsettled (many seasons in the day) warm and cold fronts. While in NSW Polar Lows aged into Occluded or Cold Fronts track up the coast.
Climate change and particularly rising sea temperatures are observed to be changing traditionally steady weather patterns. Greater intensity of flood, drought and storm frequency are anticipated. As the precedents from the historical seasonal climate record become less reliable forecasters are placing greater emphasis on variation cycles, such as El Nino/La Nina, IOD and SAM as summarised below.
The El Niño occurs every three to eight years. The trade winds weaken and the central and eastern tropical Pacific warms up. This change in ocean temperature sees a shift in cloudiness and rainfall from the western to the central tropical Pacific Ocean.
The Indian Ocean Dipole refers to the difference between sea surface temperatures of the tropical western and the eastern Indian Ocean. The IOD has three phases: neutral, positive and negative. A negative IOD typically results in more winter-spring rainfall over parts of southern Australia, while a positive IOD provides less.
The Southern Annular Mode refers to the (above-seasonal) north-south shift of the westerlies that cause cold fronts to move from west to east, bringing rainfall to southern Australia. The SAM has three phases: neutral, positive and negative, each typically lasting for around a to week or two. SAM's effect varies the rainfall on differing regions dependent on season.
Typical summer synoptic
The major pressure systems are driven
by the sun and as the sun apparently moves north and south throughout the
seasons the pressure systems go with it. Below is a typical summer map. Note
the anticlockwise air flow around the high in the Bight giving south-east
trade winds along the east coast. Also note the equatorial trough present
across northern
Typical winter synoptic
Below is a typical winter map. Note
the absence of the monsoon trough which has moved into the Northern Hemisphere.
The high pressure systems now take a line through central
Cold Fronts, Line Squalls and Southerly Change
A front is named by the active or moving air mass. Thus in the case of the cold front, it is the cold mass of air that is moving. As the cold air pushes into a warm air mass the warm air is pushed up and cooled. If this uplift occurs rapidly and there is sufficient moisture in the air, storms and squalls may occur. At other times the passage of a cold front may only be marked by a sudden change in wind direction to the south. On the East Coast this is often called a Southerly Change. Often a cold front is sandwiched between two high pressure systems and indicated by the saw tooth line on the weather map. The typical weather prior to and after the passage of a front is described in conjunction with the synoptic chart below.
Preceding the front (from Position A along the central NSW coast): As the high pressure system moves away, the gradient wind turns warm northerly and the backs into the north-west as the front approaches from NSW. The barometer will be steadily dropping over two to three days as the high moves away and often the pressure gradient will become steeper within six to twelve hours of the front, with north-west winds increasing to perhaps 25-30 knots.
At the front: The northerly wind will back right around into the south-west or the south-east. The barometer will be at its lowest. The air temperature will drop. If a storm accompanies the change prepare for severe weather.
After the front: As the next high pressure system takes over, the wind will settle into the south and be much cooler. The barometer will begin to rise and the strength of the wind will depend on the pressure gradient. On the accompanying barograph trace above from coastal south-east Queensland, note the diurnal variation, steady drop in pressure as the front approaches, sudden jump as the storm comes though, then the steep pressure gradient following the front.
A line squall is the name
given to an extremely active or severe Cold Front. The temperature difference
between the cold and warm sectors is large and that the warm air mass is extremely
unstable. Convection along the front is massive.
It must be considered that the front is not always a single wall of cold air
but often a series of ragged outlying air packages running ahead of the main
thrust as scattered rainstorms.
The line squall might not be forecast because the intensification of the front occurs during its passage. Any sharp Cold Front must be regarded with caution and a close weather watch maintained.
The line squall can be accompanied by a long low black Roll Cloud stretching across the horizon in advance of the front. This is caused by the rapid advance of the cold sector forcing the air in the warm sector at the front to be tumbled over. The warm air being close to saturation, condenses out. A particularly violent front is called the Southerly Buster.
Tropical revolving storms (TRS)
For the seaman the most dangerous of weather systems is the TRS, also called a Hurricane, a Typhoon or in the Southern Hemisphere a Cyclone. It is an intense non-frontal low pressure system which forms over tropical waters in 5-10°S Latitude (usually in summer) where sea surface temperatures are over 26° C. Hot air rises rapidly, spiralling skyward from an increasingly low pressure centre. At height the air cools rapidly creating a temperature differential and shedding energy from latent heat. This energy accelerates the system. They have a minimum mean wind speed of 34 knots extending more than half-way around the centre persisting for at least six hours and be rated in severity from category 1 to 5. Cyclones have gale force winds with wind gusts in excess of 50 knots around their centre. In category 5 cyclones, gusts can exceed 150 knots.The central eye, the Vortex, is calm though the seas here will be treacherously confused.
Once established the cyclone typically drifts off erratically on an westward path at 10-20 knots, but erratic is the key word. At this stage the cyclone may be detectable by radar at 200 nm distant, and sea conditions may be deteriorating. It is important to know your vessel's position relative to the storm centre in order to get out of its way. Survival is unlikely from a direct hit by a cyclone in a small craft. The torrent of foam and tempest of wind will preclude any navigation. The lower zone of the cyclone is called the dangerous semicircle (as its internal winds will be amplified by the easterly tracking path) while the upper navigable semicircle's winds will be diminished by the eastward path. The picture below illustrates the Admiralty advice for large vessels to steam away from the centre of a cyclone.
It is possible that the cyclone may recurve around 20°S Latitude at its Vertex, and then drift off to the South East. Sometimes they reinvigorate a Polar Low to form a Storm Bomb further South.
More typically the Cyclone will cross the coast and loosing its energy driving sea surface temperature will drop a torrential downpour. Sea levels will be higher both due to the low atmospheric pressure and the shore bashing storm surge, but now the additional downpour will ensure general flooding.
Action to take on the approach of bad weather
Warning signs of an approaching TRS
1.Coast radio stations will be issuing frequent and regular warnings, including the best information available, of the position of the centre and its expected movement.
2. A definite, unusually steep fall in the barometric pressure. If the corrected pressure is below more than 3 hPa below normal, beware! If it is more than 5 hPa below normal, there is probably a storm within 200 miles. This distance may well be within the zone where bad weather cannot be avoided. When checking the barometer to see if it is below normal, it is essential to take into account the diurnal variation.
They are the most significant indicators. The remaining warning signs are useful, (though not conclusive on their own), and may enable an earlier prediction of the proximity of a TRS.
3. High sea water temperature (over 26° Celsius).
4. Fleeing birds.
5. Appreciable change in the direction and strength of the wind.
6. Unusual clarity of the atmosphere, remarkable visibility.
7. Extensive high cirrus cloud, swirling in towards the storm centre, and reflecting lurid colouring at sunrise and sunset.
8. In the open sea, swell from the direction of the storm centre.
9. High humidity and rain increasing as TRS approaches.
Action to take
A Seafarer operating close in along the coast should, upon hearing a cyclone warning for that area, proceed without delay to the nearest port, cyclone anchorage or safe haven. Port authorities in cyclone affected northern Australian ports have Cyclone Emergency Plans in place. Should you make for a port with which you are not familiar you should contact the Harbour Control on VHF for advice. Should you enter a port or a haven which does not have a cyclone plan, avoid anchoring amongst other boats if possible. If one drags it usually picks up a few others.
If possible go as far as possible up a small creek and secure fore and aft, and athwartships to stout mangroves. Mangroves provide a tremendous wind break without being too high. Also, make allowance for ‘storm surge and ensure there will be sufficient water to get out after the storm.
If at sea and the cyclone forms rapidly in the immediate vicinity, secure for heavy weather and make for the nearest haven. Also, if scheduled weather reports do not confirm the weather in your area, advise a Coast Station and other vessels of the conditions being experienced. Should conditions become severe, maintain regular contact with a Coast Station or Volunteer Rescue group.
It is essential to keep a close eye on the storm's movement, since any individual storm may behave irrationally. They are highly unpredictable, and it is not sufficient to assume that any particular storm will obey the rules. If the storm changes course, then it may be necessary for you to alter your own course of action.
In addition to knowing where the storm is it is essential to secure the vessel before the onset of heavy weather.
In determining a safe haven for a Tropical revolving storm or rough weather in general the following factors should be amongst those taken into consideration:
· Does the area provide shelter from the prevailing and expected winds?
· It must be easily accessible.
· Sufficient swinging room.
· Sufficient depth of water at all levels of tide.
· Good holding ground if intending to anchor or a secure mooring point capable of mooring the vessel with chains or wire.
· Distance of the safe haven from your present position.
· Estimated Time to reach the haven in the prevailing conditions.
· Is it in the likely path of the storm?
· Effect of a storm surge in the haven.
· Any submarine cables or ordinance areas that make the haven unsafe.
Angle of indraft: The angle at which the wind crosses the isobars. It results from a balance of Pressure Gradient Force, Coriolis Force and Friction. The angle of indraft varies from approximately 45° at the edge of a storm to 0° in the eye of a cyclone.
Anticyclone or High: An area of high atmospheric pressure within a closed (circular) system of isobars. An anticyclone is a region where a system of isobars encloses a centre of high barometric pressure. The centres of anticyclones are generally areas of light winds, little cloud and good weather.
Atmospheric pressure: The force exerted by the atmosphere on each unit of area. This can simply be thought of as the weight of air. The unit used today is the HectoPascal, where 1 hPa equals 100 Newtons per square metre.
Backing and veering: A wind is said to back when its direction changes in a counter-clockwise direction eg. from West to South. When it changes in the opposite direction it is said to veer.
Col: An area of little pressure gradient between high and low pressure systems.
Depression, Low or Cyclone: An area of low pressure within a closed system of isobars.
Diurnal variation: The atmospheric tide which causes the pressure to be lower at about 0400 and 1600 local time, and higher at 1000 and 2200. It should be taken into account when considering whether the pressure is truly rising or falling. Diurnal variation is about 4 hPa.
Isobar: A line on a weather map which joins places having the same atmospheric pressure. Used to determine the direction and strength of the wind.
Pressure gradient: A measure of the rate of change in atmospheric pressure with distance. The closer the isobars are together, the higher the pressure gradient is and the stronger the winds will be.
Pressure and wind: Wind blows from areas of high pressure to areas of low pressure. The pressure gradient is an indicator of the speed of the wind. On a rotating earth, the wind direction is deflected to the left in the southern hemisphere. As a result, wind blows round areas of high and low pressure. In the southern hemisphere this direction is anticlockwise round a high, or anticyclone and clockwise round a low, or cyclone.
Ridge: An area of high pressure extending into or penetrating an area of low pressure. A Ridge of high pressure may extend out from an anticyclone.
Sailors rhymes: Red sky at morning, Sailors take warning, Red sky at night, Sailor's delight.
Trough: An area of low pressure extending into an area of high pressure. A trough of low pressure may extend out from a depression.
Trade winds: In Australia this refers to the South-Easterly winds which are the prevailing winds along much of the Eastern seaboard. They are caused by the movement of air from the sub tropical high towards the equatorial.
Tropical cyclone or Tropical Revolving Storm: An intense low pressure storm which may form over tropical waters, usually in the summer months.