WEATHER

The internet has opened up the weather business to anyone with a PC – and sophisticated computer modelling now delivers weather guidance for more than a week in advance. 

 

The following website links have all the information you'll need to plan ahead:

But first, a few basics. Wind is generally driven by the differences in air pressure, indicated in our weather charts by isobars but it must be remembered that the spacing and direction of the isobars indicate the wind flow above the friction layer – at around 600 to 900 metres. 

Over regular terrain, the surface winds will be less in speed and veered (more right) in direction. Air wants to escape from high pressure systems but, as it attempts to flow out, it is rotated into an anti-clockwise flow. Likewise, air circulates clockwise around a low pressure region. This is the reason that pressure systems remain as identifiable features for quite some time, since if the air did flow directly from high to low pressure, then the features would quickly disappear. However, they are not perfectly circulating columns of air; the air leaks out from the lowest levels of a high pressure system while it fills into a low pressure system.

It is very important to appreciate that the latitude is important when calculating or 'eyeballing' the wind speed from the isobar separation on a weather chart. With the same isobar spacing at Townsville (latitude 20), Sydney (latitude 34), or at Macquarie Island (latitude 55) – radically different wind speeds will result.   

Location                             Townsville (20)           Sydney (34)             Macq Is (55)

Wind speed (kts) at 900 metres           38                           23                             16   

Wind speed at the surface (10m)     25-30                     15-20                        12-15 

 

CLOUDS

Most people can glance at the sky and instantly decide on the immediate likelihood of rain. The experienced observers simply take a little more time actually doing the observation and can forecast further into the future. But from the sky alone, the forecast is limited and there is no guarantee of success beyond 9 to 12 hours, even though some cloud patterns can point to weather systems some 24 to 36 hours away. By combining the physical description of the cloud with its height, 10 major cloud types are generally referenced:

 

HIGH LEVEL CLOUDS  (ice clouds)

Cirrostratus: Diffuse, milky, overcast, often producing the halo phenomena around the sun or moon. Invading the sky from one particular quadrant and often thickening over many hours. When thick, the sun becomes diffused and shadows become ill-defined to non-existent.

 

Cirrocumulus: Small cells, ripples or grains in a pattern, often with wave-like pattern similar to patterns in the sand. The cells or cloud elements are small; most often around the size of a small finger on an outstretched hand.

Cirrus: Detached areas, patches or bands of white wispy ice clouds. Can be organised semi-pattern or hooks (mares tails) or disorganised clumps.

 

MID LEVEL CLOUDS (away from the earth surface)

Altostratus: Light grey to very dark grey layer or sheet, generally covering all the sky but invading from a particular quadrant.

Altocumulus: Cells, patches or rows. Often in a pattern or regular bands and sometimes with obvious wave like structure; e.g. mackerel sky with the size of the cells or cloud elements often about the size of your fist on an outstretched hand.  

 

LOW LEVEL CLOUDS (close to the earth surface)

Nimbostratus: Much thicker altostratus cloud with a lower cloud base. Associated with rain and dark with diffuse lower base.

Cumulus: Puffy, woolly detached clouds with sharp outlines. May grow vertically.

Cumulonimbus: Heavy dense cauliflower cloud with developing anvil top – the thunderstorm with heavy rain, possible hail, lightning and thunder.

Stratus: Low grey cloud with no definitive shape. Fog or lifting fog is the best example.

Stratocumulus: Grey or whitish layer or sheet cloud, sometimes in rounded rolls or lumps. Can be a very uniform continuous deck or layer.

 

THE BIG PICTURE – SATELLITE IMAGES

The most commonly used satellite images as seen on TV, newspapers and the internet come from a geostationary satellite. Local detail is poor, but excellent for the bigger picture in placing fronts, major cloud bands, etc. Better detail is available from polar orbiting satellite satellites.

Visible images (VIS) are as you would see if taking a photo from space – it obviously must be taken during daylight hours. Thermal images (IR) can be taken day or night and hence form the bulk of satellite imagery. Thermal images map out the temperature of the cloud, and typically look at temperatures between +5 and -50 C.

Usually, cold clouds are bright white and warm clouds are dull grey. As such, the colder clouds (cirrus) appear as bright white bands, even though in reality it may be semi translucent. Likewise, a dense low Cu cloud, or wide expanse of Sc cloud may be dull and difficult to see as they are warmer.

The common 'rain' radar sees precipitation falling from the cloud. It is possible to quantify the amount of rain falling and, of course, successive images show the exact track of that precipitation.

 

WIND 

The wind is never steady – the flow of air is nearly always turbulent and standard observations are taken at a height of 10 metres above the surrounding terrain and averaged for a period of 10 minutes. It is common for the wind to gust (stronger than average) and lull (weaker than average) by around 20 to 30% from the mean or average reading. There is no set amount by which the wind speed may gust above the mean; this depends on the nature of the air stream itself; the depth and stability of the flow will generally characterise the turbulence or gustiness of an air stream. 

GUST:  a short term fluctuation generally considered to last less than a minute.

SQUALL: a strong surge that suddenly rises and lasts for more than a minute, several minutes or longer.

WIND WARNINGS

There are four different wind warnings that are used in Australia by the Bureau of Meteorology (BOM).  

STRONG WIND WARNING corresponds approximately to Force 6 and Force 7 on the Beaufort Scale (a mean wind speed of 25 to 33 knots) and issued only for coastal waters.

GALE WARNING

Covers Force 8 and Force 9 on the Beaufort Scale (a mean wind speed of 34 to 47 knots) and may be issued for land, coastal waters or ocean waters. Gale force winds are common around intense low pressure systems and intense frontal systems. The waves become significantly larger and it becomes difficult to differentiate between the locally generated waves and the swell waves that propagate into the region from other areas. The strength of the wind tends to lift the tops of the broken waves away as 'spindrift'. Gale strength winds are generally the limit that most sailors experience during their sailing.

STORM WARNING

A Storm Warning is the most significant warning that can be issued away from the tropics. It covers wind strengths of Force 10, Force 11 and more if necessary (greater than 48 knots). Storm force winds are not very common over the land and found usually over the ocean. These winds rarely occur with 'straight' isobars, usually requiring an intense low pressure system where the mean wind speed reach 50 to 70 knots. It is rare for the mean wind speed to exceed this amount away from the tropics, but exceptionally intense low pressure can sometimes form with mean wind strengths of 70 to 80 knots or more. These are most likely in the mid latitudes; perhaps as a result of a tropical cyclone moving away from the tropics but still with an exceptionally tight pressure gradient.  

TROPICAL CYCLONE (HURRICANE OR TYPHOON)

This is also an open ended warning used in tropical waters – with a wind of Force 12 – 64 knots and above!  Seas are described as 'phenomenal' and the visibility is seriously affected. Hurricane strength winds are rarely experienced on land; only the near coastal land will receive the full fury if a tropical cyclone crosses the coast for the low pressure system quickly loses its intensity as it moves inland. 

LOCAL WINDS

Cumulus clouds generally suck from bases, while wet ones blow air out from their bases. As a general rule, the dry ones have less impact on the surface winds; the base of the cumulus cloud needs to be below 2000 feet to influence the surface winds, or the depth (thickness) of the cloud needs to be greater than its height above the surface. On hitting the surface, the outflow of air from a wet cumulus spreads asymmetrically, generating a surface wind that favours the cloud's leading edge.  

SEA BREEZE

Because the sea breeze is temperature driven, it is natural to conclude that a high afternoon temperature (compared with nearby sea surface temperature) in the coastal regions will result in a stronger sea breeze. This is NOT true as the sea breeze potential and development depends on a number of other factors. 

The air must warm over the land, but the direction and strength of the prevailing gradient (isobar driven) wind is actually the critical factor in determining whether the sea breeze is strong or soft. Other important factors are the shape of the coastline, the inland topography as well as the synoptic pattern and vertical velocity (rising or subsiding air) in the lower levels of the atmosphere.

Most sea breezes develop under a prevailing gradient wind flow – some of those gradient winds assist the sea breeze to grow while others hinder the development.  Occasionally, the gradient wind may be near calm (as is found under the centre of a high pressure system) and here a slightly less complicated and less dynamic sea breeze may develop.

When the gradient wind assists the sea breeze development it is said to be reinforced. The strongest sea breezes occur when the gradient wind is offshore, but closest to the final sea breeze direction – i.e., facing seawards again with your arms outstretched, the most favourable ambient wind will come from the land, but not far to the left of your left hand. These reinforced sea breezes tend to be stronger, reaching maximum wind strength in the late afternoon or evening and they persist well into the night.

A wind directly offshore (hitting the back of your head) will produce a later starting and earlier finishing sea breeze with less maximum strength. Finally an offshore wind on your right hand side generally produces an even weaker and later sea breeze, often with a calm or 'glass-out' before the onset of the breeze.

Onshore winds do not promote sea breeze conditions, though some thermal or sea breeze influence can often be seen where the onshore gradient flow may increase a little in speed during the afternoon and also rotate left (in the southern hemisphere).

The weaker sea breeze (with gradient wind coming directly opposite the known sea breeze angle) will start late, end early and never attains the windspeed seen with the reinforced sea breeze. It is often under these conditions that the weaker sea breeze may develop some 5 to 10 km offshore and slowly push towards the coast. Thus as you sail along the coast, the offshore wind maybe seen to ease (often in a very erratic manner) before a calm zone or 'glass out' to be followed by the sea breeze arriving from the sea. 

The most common 'local' offshore flow along the coast at night is cold air drainage, katabatic flow or downslope winds. The mechanism is cold sinking air, created when the earth's surface cools overnight which in turn cools the adjacent lower layers of the atmosphere. Large hills or mountains are important, but a consistent sloping terrain is best, especially if it focuses or funnels the airflow near the coast.  

SEVERE THUNDERSTORM WARNINGS

Not to be confused with the wind warning, the Bureau of Meteorology also warns of severe thunderstorms or cumulonunimbus cloud systems. Any thunderstorm can produce lightning, hail and locally squally winds, but a severe storm (by definition) can produce large hail, flash flooding and destructive squally winds... including tornadoes (over the land) or waterspouts (over the ocean). The average thunderstorm has a lifespan of 1 to maybe 2 hours, while a severe thunderstorm may live for 3 to 6 hours and travel several hundred kilometres. Severe thunderstorm warnings are always short lived and most frequently based on radar observations.   

WATER

 

Waves are generated by the wind blowing across the surface of the water. The fetch is the distance or length of ocean that the wind has traversed and the duration is the amount of time it has taken the air the travel this length. Thus the fetch and duration convey a 'runway' length and time that is available for the seaway to develop. From this comes the concept of a 'fully developed sea' – where the waves have grown to their maturity for a given wind speed.

  

SWELL

Swell waves propagate from a locally generated sea to another place. Swell waves travel at vastly different speeds; the long swell waves (with periods of 15 to 20 seconds) travel at speeds of 20 to 30 knots, while the short wavelength (short period waves <6 to 8 seconds) travel much slower at 8 or 10 knots. Swell waves can travel across entire oceans many years ago, a large long swell was sometimes the only clue that a severe weather system lay somewhere over the horizon.

(Photographs and Text courtesy of the Australian Bureau of Meteorology)

 
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