Working Out The Weather

The internet has opened up the weather business to anyone with a computer - and large computer models now deliver weather models guidance for more than a week in advance. The following websites have all the information you'll need to plan ahead, simply click the text to go to these sites:

Australian Bureau of Meteorology
Sea Breeze
Magic Seaweed
Buoy Weather

But to better understand what we're dealing with, here are a few weather basics.

WIND

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.

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.

SEVERE THUNDERSTORM WARNINGS

Not to be confused with the wind warning, the Bureau of Meteorology also warns of severe thunderstorms or cumulonunimbus cloud systems (see CLOUDS below). 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.

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 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.

(Photographs courtesy of the Australian Bureau of Meteorology)