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Page 6 Wind and Weather Although we cannot see the wind but as paddlers we are always aware of its effects on us and on the waters we paddle. A gentle breeze in the face paddling upwind can be quite refreshing. The same wind seems to disappear when we turn and paddle downwind. This occurs when we are paddling about the same speed as the wind. The effect can be quite eerie and in summer we lose the cooling effect. Wind at higher speeds can make paddling quite exiting and invigorating but as wind speeds increase paddling can become quite hazardous. A knowledge of wind and its effects is a valuable asset for paddlers comfort and safety. This article is the first of a series designed to help paddlers make sound risk management decisions concerning when and where to paddle. The science of wind and weather meteorology, is large and complex and meteorologist are still researching the causes of weather and trying to improve forecasts. Clearly then this article can only give a simplified overview of the broad principle of wind. The big picture Wind is the movement or flow of air. The large wind systems occur due to the different temperatures across the earth from the tropics to the poles. Air over the tropics heats up and rises. As it does so it cools and descends again and the cycle restarts. This simple pattern is complicated by the difference in temperature over the oceans compared with the continents. As air rises the pressure at sea level falls. As air falls pressure is increased. These air pressure is recorded and mapped by meteorologist and published as the synoptic charts seen on the TV weather or newspapers. The maps are made by joining points of equal pressure called isobars (see fig. 2). The H indicates the centres of high pressure systems (anticyclones) and L the centre of low pressure systems (cyclones or depressions). In each hemisphere working out from the low pressure belt around the tropics (Doldrums) are a alternating bands of Alan Jones AC Instructor high and low pressure systems. The belts which most influence us in and around Sydney (latitude 33 deg) cal cyclones. High winds are common in low pressure systems. In a high pressure system, air is settling slowly downwards as it spirals gradually outwards . This gentle subsidence tends to give rise to clear air and often sunny days. Sometimes however stagnant air can trap low cloud giving a murky cloudy day. This gentler air low usually means gentle winds. are the Sub-tropical High Pressure belt (approx. 24 -45 deg) and the Westerlies and Roaring Forties (approx. 40-50 deg). These belts migrate north and south following the suns seasonal pattern. In summer the high pressure belt moves south over Sydney. In winter this belt migrates north with sun allowing the westerlies low pressure belt to move up over southern Australia. All of the systems gradually move around the earth from west to east. Hence we say that our weather comes from the west. We may get Perth weather a few days after they do and New Zealand a few days after us. The wind and the earths rotation Air flows flows from high to low pressure but it does not do so directly. The rotation of the earth deflects the flow to the left in the southern hemisphere and to the right in the northern hemisphere. This is known as the Coriolis force (after its discoverer). In the southern hemisphere, the net effect is that air flowing from high pressure to low is deflected left and circulates gradually outwards anti-clockwise around the high pressure cell and clockwise around the low pressure cell. Wind speed. The higher the high and the lower the low the faster the air (wind) will travel. Also the greater the pressure gradient the faster the wind. The closer the isobars are together, the steeper the gradient, the faster the wind. Fronts Warm fronts are rare in the southern hemisphere. Cold fronts are masses of cold air from the southern polar region usually moving clockwise around a low with the whole system moving west to east. Cold fronts are marked on the weather map as a thick line with bards or spikes on its leading edge (see fig 2). In summer when the lows are usually way south the fronts are associated with the troughs of low pressure between highs. Two such fronts can be seem in fig.2, one south of perth and the other extending from Adelaide and continuing south through Melbourne. This front reached Sydney a couple of days later. Fronts, wind and rain A cold front is mass of cold air which crawls across the countryside like a wedge, forcing the warm air in front of it to rise. At Sydney’s latitude the Coriolis force is such that the wind approximately follows the isobars. Highs and lows As air spirals into low pressure cells it has to go somewhere. It rises. As it does so it cools and water vapour it is carrying condenses and forms clouds which often bring rain, hail or snow. These are sometimes referred to as rain depressions or to our north deep depressions sometimes become tropi- The above diagram shows a typical pattern. As the warn air rises condensation takes place forming large fluffy cumulus clouds which may bring rain, hail or snow. Behind the front subsiding air may trap a layer of low cloud Page 7 and coming from the north-east. A fairly typical pattern in late summer in Sydney. Since the wind has passed over the warm northern Pacific Ocean it picks up moisture. As is reaches the land which is hotter than the ocean the air rises forming a mass of coastal cloud seen over Queensland and northern NSW. Also in the two figures the cloud masses associated with the tropical lows to our north can be seen. These will form the monsoons and the big wet in the north. Wind speed and paddling Wind speed is often give in Knots i.e 1 nautical mile/hour. Knots are used because 1 minute of latitude is 1 nautical mile. One nautical mile is 1.8 kilometres. Conversion is easy, double the knots and take about 10% off e.g 10 knots is (2 x 10) - 2 = 18 kph. Source: Japan Meteorological Agency via Bureau of Meteorology, Australia Australian Region Satellite Image - Wed Feb 05 12:30pm EST - Fig 1. Paddling speed can be about halved heading into a 10 knot wind, so paddling back from somewhere in a 10 knot wind will take twice as long or be very tiring. Paddlers should also note that wind gusts can be 40% higher than forecast. The Beaufort Scale The beaufort scale uses an index for wind speed and associated term as below; 0 Calm 0kn (0kph) Sea mirrorlike Source: Bureau of Meteorology, Australia Mean Sea Level (MSL) Analysis - Wed Feb 05 2003 10am EST - Fig 2. bringing continuous rain. There is a wide variation associated with the passage of cold fronts. The temperature drop is more noticeable in summer as the difference in land and sea temperatures is greater. This is sometimes referred to as a ‘cool change’ with less likelihood of cloud and rain. In winter the change in temperature are usually quite small. Weather map and satellite images. Satellite images are useful in forecast- ing the major weather patterns. In the image for 5 February 2003 above (fig.1) the Band of cloud associated with the cold fronts (see fig.2) can be clearly seen. Note the clear air over the highs. Which way were the winds blowing in Sydney that day? The air spiralling out of the high in the tasman sea roughly follows the isobars. The isobars are fairly widely spaced and the difference in pressure between the trough and the high is not great. So the wind is light 1 Light air 1-3kn (1-5kph) Ripples like small scales without crests 2 Light breeze 4-6kn (6-11kph) Small wavelets, glassy appearance but do not break. 3 Gentle breeze 7-10kn (12-19kph) Large wavelets; crests begin to break; foam of glassy appearance; some scattered white horses. 4 Moderate breeze 11-16kn (2028kph). Small waves 1-1.5m, fairly frequent white horses 5 Fresh breeze 17-21kn (29-38kph) Moderate waves 2-2.5m; many white horses some spray. (...and so on to force 12 Hurricane) Sources: The Wonders of the Weather, Bob Crowder. Bureau of Meteorology 2000 Bureau of Meteorology www.bom.gov.au web site. Page 6 Wind and Weather 2 - Alan Jones AC Instructor Despite this ‘big picture’ system of winds, the wind sometimes seems to defy the larger pattern and seems to be blowing from a different direction. What other forces are at play? The major wing patterns are driven by the differential heating and cooling of the earth’s surface by the sun. So too are local wind systems. Fig.1 High and Low Pressure Systems Source: Wonders of the Weather,Bob Crowder. Bureau of Meteorology, Australia As mentioned in the last article the winds generally follow the isobars at our latitude. The arrows in Fig.1 show the general wind direction. Air travels (wind) clockwise round low pressure systems (L) spiralling slightly inward towards the centre. Outside the tropics these low pressure systems are called sub-tropical cyclones. The low to the east of Tasmania ,see Fig 1, is producing south-easterly winds in Sydney. Since the air has just passed over a large stretch of cool ocean the air is moist, rising in the low and rising over a warmer land mass land it probably means cloud or rain in Sydney. The isobars are relatively close showing a steepish gradient from high to low, consequently the air flow (wind) will be quite fast i.e Sydney will be experiencing strong southerly winds along the coast. Air moves anticlockwise around the the high pressure systems (H), anticyclones, spiralling outwards slightly from high to low pressure. Fig.1 shows a large high pressure cell over the Great Australian Bight. Adelaide will have light (isobars far apart) southerly winds. Air is falling in the high so it will probably be cold and dry. Perth will have light N-NW winds. This high will move west bring clearer skies as it does so. Pressure systems generally move from west to east. WE may get Perth weather three days later and New Zealand cops it a couple of days later than Sydney. Summer patterns In summer a belt of high pressure cells For example the ‘sea breeze’. On a hot summers day the land heats up much more than the sea. As a result air over the land heats up, becomes lighter and rises. Cooler air over the sea is sucked onshore to take its place. The bigger the temperature difference between the sea and the land, the stronger the wind. The sea breeze initially blows at right angles to the line of the coast. On the east coast the direction is from the east. However the earths rotation (the Coriolis effect) deflects the wind to the left i.e the wind comes from the NE. This Nor-easter is a common occurrence in Sydney summers. It strengthens as the day heats up and dies off in the evening as the sun sets. The opposite affect occurs at night. The land cools faster and the air over the sea is relatively warmer and rises sucking the air off the land. An off shore breeze. This wind is usually weaker than the sea breeze as the temperature differential is less. Systems Interact Fig.2 High pressure belt Source: Wonders of the Weather,Bob Crowder. Bureau of Meteorology, Australia are generally centred over the south of the continent with the lows well off shore to the south (see Fig 2.). The prevailing winds are generally from the north east following anticlockwise around the highs. In winter the high pressure belt moves north following the sun. This allows the lows to move north closer to and over the continent bringing colder southerly winds. Sea Breezes The strength and timing of the sea breeze depends upon the strength and direction of the prevailing wind (the big system). Take for example a high over the Pacific producing a light NE wind on a Sydney summer day. Highs tend to produce clear skies so the land will get progressively hotter. The sea breeze will also be from the NE so it will augment the prevailing Northeaster. This effect is called a ‘Black’ North-easter. The opposite is true if the prevailing wind is from the south. This will oppose the sea breeze from the NE and reduce wind strength. Winter Patterns Page 7 Myall Lakes 2003 Annual Camp To the south of Australia , the Southern Ocean extends around 3000km to the Antarctic continent. Parcels of very cold polar air move northward, warming up somewhat as they pass over the ocean. The air is still cool when it reaches Australia. These cold air masses form the cold fronts which bring the cold wet winter days. Southern Ocean cyclones (lows) are giant storms, often 2000km across, and as mentioned earlier generally circling the earth from west to east. This cyclone belt moves further north in winter bringing cold southerly winds and storms to southern Australia. 22 July A well executed reverse sweep - Cassie 2yrs A typical winter pattern is illustrated by the 4 day synoptic charts for 22 to 25th July 2000. On the 22nd July a high is centred over eastern Australia and lows in the Bight. The wind circling anticlockwise around the high and clockwise around the lows, and roughly following the isobars, will be producing westerly winds over Sydney. Since this air has just passed over the cold arid central Australia it will be cold and dry. The dry air is descending into the high so it is likely that Sydney is experiencing a cold clear day. Two cold fronts (toothed lines) can be seen in the Bight moving towards Melbourne. 23 July A cool dude kayaker - Christopher 7yrs 24 July By the 23rd the high is moving offshore to the east and the lows have converged. The cold front will be bringing cloud and rain to Adelaide and Melbourne. The wind is Sydney has shifted to the NW and the skies clear will still probably be clear. By the 24th July the lows are dominating the weather pattern bringing cloudy, cold and rain along the Victoria and NSW coast and possible storms. The wind will be fairly strong (close isobars) from the W to SW. By the 25th July the low is still hanging around south of Tasmania with southerly winds in Melbourne tending W to NW in Sydney. Since the wind has now come from the southern ocean it will be a cold wind with some moisture. As it hits the continent, which is warmer than the ocean and rises over the dividing range it may produce cloud and rain from the west. A cool dude canoeist - James 9yrs 25 July Fig.3 Typical winter synoptic charts Source: www.bom.gov.au/info/ftweather/contents.shtml Sources: The Wonders of the Weather, Bob Crowder, Bureau of Meteorology. Priced at around $40 this is 260 pages worth of excellent reading and reference from mid-teens to adult. It is written in ‘plain English’ without compromising the science. Bureau of Meteorology Web site; Myall views through the paperbarks Perfect one day, perfect the next. On again next year mid March, Violet Hill, Myall Lakes National Park.