Winds [46]

By Sundra John

Wind has always been the one of the ‘most’ determining factors on whether we kayak or not.

I have always been aware of wind conditions when checking the forecast, but never really understood its origin.

As part of my Sea Instructor training, the request to contribute an article to the magazine on this topic has been a self-learning exercise. The objective with this article is to generate interest and receive feedback to create a better understanding of winds and weather. For the purpose of sea kayaking I have focused on local wind patterns.

In definition, wind is the movement of air measured in relation to the ground. Wind originates when air moves from an area of high pressure to low pressure. Weather patterns are generally dominated by areas of low pressure called depressions.

In the southern hemisphere air flows clockwise around low pressure systems and anticlockwise around high pressure systems. This rotating effect around a cell is caused by the earth’s rotation called the Coriolis effect. Therefore, if you face the wind, the low pressure will be on your left, with the high on your right. By looking at the synoptic charts below we can better understand these pressure systems.

Figure 1 - a fairly typical summer weather map

Figure 1

A fairly typical summer weather map is shown in Figure 1.

Northerly winds over eastern Australia on the western flank of a Tasman Sea high. They carry hot, dry air from inland Australia southward over Victoria and Tasmania. With winds strengthening ahead of an approaching front, this represents a classic weather situation with extreme bushfire risk.

Moist, easterly flow from the Coral Sea onto the Queensland coast causes very warm, humid and sultry weather east of the Great Dividing Range. This air, often susceptible to the development of showers and thunderstorms, is described as ‘unstable’.

The cold front passing South Australia replaces the hot, dry north westerlies with southerlies carrying cooler, often relatively humid air from waters south of the continent.

Such summer fronts are often quite shallow and may not penetrate far inland, particularly if they are distorted and slowed over the Victorian mountains.

Figure 2 - a relatively common winter weather map

Figure 2

In Figure 2 a relatively common winter weather map shows: Very cold, unstable air from well south of Tasmania flows northward over Tasmania, Victoria and southeast New South Wales, reducing normal day temperatures typically by five degrees or more. Note the cold front, the deep low pressure (pressures below 976 hectopascals) south of Tasmania and the high (1020 hectopascals) south of the Bight. Occasionally, rapid interaction with other weather systems around the southern hemisphere can almost halt the pattern’s eastward movement, causing successive cold fronts to bring a prolonged spell of cold, showery weather to southern Australia. Easterly winds dominate over inland Australia. Although southern cold fronts become shallow and diffuse as they move into northern Australia they often trigger a surge in the strength of the easterlies and this, combined with their extreme dryness, creates a very high fire danger in the tropical savannah region.

An active low pressure system near Perth is ‘cut off’ from the southern westerlies. Situations of this type may cause rain and rather cold weather over southern parts of Western Australia.

Figure 3 - wind strength of 4

Figure 3

Figure 4 - wind strength of 8

Figure 4

Wind strength is determined by the difference in pressure within a depression (low), i.e. where the isobars are spaced closely together indicates a deep low and therefore strong winds. Figure 3 indicates a wind strength of force 4 (11-16 knots) at Rockhampton on an approaching cyclone. Figure 4 indicates a wind strength of force 8 (34-40 knots), a gale although not the highest speed for the day. Wind strength is also the highest at the core of the low.

Figure 5 - wind speed to pressure relationship

Figure 5

Figure 5 indicates wind speed to pressure relationship. Notice the highest wind speed (124 km/h) is reached as the pressure is still falling (970 hPa). Note: Figure 5 is related to Figures 3 and 4.

Additional information;

  • Wind speed is highest at 500 metres above sea level.
  • Wind speed at sea level is 70% the speed at 500 metres.
  • Wind speed on land is 50% the speed at 500 metres.
  • The reduction in speed is caused by surface drag.

From the information above we can therefore assume that the wind we can feel on land would be stronger at sea.

Figure 6 - how wind affects our speed

Figure 6

Figure 6 is a useful indicator of how wind affects our speed.

By averaging a paddling speed of 3-4 knots we can see a 20 knot headwind would cut our paddling speed down to 2 knots.

Sea conditions in Table 1 are indications of sea state for wind influence only. Conditions change (generally worsen) when combined with other factors such as tides, topography (under & above water), current and swell.

A barometer is an invaluable device for monitoring pressure changes, and has been used by the experienced to provide accurate short term forecasts.

Micro weather patterns generated by seasonal change, land/water temperature difference and topography create the land/sea breezes we commonly hear of in reports.

From studying the table on the previous page most of us should be able to judge the wind speed we can cope with. Knowing the wind prediction will help us determine:

  • Whether we paddle or not
  • What distance to target
  • Which direction to paddle

In conclusion, I would encourage any further contributions towards this topic to make it as resourceful as possible.

Material used in this article has been sourced from the following:

Table 1: the Beaufort Scale (from the Mobile Aeronautics Education Laboratory Weather Workstation)