Weather tips by Mike Bailey

In some ways, the words here this week are something of a follow up to those of last week in relation to snow and rain – almost like the weather itself: a continuing story.

Last week, this site warned of the need to carefully evaluate information regarding the likely spread of rainfall, which sometimes can be spectacularly promising for farmers or conversely rather disappointing for those who may be planning a visit to our Crown caravan and camping sites at times like this winter school holiday break.

Maps showing projected rainfall areas indicate the total space over which rain is likely to fall and give only general indications of likely volume, but they usually don’t allow for parts within those broader areas where it may remain dry.

If the promised rain is as heavy and widespread as the maps may suggest, farmers will be happy but not the holidaymakers; and if the rain fails to hit specific areas the farmers will be disappointed while those on vacation will be smiling.

It needs to be appreciated that those maps are not detailed enough to display precisely the amount likely to fall over any individual part of a much larger given area.

Indeed, forecasting as a whole still has some way to go before it will be able to be so precise – but it is getting much better than it was even a couple of decades ago.

These points are relevant in considering the likely affect of rain on any holiday visit to the many excellent Crown sites through New South Wales, and they should carry even more weight at this time when skiers are anxiously studying weather movements to try to determine the best time to head to the slopes.

Snow volumes and spread can be even more difficult to predict than rain, with some systems that bring the widest spread of falls being those that fail to bring the heaviest drops to locations where skiers need them most.

Naturally, temperatures play a major part in affecting snowfalls.

Air temperatures at ground level need to be less than 7 degrees Celsius for snow to settle, and obviously lower again if a large volume of snow is to remain for a lengthy period.

Many local factors have a bearing on what the thermometer will show at a particular location, and that is why some weather systems will bring perhaps moderate snowfalls to the Snowy Mountains, very little around the Central Tablelands, but still occasionally manage to produce some notable drops much further north on the Tablelands.

Just as temperatures at ground level are not always uniform across a particular area, they also don’t have such uniformity at much higher levels of the atmosphere – and it is there that what falls to the ground as snow really has its origin.

Consideration of these points should demonstrate why forecasters examine much more than just the surface level maps that appear daily in newspapers or on television screens when they set out to make their predictions.

They also look at charts that advise what is happening in upper levels of the atmosphere, to check temperatures and other influences there.

For example, steerage wind flow can be different at higher levels of the atmosphere, which explains why you may be feeling a north-easterly seabreeze at a point on the coast but at the same time you see that clouds streaming across the sky from the north-west.

This means winds at those higher levels are blowing from the north-west, while those closer to the ground are from the north-east.

That last paragraph also explains the way we name winds; something that should be obvious to most readers, but friends of mine who have marked major high school examination papers say that students are often confused about the issue.

Put simply, winds are named after the direction from which they blow and not after the direction where they are heading: hence a southerly is a wind out of the south; a northerly is a wind out of the north, etc.

To gain insight about the likely wind strength, check both the forecast and the weather map.

If the map has many isobars tightly ringing a pressure system, the winds will be much stronger than if there are just a few well spaced lines around that particular system.

And, as has been pointed out on this site in the past, winds around high pressure systems in our hemisphere blow in an anti-clockwise direction; while those around lows blow in a clockwise direction.

Weathering July

Winds can be strong at any time of the year depending on the alignment of pressure systems, but they are especially strong in what is often referred to as a textbook late winter weather pattern.

Again, such a pattern is not necessarily confined to that period, but it is more likely in the early months of the second half of the calendar year.

Skiers welcome such a pattern because it usually sees a strong low pressure system located around Bass Strait bringing cold and gusty air from regions further south and producing snow over the Snowy Mountains of New South Wales, and the alpine regions of both Victoria and Tasmania.

Watch for such patterns if you are thinking of visiting the snow country over the next few weeks, and through the remainder of the skiing season

These patterns will generally deliver moisture to the western side of the ranges, while the coast remains dry: a reflection of the differences between the windward and the leeward sides of a mountain range.

The winds become gusty as something of a fight develops between pressure systems as cooler air whipped up by the lows down south seeks to break though the barriers created by a strong high further to the north.

There is further enhancement of wind strength as the next high begins to move or ridge through across the top of the low, maintaining the flow of cold south-westerly air with the prospect of further snowfalls.

And, all the while, those visiting coastal Crown caravan and camping sites can generally enjoy clear skies with the Great Dividing Range providing some easement for the winds – although the movement of the air does generate something of a chill factor.

To avoid the worst affects of that aspect of this pattern, it is advisable to find beaches where landforms provide further natural shelters from buffeting winds because the chill factor can be especially marked once a bather takes to the water – even if the temperature of that water is pleasant, and possibly even higher than the air above it.

The opening days of this month provided a demonstration of exactly the conditions being discussed here, and brought a valuable top up of snow over the Snowy Mountains.

That chill was in sharp contrast to some extremely mild temperatures in June, which for Sydney was the warmest sixth month of the year since 1991.

Combined minimum and maximum temperature averages for the month came in at 15 degrees, which was 2 above the long term norm.

Maximum readings were also 2 degrees above at 19, with the warmest day on the 12th producing a high of 22 degrees – the highest for the month in 3 years.

A minimum of 15 degrees on the 4th was the highest overnight figure in June for 6 years.

Hobart also had its warmest June since 1991, and Melbourne its warmest since 1957.

Away from the capital cities, there were some isolated heavy rainfalls – with the Hunter Region scoring highest.

Nelson Bay recorded 307 millimetres, which was more than double the June average of 151; and Cessnock scored 119 millimetres – almost 4 times its average of just 31.

Climatologists are not expecting widespread heavy falls through July, but big drops are always possible in isolated locations.

Predictions are also for at least some moderate falls developing from the west through the first full week of July, and there will be more of the cold fronts that brought very gusty winds and snowfalls at the beginning of the month.

Also, as explained on this site last week, July is usually our coldest month.

This makes the carriage of and access to appropriate winter clothing a necessity for all school holiday visits to the many great Crown camping and caravan sites across New South Wales.

To further add to preparations for getting the most out of such visits, keep in touch with the latest weather information where possible through the Bureau of Meteorology’s website at www.bom.gov.au or by telephone, and monitor up to date weather information as it is presented through various media outlets.

As recent developments have shown, changes can be significant and that battle in the atmosphere referred to earlier can produce damaging winds capable of impacting on outdoor activities on both land and water.