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Guide to understanding weather maps

Adventures
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The weather can make or break a trip. Learn how to read weather maps and cloud patterns so you can prepare accordingly.

Sure, most people usually don’t head for the ski fields without listening to a snow report, and don’t go fishing in a tinny without looking up wind and wave forecasts, but many travellers head off on a caravanning holiday without the faintest idea of likely weather.

The consequences of being in the right place at the wrong time can cause anything from a caravan sale to a family break-up. Even the cushiest of caravan or motorhome is somewhat claustrophobic after four days of constant rain, and sightseeing can be a damp and pointless pursuit.

SEASONS

Broadly speaking, the time of year dictates where you should go, based on the likely weather conditions. As a general rule, you head north of the Tropic of Capricorn and to the Red Centre during the cooler months and turn south of the Tropic for coastal and mountain destinations during the warmer months.

The most restricting annual weather pattern for Australian travellers is the wet season that hits the northern tropical areas during the southern summer. Unless you plan to stay in the far north for the Wet – and yes, it can be an interesting time to be there so long as you’re prepared for lengthy travel restrictions due to flooding and cyclones – don’t go there at that time of the year.

The inland desert regions are no-go zones during the summer: temperatures can reach the 50s and even a minor vehicle problem can become life-threatening. As of 2009, the Simpson Desert tracks are closed during the hot months.

Conversely, the Alpine regions that are snow-covered during winter are open for touring during summer months.

Within that broad travel framework there are changing weather patterns to be aware of. In these days of laptops and wireless broadband it’s possible to consult long-range and short-range weather forecasts for most of the regions you’re visiting.

WARNINGS

bushfire-season

The most critical item on any weather site is the “Warnings” information. Warnings are issued to cover weather extremes and are typically strong wind, frost, damaging hail, snow, ice and sleet warnings. There are also warnings of extreme heat conditions that can provoke dangerous bushfires.

All these weather conditions should be noted and acted on by travellers.

A strong wind warning covering the coastal area you intend driving through should make you reconsider your plans, because strong winds have adverse effects on van and motorhome stability. Relatively light, box-shaped structures are easily destabilised by strong wind gusts, especially on exposed sections of highway.

Normally, strong wind systems blow themselves out in a day or two, so if you can postpone our drive you’ll have a safer journey. Hail, snow, sleet and ice warnings should also make you reconsider a planned drive until conditions improve.

TEMPERATURE

Forecast temperature should be a guide to daily activity during a bush trip. If it’s going to be stinking hot, don’t plan a bushwalk during the middle of the day: break up your activities so that you’re active when it’s coolest.

The temperature forecast should indicate what clothing and supplies you’ll need for the day’s planned activities. In cold weather, combine the temperature forecast with the predicted wind chill factor when you’re planning what to wear.

As is made all too obvious every bushfire season, the day’s weather forecast can be a good indication of bushfire risk, but many people ignore the warning. If you’re camped in a fire-prone area and forecast favours wildfires, think about moving to safer position – certainly if a total fire ban is declared.

ISOBARS

isobar-map

The most common weather pattern charts are black-and-white drawings of Australia, with curved lines that look like map contours; shorter, heavier, curved lines with little shapes on the front of them; and the letters L and H. These drawings are known as synoptic charts.

The curved lines are called isobars and encase either the letter L, indicating a low barometric pressure zone, or H, indicating a high pressure zone. On some weather maps the isobars have barometric pressure numbers on them.

In the southern hemisphere the wind flows around a low pressure zone in a clockwise direction and around a high pressure zone in an anti-clockwise direction. So, when you see one of these zones on a synoptic chart you can visualise the wind following the isobars around the centre.

The spacing of the isobars gives a good indication of wind strength: the tighter the spacing, the greater the wind strength. Wind strength is also indicated by a “wind barb”, which is a dot with a tail. The direction of the tail is the wind direction and the number of barbs on the tail indicates wind strength.

PRESSURE

Weather forecasters talk of “low” and “high” barometric pressure zones. Atmospheric pressure is the weight of the air above us, and although we’re not conscious of it, variations in atmospheric pressure contribute to weather patterns.

Atmospheric pressure is measured on a barometer and its rate of change indicates the speed of a change in the weather.

Normal barometric pressure at sea level is 101,325 Pascals, or 101.3kPa, or 760mm (29.92in) of mercury, or 1 Bar, or 1013.25 millibars, depending on which measuring units you prefer.