Watch the weather

Doug King | VOLUME 31, ISSUE 3
This ‘cloud tsunami’ – a wedge of cloud which forms at the leading edge of a thunderstorm – rolled across Sydney in November 2015.
Fair weather or foul, it’s vital to always check the weather forecast and assess what conditions might await you on the water.

All boating-safety agencies and organisations emphasise the importance of consulting a weather forecast before heading out on the water. But why is it so important?

Three weather-related factors are important to boaters: wind, temperature and rain. Wind will create waves, temperature relates to your comfort on the water, and rain or precipitation can affect visibility. Understanding how weather affects your waterway will enhance your comfort, safety and confidence.

HOW WEATHER IS GENERATED

The earth’s atmosphere is heated by the sun during the day and cools at night. As air heats it becomes less dense and rises and is replaced by colder, denser air. It’s a continual heating and cooling process, with air movement also influenced by latitude and the earth’s rotation. Unequal heating of air causes areas of high and low pressure relative to each other. These are called ‘highs’ and ‘lows’.

Because air is always trying to equalise its pressure, there is continual movement of air from high to low pressure centres. In the southern hemisphere, air circulates clockwise around low pressure systems and anticlockwise around high pressure systems – in the northern hemisphere, they go the other way around. The lines on a weather map join areas of equal pressure and, while circulating air does not exactly follow the lines, looking at a weather map can give you an idea of the wind direction. Also, the closer together the lines, the stronger the wind will be.

It also helps to understand where the air mass coming toward you has come from. Our weather comes from the west and moves to the east. In southern states, westerly and southwesterly winds have been dragged over the ocean and can be cold and moist. This can create strong winds because of the temperature and pressure differences to the air over the land. In northern areas, air coming from over the sea is moist and may be warmer, while air coming from over the land will be dry.

Locally, the weather is greatly influenced by land masses and water areas that cause the air to heat and cool at different rates. This can create localised patterns that can be relied upon to predict local weather fairly accurately.

WIND AND WAVES

Waves are a source of discomfort and danger to all vessels. Waves can store or dissipate large amounts of energy – a 3m-high wave has nine times more energy than a 1m-high wave, for example.

Wind is key to wave formation. The most common types of waves are caused by wind and – when measured on a single body of water – the stronger the wind, the larger the waves. The term ‘fetch’ describes the length of water over which wind has blown. How far (distance) and for how long (time) the wind has blown over water are important factors of wave generation – generally, the greater the fetch, the larger the waves.

The wind direction always describes where it’s coming from – for example, a westerly wind blows from the west toward the east.

When reading the weather forecast, it’s important to understand how the wind direction and speed will build up waves in your location. In areas sheltered from wind or close to the upwind shoreline, there may be little or no effect, while downwind the seas may be quite rough.

The local topography also affects the wind and waves. Wind can speed up if it is squeezed between valleys, while waves can be short and steep in confined waterways such as bays, inland lakes or areas with many small islands. And when waves hit the shoreline they bounce back, which steepens and confuses seas.

Boaters should be aware of onshore winds. Whether it is a southwesterly from the Southern Ocean or a southeast trade wind from the Coral Sea, onshore winds need to be carefully assessed due to the long distance the wind has had to affect the water.

Check a weather forecast for information about predicted winds and a marine weather forecast for predicted wave heights. Combined with local knowledge, these forecasts can give boaters a good idea of what to expect on the water.

CLOUDS

Clouds can also help with weather interpretation as they’re linked with weather systems, so take note if clouds are increasing or decreasing.

Generally, thickening or lowering clouds indicate rain and deteriorating weather. When clouds show signs of evaporation (when they’re breaking up or holes appear in them) improving weather is generally expected.

The sequence of cloud formation is another sign of changing weather. High-level feathery clouds are often a precursor to changes in weather. If these clouds thicken and join up while becoming lower, then a change is on the way.

WEATHER FRONTS

Fronts are boundaries between high and low pressure systems. Where there is a significant difference between the air masses, such as in temperature or moisture, fronts can bring extremely strong winds and squalls and can cause rough seas.

Because of the different directions in air rotation between highs and lows, the wind direction shifts suddenly and can cause confused, steep seas and difficulties for sailing vessels and small craft. Across Australia and New Zealand, these are called cold fronts, southerly busters and cut-off lows.

A barometer can help show the approach of a cold front – the air pressure will fall with the approach of a cold front and rise as weather fines up. If the pressure drops rapidly, the change will be stronger than if it falls gradually.

CYCLONES

Between November and April, tropical cyclones may be encountered in northern Australia. These are intense low-pressure systems that are started when warm moist air and cold air collide. Fully developed cyclones have a centre of low pressure and extremely high winds that create rough seas. We have excellent cyclone warning systems and keeping up to date with weather forecasts is vital for those boating in the north.

If you are on the water when a cyclone warning is issued, make for a safe port. Most ports in cyclone-prone areas have a cyclone contingency plan, so if you operate in these areas check with the port authority.

SEA BREEZES

Air warming up over land during the day rises and by afternoon, cooler air over the water moves to take its place, resulting in sea or onshore breezes. These can be quite strong and can quickly develop waves on a previously calm sea. Western Australia’s Freemantle Doctor is a classic example of a sea breeze. In tropical areas the reverse can happen at night, causing offshore or land breezes.

FOG

There are two types of fog. Advection fog occurs when warm moist air is drawn across water, while radiation fog occurs when the moisture dew-point corresponds with the air temperature. Advection fog can occur any time, while radiation fog usually occurs over water in the southern half of Australia during autumn and winter. In both cases, moisture becomes visible as fog at sea level and can create navigational problems for mariners.

THUNDERSTORMS

Thunderstorms can cause problems for mariners because of the unpredictable high winds and possible lines of squalls. Look out for cauliflower-type clouds (cumulus or cumulonimbus clouds) building up to a high altitude that will often have an anvil-shaped top. When thunderstorms are forecast or you spot theses clouds, head for shelter.

WEATHER FORECASTS

For up to date weather forecasts, go to the Bureau of Meteorology webpage (bom.gov.au). In the ‘Our Services’ section, click on the ‘Marine and Ocean’ button and then select the state and waterway applicable to you.

BOM also offers an interactive forecast tool called MetEye. It is very good and worth exploring.

Club Marine’s new app provides access to weather information, weather alerts and real-time weather conditions. Other weather apps are good, but they can be very localised.

When on the water, listen for updates and weather warnings on the radio. These are normally issued by local Volunteer Marine Rescue groups (VMR).

Forecasts in newspapers and general TV weather bulletins may not be as up to date or detailed as you would like.

Marine weather forecasts have a fixed format: wind strength, wave conditions and general weather. Wind speeds are given in knots (one knot equals 1.8km/h) and forecast winds are an average. The Bureau of Meteorology issues this notice with its forecasts: “Wind gusts can be 40 per cent stronger than the averages given here and maximum waves may be up to twice the height.”

Due to many local variations, winds increase and decrease compared to the forecast. Waves will also vary because of the underwater topography, the shape of a coastline they might encounter and the time between successive crests.

Weather warnings are provided when the forecast is for strong winds (26 to 33 knots), gale-force winds (34 to 47 knots), storm-force winds (48 to 63 knots) and hurricane-force winds (over 64 knots).

The general component of the forecast will also contain thunderstorm and squall warnings, if relevant.

If you are unsure how the forecast weather will affect the waters you want to go boating on, seek some local knowledge. The local Coastguard or VMR are most helpful with this.

Marine weather basics

• Always get a current forecast before you go boating

• Get updates while on the water

• Know how weather affects your local waterways

• Learn how to read the clouds for signs of changing weather

• If in doubt, don’t go out

As with most things in boating, knowledge and experience build up an understanding. By observing the actual weather and relating it to the forecast, you’ll soon confidently assess how conditions may be. This makes it easier to decide whether to go boating and if so, where to go.

Clouds that indicate clearing or continuing weather

• Cloud bases rising

• Rain stopping and clouds breaking up at sunset

• Altitude of cloud bases over mountains rising

• Clouds dotting afternoon summer sky Clouds that indicate deteriorating weather and wind

• High cirrus or ‘mare’s tail’ clouds

• Clouds lowering and thickening

• Rapid vertically developing cumulus clouds growing very tall into the atmosphere

• Dark, threatening sky in the direction from where changes develop

• Clouds moving in different directions at different heights

• Dark, low clouds with a greenish colour – this indicates very cold air with possible microbursts


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