These signs indicate a future shift in wind direction or speed:

• A change in the sky - more clouds, fewer clouds, or different kinds of clouds.
• Sailboats on the horizon heeling in a new wind.
• Flags or smokestacks on shore indicating a new wind direction.
• A rapid change in temperature.
• A change in the visibility on the horizon. For example, in the Whitsundays a clearing haze means that the wind will come from that direction.
• A change in the water surface. Darker water means waves and more wind. Refer to the Beaufort Scale
• A changing barometer reading - falling at 2-3 hPa hourly would indicate deteriorating conditions.

These are all signs of stormy weather approaching

• Dark, cumulonimbus clouds approaching on the horizon.
• Thunder and/or lightening.
• A falling barometer reading.
• A change in wind speed - especially when the wind dies off or blows gently toward the big, dark clouds.

The strength of the wind determines whether or not to put to sea. For small craft, a wind velocity of more than 15km per hour will chop up shallow water to make for uncomfortable boating. When over 20knots, it is time to seek calmer water. Larger craft, or course, and particularly deep keelers, can handle 15-20knot winds comfortably. Above 20-25knots per hour, bareboats are advised not to be out.

Now let us consider other important phenomena mentioned in forecasts - clouds. There are very few days when we don't see clouds. They are the signposts of the sky and can, if understood, be a very useful guide to prevailing and future weather conditions.

Before looking at clouds, however, let's just discuss fronts. They are the boundaries between huge masses of air of different characteristics. These fonts travel from West to East South of Australia and at the boundaries you can get cold air undercutting warmer air, or warmer air riding up over colder air. They are known as cold or warm fronts. You can imagine the enormous volumes of air being moved at these fronts, particularly the cold fronts. If the air is pushed upwards it will be cooled and clouds may form. The approach of fronts is often clearly visible by means of clouds. You can't avoid fronts as they cover hundreds of kilometres in length, but you can prepare for their passage. The most frequent front we experience in Australia is the cold front.

Clouds are formed by the condensation of water vapour present in the atmosphere, if cooled to a temperature where a change of state from a gas to a liquid takes place, There are two basic forms of cloud, cumuliform and stratiform.

• Cumuliform clouds are caused by air rising by being heated from below and look like large balls of cotton wool. Cumulonimbus clouds, for example, can tower to enormous altitudes and bring extremely heavy showery precipitation (nimbus means a rain cloud).
  
  
• Stratiform clouds are formed by advection, that is to say, by the cooling of horizontally flowing air. There are considered to be ten basic variations of these two types, divided into three layers or heights of the bases of the clouds above the surface, as shown in the diagram and table below.

Most common clouds in Australia.

Clouds come in all sizes and shapes, but they can all be categorised as:

• Cirrus: The highest clouds are all pure white and are wispy and thin. They signify fair weather - for the next day, at least.
  
  
• Cumulus: Puffy clouds like cotton balls. The associated weather depends upon the clouds' colour and size. Cumulus clouds mean fair weather when their bases are high in the sky or when they're relatively thin and bright white at lower altitudes. The taller (thicker) and darker ones with low-altitude bases like a mountainous looking cauliflower, are the Cumulonimbus variety, which foretell ominous weather, including sudden thunderstorms with heavy rain and lightening.
  
  
• Stratus: Low layered clouds (if on the ground it's called fog), very even-looking from underneath. Stratus comes from the Latin word for "spread, stretch out, or cover," which is what these clouds do. Their associated weather depends on their colour, thickness, and altitude - the lower, thicker, and darker, the more they're associated with rain, wind, and (sometimes) low visibility.
  
  
• Nimbus: Or rain-bearing for the purists, is similar to a giant vacuum cleaner sucking sea water directly into the cloud body. Sometimes, not always, this same cloud is the first in the area to produce rain as it reaches it's saturation point much quicker than it's mates. If this same rain doesn't reach the sea when it falls, but kind of tapers off, it is called 'Virga'... Great stuff to know!

Thunderstorms or squalls, more common in our summer months, are sudden, sometimes severe storms that are usually localised in size (covering about 10km). Although they may precede the passage of a cold front, they can also be caused by a landmass heating up (with hot air rising into cooler air aloft) during a humid summer day. Winds can build to over 60 knots quickly and come from any direction, and lightning is common. Squalls are relatively unpredictable but dark cumulonimbus clouds and distant thunder are signs that one may be near.

Because sound travels about one-fifth of a mile per second, you can tell how far away the thunder and lightning are by counting the seconds between the lightning and the thunder and dividing by five...five seconds means the lightning is about one mile away.

If you're metrically inclined, count the seconds and divide by three to get the distance in kilometres... three seconds means the lightning is one kilometre away.

The progress of a mature thunderstorm.

The barometer
This instrument, measures the weight of air above a location, and is often part of the instrumentation on a bareboat.

It should be remembered, however, that a barometer tells you only what the weight of air above you is at the time of reading. The corresponding weather printed on the dial (Fine, Fair, Change, Rain) cannot always be relied upon except at the extreme ends of the scale.

However, when it is noticed that the barometer reading is falling rapidly eg 2-3hPa in an hour, then this will alert you to seek the latest forecast as strong winds could be on the way - either from an approaching thunderstorm, freshening north-easterly winds ahead of the southerly change or, at worst, the development of a small low.

How air pressure changes daily.

Just as there are two high and low tides of water every day, there are also two high "tides" and two low "tides" of air each day. However, whereas the times of high and low water tides change from day to day, the "tides" of air do not.

Every day about 10 am and 10 pm a high air tide (highest barometer reading) will occur and the lowest will occur at 4 am and 4 pm (it wall normally be falling by about 1.5hPa per hour).

However, if a disturbance is on the way (such as a thunderstorm, southerly change) then there will be a much larger fall in pressure than the normal 1.5 hPa, so a fall of 2-3hPa hourly would be an indication that the weather could be deteriorating. If in doubt, check the latest forecast.

Temperature differences can also cause changes to the weather on a local scale. Sea breeze is the name associated with a family of winds generated on sunny and partly sunny days, when the extra heat of the land causes a cool breeze to blow inland from the water. In the Whitsundays, the sea breeze is so predictable during the summer that you can almost set your watch by it.

Breezes blowing from the land onto the water (land breeze) are strongest (up to 10-15 knots) early on a winter morning while a sea breeze on a summer afternoon averages 15 knots.

How sea breezes work

As the land heats, the air rises (sometimes creating puffy, cumulus clouds), and by midday an area of low pressure is created along the shoreline.

Meanwhile, because the water remains cooler, as does the air above the water. Cool air from the water blows (or, really, is sucked) into the low pressure area over the land.

Sea breeze wind blows from sea to land during the day.

A land breeze
As the sun drops on the horizon, the heating of the land diminishes, and so does the sea breeze. In Queensland waters watch for the sea breeze to build until 3 o'clock or so and then slowly die away.

Land breeze wind blows from land to sea at night.

Use your understanding of weather and the information provided by forecasts to make your cruising holiday safer, faster, and more comfortable. But first, we want to establish some general premises about your sailing:

• More wind = more speed: Wind is like boat speed - more is better, but only to a point. Each boat is different, but when sailing on a beam reach or closer angle to the wind, most boats benefit from increased velocity up to about 15 knots. By then, of course, beginning sailors should be at the dock or at anchor. Going downwind, most boats respond favourably as the wind picks up to 25 knots, but over that, control becomes a concern.
  
  
• Sailing on a reach is fastest: If you had a particular destination in mind and could make the wind blow from any direction, you would make you course a reach. That way, you could sail on your boat?s fastest point of sail and steer a direct course.
  
  
• Sailing straight downwind is often slower: Many boats actually sail directly downwind (on a run) more slowly than if they head up and sail on a reach and then jibe over.
  
  
• Sailing upwind is slower: Because you have to zigzag your way back and forth to reach a destination directly upwind, you end up sailing extra distance.
  
  
• Waves matter: Waves slow you down when sailing toward them and can speed you up when sailing with them.

To make your sail safer and more comfortable, use your wind knowledge to avoid going out when it's too windy or stormy.