We all know that bad weather can spoil a holiday or an outing, but it is, generaly, no more than a nuisance. Boating, however, presents a quite different picture. You should not even contemplate going out for a trip in your boat without having found out the likely weather conditions. It doesn't matter that you won't be far from shore, suddenly and seriously changed weather conditions can cause disaster on the water. This means that you need to have some knowledge of weather and what causes it.

Barometers and air pressure

Weather is caused by air travelling around areas of high or low atmospheric pressure. Atmospheric pressure is commonly called barometric pressure because we usually measure it on a barometer.

Atmospheric pressure is constanly changing. Change, therefore, should beconstanly noted. The unit of measurement is now called the hectopascal although the old, familiar term milibar slips out occasionally, even from meteorologists. If there is a change downwards of more than about 7 hectopascals in the 24 hours since the previous reading then it is likely that, as a low pressure area is approaching, poor and certainly changed weather conditions can be expected. As a rule of thumb poor weather is generally associated with low pressure systems but this is not always true. Reference to weather forecasts, therefore, become essential.

Weather reports and forecasts

Wind, or more correctly, wind strength, is the weather condition likely to be of most concern to small craft. Wind strength can radically alter the length and steepness of waves, particularly if the wind and tide are coming from opposite directions. Wind strength and direction can be forecast quite accurately over the open areas of the sea. It is, however, much more difficult close to land or in enclosed boating areas. There local effects can seriously change the wind direction and strength which otherwise could resonably have been expected by studying the direction and spacing of the isobars on the weather map.

There are quite a number of apparently everyday words used in forecasting which have very specific meaning when used by meteorologists. It may save you a problem, an inconvenience more than a hazard, if you understand the meteorological meaning of these words:

• Fine - simply means free from absence of precipitation.
• Gust - a gust is a sudden increase of wind strength, generally of only a few seconds duration.
• Squall - a squall is a strong wind which arrives suddenly, lasts usually for some minutes, and then dies down as suddenly as it arose. Its precise definition may help to illustrate the danger of a squall to a small craft: a sudden increase in wind strength of at least 16 knots to not less than a speed of 22 knots. These are minima. Picture, therefore, what a minimum increase of these proportions on top of prevailing wind strength might do to the safety and comfort of you, your family and friends. If a squall are squalls are forecast then be sensible and seek shelter.
• Veering - veering is when the direction of the wind changes in a clockwise direction.
• Backing - backing is when the direction of change is anticlockwise.

Sometimes wind strength is described by particular phrases an/or wind force numbers. The interrelationship between these particular phrases, numbers and corresponding range of speed is known as the Beaufort Scale, after its inventor Admiral Beaufort.

Waves

Waves are caused by waves. There are, basically, two sorts of waves: those caused by local wind effect and those caused by wind effect a long distance away. They are known as sea waves and swell waves respectively.

The height of a wave depends not only on the wind strength but the distance it has been blowing over open water, referred to as fetch, and the time it has been blowing.

When the direction of the wind is opposite to the direction in which the tide is flowing, the result is noticeably steeper waves than the prevailing wind strength might indicate.

When you are close inshore or near harbour walls or headlands, waves may change direction, or be refracted. They can also bounce back off the land surface and cause a nasty confused sea, sometimes called a backsurge.

Clouds

Clouds are formed by the condensation of water vapour present in the atmosphere, if cooled to a temperture where a change of state from a gas to a liqued 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 can tower to enourmous altitudes and bring extremely heavy shower precipitation (nimbus means a rain cloud)
Stratiform clouds are formed by advection, that is to say, by the cooling of the 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.

Visibility

Poor horizontal visibility at sea level is generally the result of rain, fog or occasionally hail. Radiation fog forms over land on clear cold nights. Sea or advection fog occurs when a body of warm moist air moves over a colder sea surface and is cooled to a tempearture, known as dew point, when condensation takes place. Fog in other words is low cloud.

Tides

As you know, nautical charts and maps show the depth of the water at various places. The depths given on the chart are the minimum depths that can be expected at any time at those places. In practice there will nearly always be a greater depth of water than is shown on the chart. This is because of tides.

Tides are the regular rise and fall of the level of the sea surface around our coasts and in our harbours. When the level of the water reaches a maximum height high water occurs. When the level falls to a minimum height low water occurs. Most places experience two high waters and two low waters in each 24 hour period.

The time interval between one high water and the next low water, or vice versa, is called the duration of the tide. The average duration of a tide is approximately six hours. That is, the tide rises for approximately six hours and then falls for approximately six hours. The difference beteen the height of the tide at high water and the height at low water is called the range of the tide.

It is obviously important for you to be able to find out the heights and the times of high and low water, especially if you are boating in shallow waters or if you are intending to cross a bar.

Be aware of the importance of having a basic knowledge of the weather. Specifically, know what to expect for your neck of the woods, by reading forecasts, listening to broadcasts and observing the sky and wind directions. Above all don't take chances.