Being Weatherwise for Sailing (Part 1)

Weather and sailing. Where do you start? The internet now gives everyone access to huge amounts of weather information, but for an enjoyable and safe weekend potter on the harbour you need a few weather basics. In this series of features we will discuss how to read the large scale weather, how to understand the many forecasts available, where to get the forecasts and then how to transfer all this information onto your local sailing waters.

This week we begin by looking at some basics all sailors need to know:

The Highs and Lows of UK Weather
The UK's mid latitude position between 50 - 60°N leaves it between the warm south and the cold north. The atmosphere is driven by a need to reduce this temperature gradient by sending warm air north and cold air south using cyclones and anticyclones, or the highs and lows which make up our 'synoptic-scale' weather, and which we see on the pressure charts.

When low pressure systems form in the western Atlantic, they pick up warm moist air which is then mixed with cooler air from the north as they move east across the top of the Azores high. The boundaries between these air masses are marked by fronts.

A typical anticlockwise rotating low centre will reach western Ireland and then curve north up the western Scottish coast. The warm front moves across the UK first, with warm moist air behind it bringing spells of rain and often reduced visibility with south to south westerly winds. The cold front chasing along behind is the boundary between this warm moist air, and the cooler more unstable brighter and more showery weather behind, with winds typically from the south west to north west.

The transition of these fronts is something the sailor has to take note of as the passage of these fronts over head will bring strong winds and occasionally heavy rain which can quickly turn a pleasant day's sail into a dash for shore.

High pressure usually follows these low pressure systems. These anticyclones are clockwise rotating sinking air masses which we associate in the summer season with light winds and sunny skies, and often building afternoon onshore or sea breezes.

The pressure imbalance created between the high and low pressure areas is what drives the wind as it tries to reduce this pressure gradient. Wind which we see on the water is, in the northern hemisphere, more backed (further left looking upwind) than this 500m pressure gradient wind due to the effect of friction or drag on the wind by the surface. The rougher the surface the more drag on the wind, i.e. winds are more backed over the land (20 - 40°) than they are over the smoother sea (10 - 20°). This is something to look for on the water when winds are blowing off the shore, as they will veer (clockwise, or shift right looking upwind) downwind of the shore.

Heat from the sun has a noticeable effect of the surface winds. How many mornings have you looked out the window to see calm conditions but by lunchtime the winds have picked up nicely? During the night the earth cools and so in turn it cools the air directly above it, which reduces its 'energy' or mixing and so reduces the wind speed. As the sun heats the earth, the air next to the surface begins to warm and so begins to rise and mix as cooler air sinks to the surface to replace it, which results in an increased wind speed.

Friction and heating are just two examples of the many factors which we must take into account when on the water. The forecasts we receive are often for a much wider area and there are small scale processes which affect the local winds. We will look at the forecasts themselves in part 2 and their strengths and limitations before we move on to have a closer look as to how the sailor can adapt them to their local shores.