Floating in air

Rupert Holmes explains some essential weather knowledge to help interpret forecasts on the water

- Rupert Holmes

Weather forecasting has changed enormously over the past few decades – there’s now a bewildering array of apps and websites that have the potential to give quite fine-grained data.

But at the same time, the way in which we receive and interact with weather data has also changed along with our expectations.

For example, despite the massive improvements in accuracy, forecasting still gets a bad press. Today’s predictions for six days’ time are as accurate as the 48-hour forecasts at the time of the 1979 Fastnet Race disaster and that ability continues to improve at the rate of a day per decade.

Yet interpreting what a forecast will actually mean on the water still requires some understanding of the basic elements of weather systems.

Weather is fundamentally created by the interaction between two distinct air masses with different properties: warm and wet vs dry and cold. Warm, moist air in the earth’s heat engine around the equator – the tropical regions – rises, while cold, dry air at the poles falls towards earth. The boundary between them – the polar front – can be very clear at times (see synoptic chart, above).

If the polar front passes over you, some rain will typically fall. This is because the warm, moist air is cooled on contact with the polar air turning the vapour into droplets that fall as rain.

The rotation of the earth causes these bodies of warm and cold air to spin, which in turn creates a kink in the polar front. This is the seed of a new low pressure system.