I have a lovely old instrument at home which belonged to my grandfather; using dials on concentric circles you can set the direction of the wind, barometer reading and trend, along with the season. This then gives a basic weather forecast by identifying the air mass and what is likely to happen next, similar to the forecasts found on home weather stations.

So what is an air mass? If you go vertically up through the atmosphere you will get some big changes in temperature, humidity and wind over a short distance. However, if you move horizontally through the air you will find that the air’s characteristics will stay very similar often for hundreds of miles. The changes will usually be at a frontal boundary. The lower levels of the air will take on some of the characteristics of the surface that it is passing over.

As the air is heated, or cooled from below, the temperature of the air becomes close to that of the surface it is crossing. If I take an example of the air in a south-westerly airstream heading for the British Isles, this air originated somewhere near the Azores; it is warm and there will be plenty of moisture held in it.

Tropical maritime

As the air moves north-east it is travelling over an increasingly cool sea, therefore the air cools and, as it does so, water condenses to form low cloud and possibly fog, particularly in springtime when the water is cold. This airflow is called ‘tropical maritime’ from its source region and will vary little in strength or direction.

For each wind direction we can determine where the source region is and therefore its likely characteristics; the relative temperatures of air and sea will tell us if we can expect low cloud and fog, or towering cumulus and showers.

The tropical maritime air (Tm) from the south-west is quite different from the air coming from the north-west which is polar maritime (Pm). This air was originally cold and dry, but picks up some moisture over an increasingly warm sea as it tracks south-east, producing large cumulus clouds, sunshine and squally showers. This will be most aggressive on the west coast and less so after the air has crossed the UK. This polar maritime air will give great visibility, except during the squalls, as it has little in the way of pollution in it.

Our main air masses are either tropical and warm and wet, or polar and cold and dry; however when the air is from a land mass it is much drier with a greater temperature difference between summer and winter.

So the direction of the wind gives a good indication of the weather we get and this holds true wherever in the world we are. Once we venture offshore our sources of weather information is limited and for most of us is restricted to GRIB files and synoptic charts on slow satellite downloads.

Both are very useful and I would not like to be without either. GRIB files have revolutionised routeing, but you can gain a competitive advantage, or more comfortable passage, by using the additional information found on weather charts.

Identifying the type of wind

We are familiar with fronts, but by identifying the air mass we can identify the type of wind to expect. Transatlantic records are usually set by staying in the more stable Tm air mass where a steady wind speed and direction allow boats to be pushed harder, rather than in the stronger, squallier conditions found post cold.

Sometimes, however, our options are limited as the whole passage is within one air mass. During one Round Britain Race I did the wind was east-north-easterly and there was little change expected in the air mass, giving a complete circuit of the UK in poor visibility. Approaching the Lizard at dusk were three Open 60s; with this stable air mass (warm air, cold sea) we knew that there would be a shut down near the coast, but to maintain our speed we kept heating up and got too close to the land at five miles. A competitor ten miles offshore was 30 miles ahead at the next sched, while the inshore boat was 30 miles behind.

Chris Tibbs is a meteorologist and weather router, as well as a professional sailor and navigator, forecasting for Olympic teams and the ARC rally. He is currently on his own circumnavigation with his wife, Helen. His series of Weather Briefings can be seen here

See also:

Chris Tibbs prepares his own boat for an Atlantic crossing

The best route for an Atlantic crossing

Taking the northerly route across the Atlantic

Offshore weather planning: options for receiving weather data at sea

as well as Chris Tibbs’s series of Weather Briefings