Weather is one of the more difficult areas of sailing theory to understand - the weather patterns we see are mainly due to where the air (known as air masses comes from.
Sources of air masses
The source of air mass depends on where they come from. Warm source regions are:
- Sahara Dessert (dry and warm)
- Tropical oceans (moist and warm)
Cold source regions are:
- Arctic Ocean (moist and cold)
- Siberia (dry and cold)
- Northern Canada (dry and cold)
- Southern Ocean (moist and cold)
Stability of the air mass
Depending on where the air has come from also controls its stability. Unstable air produces strong winds and rain. Air masses (like polar air) are heated from below which makes them stable, and tropical air is heated from below which makes them unstable.
As air masses move towards us they are modified. As they move over:
- the sea increases the moisture (as the water evaporates from the surface) - known as a maritime track
- the land remains relatively dry - known as continental track
- a warmer surface they cause a colder air mass to become unstable as the surface is warmed from below
- a colder surface then a warm air mass is cooled from below it becomes stable.
Types of air mass
The combination of the source of the air mass and how it reaches us governs its type.
Originating over the Sahara this type of air mass reached the UK most often in the summer months.
Visibility is often moderate or poor due to the sand particles in the air (in the Canaries this type of air mass is known locally as a calima).
When the air mass originates from Canada (where it is cold and dry) it is known as Polar Maritime as it has a long passage across the North Atlantic picking up warm air from the sea and becoming unstable with frequent showers often heavy.
Returning Polar Maritime
Returning Polar Maritime is when the passage of the air mass goes further south giving it a longer track south over the North Atlantic (causing the air mass to become unstable as it picks up moisture) before turning north east towards the UK (where the sea track causes it to become more stable).
Originating from the Caribbean (unstable air), and the crossing the warm waters of the Atlantic on South-Westerly winds, the air becomes stable as it passes over the cooler sea and picks up the moisture. This is the most common type of air for the British Isles.
This type of air mass typically has warm and moist air, with low cloud and drizzle.
The Polar Continental air mass originates from Siberia (with its cold land mass) in the winter months (in the summer it is known as Tropical Continental as the land mass is warm). The weather produced depends on the sea track: in the south of the UK with the shorter sea track over the English Channel you get clear skies and frosts; when the air mass crosses the North Sea to Scotland and the North of England the air becomes unstable (with rain/snow) due to crossing the longer stretch of water.