Due to the very nature of this sport of surfing that we have all somehow got ourselves incredibly obsessed with, and it's complete dependency on weather conditions, our lives have ended up being ruled by rising and falling air pressures over the North Atlantic, and for others - their own particular oceans.
Every day in the newspaper, you can see a european weather map with isobars (the numbered rings that join areas of equal atmospheric pressure - looking like the VictorKilo logo), and most of us know that high pressure (a big H, anticyclone) over Ireland tends to give us good weather, and low pressure makes it windy and rainy. (We actually do get high pressures...sometimes).
A low pressure area is a storm, and over the sea it's like dropping a stone into a lake, ripples (swell) move out from the stone until they eventually hit land and break as waves.
Something fewer of you may have noticed is that isobars that are closer together give stronger winds, and that the wind direction is due to the pressure that the isobars form. In a low pressure area the winds move anti-clockwise and in towards the lowest point.
Conversely the winds move clockwise and away from the centre in a high pressure area. In the summer the air pressure is generally higher than in the winter, because of the angle the Earth makes with the Sun - this is also the reason why there's time in Summer to go for 3, even 4 sessions a day, whereas in winter you're lucky if there's time for 2.(This does give us more time to spend in the pub...which is nice).
It's because of the stronger Sun that the air heats up more and thanks to Boyle's Law that this means higher air pressure (Boyle was Irish, as it happens - drank Stout, and went clubbing on Tuesdays and Saturdays with his mates).
There tend to be long periods in the Summer when a high locks in over Ireland, and have a chance to repair boards etc...for 3 WEEKS. But then it appears, a baby low pressure begins to form off the east coast of Florida, and then rolls on up the east coast of the US.
An ocean low pressure can be quite happy to roll up along a coast without moving over land, since land is warmer, creating high pressures that form winds blowing out to sea keeping the L away. All the time the low rolls up the coast it it spins faster, and faster until it reaches down to the 960-970 level that we like to see. The low will then move on up to between Ireland, and Iceland, then up to the Scandinavia, where it fades away to nothing. When we see a low pressure - a swell, in the ocean, there are a number of questions we ask:
1) How big is it? - what is the difference from the 'outside' to the 'inside' (read the numbers - 70 is a LOT, 10 isn't).
2) How long will it take to get here? To the west coast of Ireland - a low off New Foundland might take 3 days, bigger means faster.
3) What winds will we have when it arrives? - any weather forecast will tell you that, and we all know that offshore is good (that's normally wind from the east for the west coast, but there are places that face north that will be offshore on a big swell with a southerly wind)
4) Who's going to drive you there? Happy hunting