Sucking or blowing, that's the question.

This post is all about clouds. And winds that go with them...

I think for us, sea-faring people, the difference between "sailors" and "Sailors" (similarly to the difference between "men and boys", "girls and women", "chickens and eagles", "Bambi and Rambo") is (partly) made by "how well we can read the clouds".

Why is "reading" clouds important?

After 50,000 Nmiles of open ocean passages, I have experienced first hand how sailors look at weather apps, read something like "sustained NE-lies of 20 knots, gusts up to 25 knots for the next 24 hours", and interpret this as if it was the Holy Truth. And yet, at 2 AM, in the pitch dark, we get hit by an hour of gusts up to 40 knots and 90° wind shifts, having us struggling to reef our sails.

Likewise, when sailing in-between islands, all too many times, we rounded a corner or headland, and the wind changed from 12 knots to 35 knots, with an almost 180° shift, with the crew scrambling from bikini-sailing to survival mode with sails flapping in all directions. And that, while the weather prediction called for a 12-15 knots stable wind...

With weather apps like Predictwind or Windy (that is "windy.com", which rocks and not "windy.app", which sucks), getting better and better, in my opinion, we have become too reliant to sail solely on these apps to predict our wind and weather.

We forget that these apps are mostly based on larger scale atmospheric predictions - the dynamic between low and high pressure systems mostly (I am over-simplifying bit here).

But we forget that these apps are not that good at predicting precise local weather conditions, conditions defined by e.g. local topography or clouds. Many of us have experienced weird wind shifts caused by funnelling or katabatic winds when sailing close to coastal mountain ranges or in between islands. And likewise, any experienced sailor knows how clouds can cause similar wind changes as we approach them. Weather apps, in my experience, can make a "guess"-timate of those local conditions, but can not predict them precisely. At all.

This is no blame on those prediction apps, as these local conditions can be ..eh.. very "local": When sailing between islands, a minimum wind direction change can alter the funnelling effect between islands significantly. Likewise, a cloud, based on its density and saturation, at the moment it passes us, can make a very significant difference in wind direction and strength.

On this post, we'll forget about coastal sailing, islands and mountains, but we'll concentrate on "reading the clouds" while sailing offshore.

There are two types of clouds

There are "blowing clouds" and "sucking clouds" (and that is my own terminology). "Blowing clouds" are spilling energy outwards, and "sucking clouds" absorb energy inwards. End of paragraph.

Blowing clouds

"Blowing clouds", are mature clouds. They have accumulated enough energy to become saturated, and they will be spilling out that energy. That "spilling energy" will be in the form rain and wind.

You can recognize a "blowing" or "mature" cloud, by looking at its shape and colour, and looking what is happening under that cloud.
A blowing cloud towers high above the horizon. In its more extreme form, they form a "T"-shape.
Typically they are also darker in colour than their "sucking" sisters which are still building up that energy.
If you look at the air below "blowing clouds", you will see the dark shade of rain underneath.

A note of wisdom here: sometimes a dark area below a cloud can be the shadow of the cloud, rather than rain. To distinguish between "rain" and "shadow", I look at the position of the sun versus the cloud: If the shadow beneath the cloud is in a straight line from the sun to the cloud, there is a good chance the dark area below a cloud is a shade, and not rain.
Another way to look at it, is to look at the direction of the shade: if you look at the shade from the bottom of the cloud towards the sea, and the shape of the shade follows the prevailing wind direction, there is a good chance, it is rain. If not, it might be a shade.
Rule of thumb, but no guarantee.

Back to blowing clouds: as these are mature and saturated clouds, they spill energy. If you would be an eagle, and look at the cloud from right above it, the winds would spill, from the centre of the cloud, 360° outwards. The higher and the darker the cloud, the more mature/saturated it is, and the stronger the winds it will spill from the center outwards. Right underneath the cloud, there will be a strong down-draft, with rain, and hardly any wind.

What does this mean for you, sailing transatlantic and thus sailing downwind: You see a dark and large cloud coming in from behind. You see the darker colours of the skies below the cloud, showing rain. This means that in front of the cloud, as you are sailing downwind, the spilling ("blowing") wind will amplify your prevailing (atmospheric) wind. That "amplification" might be anything from 10 to 20 knots: so if you have a prevailing wind of 20 knots and a "blowing cloud" catches up on you, right in front of the cloud, you might get gusts of 30 or 40 knots. Right underneath the cloud, you will get the downdraft (and the rain), reducing your wind significantly, or reducing it to almost nil...
And once the cloud passed you, its "blowing" behaviour, the wind it spills, will kill your prevailing wind for some time, with either "less wind", or "no wind", or in extreme cases, headwind as the spilling wind is blowing against the prevailing (atmospheric) wind

And mind you, this is an example when sailing downwind, and being hit from a "blowing" cloud from behind (sailing downwind, all clouds will hit you from behind).

One note here, though: not all "blowing clouds", will hit you overhead. You will have just as many "blowing clouds", passing you on the side (and I prefer these clouds to pass on my side, I hate to be hit directly with 35-40 knots squalls directly). But by observing the size and colour of the cloud, the colour of the shade below the cloud (density of the rain), you can estimate the strength of the spilling winds it will bring to you. And if you follow my mental eagle-eye picture of "blowing clouds spilling wind 360° from its center, you can then predict what windshifts you might have, as that cloud passes left or right from you.

Sucking clouds

Sucking clouds are young clouds, which are not "mature" or "saturated" yet. They are lighter in colour. They typically do not tower that high. And they don't have signs of rain underneath them.

If you'd "fly like an eagle" (Reference to a 1970'ies song by the Steve Miller Band, but most of the youngsters amongst you, won't remember neither the song, nor the band. - hey remember their song "The Joker"? - "Some people call me the space cowboy. dddadadadum.. " ). Anyways, if you'd be an eagle and flew above a sucking cloud (oh man, I can't get that song out of my head "Time keeps on slipping, slipping, slipping")...

Anyways, so, if you'd be an eagle and fly right above a sucking cloud, you'd see, the air from 360° off the centre of the cloud, being sucked into the cloud. So, if you'd be on a transatlantic crossing, with the prevailing winds coming from behind and a "sucking cloud" approaches you, then, as the cloud approaches you, you will have less wind in front of the cloud.
Underneath the cloud, as air is sucked upwards, you will have slightly less wind, and once the cloud passes, its "sucking" will amplify the prevailing wind speed, and this will increase the wind AFTER the cloud passes (versus "increased wind in front of a cloud, if it is a mature blowing cloud")....

Sucking clouds are typically not as violent as blowing clouds, but they can still kill, shift or amplify your prevailing winds.

Sailing transatlantic East-West. What does this mean for you?

Those who know me, know that E-W transats, "are my thing". Not only because, by now, I've done six of them, but also, because for many cruisers and crew, this will be their first (and for many more, last), major open ocean passage. Being a moderator for the Noforeignland's "Crossing the Transat East-West" Facebook group, I see firsthand, how this is on the bucket list of many people, and the only open ocean passage they will ever do... Thus, time to spill some wisdom in that context, then.

So... what does this sucking and blowing mean for you, and how do you "work those clouds?"


(1) Clouds on an E-W transat, are born off Africa, and mature as they go across the Atlantic. Thus, the closer you get to the Caribbean, the more the water temperature raises, the more water evaporates in the air, the more mature clouds you will get, and the higher the chance you will be hit by a squall (a "blowing cloud"), thus the higher your vigilance should be to monitor for these "blowing clouds".
(2) During the day, visually check the clouds behind you. Is it a sucking or a blowing cloud approaching you? Can you change your course to avoid blowing clouds?
(3) As the dew point lowers during nights and early mornings, the higher the chances to get hit by a violent "blowing cloud", forming into a squall. In the dark, at night, check your radar every 30 minutes or so. If you see a dense cloud forming, try to change course a little, try to avoid the center of it. If the cloud is that big that you can not avoid it, be prepared. Reef properly. Way on forehand. Get some hands on deck to deal with the squall.
(4) As you approach the Caribbean, and clouds get more mature, and the probability to be hit by a squall gets higher: avoid flying spinnakers during night time. Nobody will punish you if you arrive a day later than planned, because you went slower during night time. But you surely will have more peaceful nights looking at the stars, while sailing on less canvas during the night, rather than trying to gain speed with your spinnaker up at night. As we approach the Caribbean, I always urge us to reduce canvas,

Have a safe transat everyone. And learn to read the clouds. Weather prediction apps are good, but keeping a good eye (or radar-eye) on the clouds behind you, and interpreting the blowing or sucking clouds well, will save your day...