The ITCZ and Its Friend, the Squall April 18, 2012

Closing in on the ITCZ (the intertropical convergence zone) we're still unable to get Hawaii weather except by email which is slow and power consumptive. I would be happy with about eight different weather products: surface analyses, surface forecasts for 24, 48 and 72 hours, wind and wave charts and the East Pacific Discussion which is a text file. But I'm settling for the text discussion and a gridded binary (GRIB) model of wind and waveforecasts.

Robbie sent an email to Lee Chesneau asking what's up with the weatherfax Hawaii signal being gone and Lee forwarded it to the weather office there. Just a defensive reply suggesting that we didn't have current data because our email referred to the guy no longer in that position . (Of course, we got his name off the current instructions for ftp weather downloads, so, let's see�... who's not current?) Bottom line seems to be that the Navy has shut down the Hawaii radiofax activities right in the middle of the season when short-handed sailors cross the Pacific in small sailboats. The Navy has its priorities. They are not quite the same as my priorities!

The NE trades are still blowing fresh and we're making decent progress at 6-7 kts even sailing nearly dead downwind. This point of sail is a very rolly position and we'd rather be at a broad reach, but we just can't get there. Convective activity at the ITCZ has narrowed the window through which we want to cross it, so we'll stay downwinding for another day or two and enjoy the rolling.

I think I'm beginning to understand the Intertropical Convergence Zone, at least I'm getting a big picture of what's going on here. I've learned everything I know about meteorology from Lee Chesneau, and Lee will probably kill me for cooking up my own ideas of how it works, but this is my mind in action. I'm sitting here at 6 degrees North wondering why the weather is acting so oddly and my mind is going to come up with a hypothesis. Lee's not here to tell me how it really works, so I shall be creative and let him correct me later. Sorry, Lee. Here's Bev's simplified and ignorant ITCZ primer.

NOTE: Inserted April 19, 2012. OK. The next day's experience was different from what I'm about to describe and it blew this theory out of the water. But I still think it happens this way some of the time. Read on for other options.

There's a lot of moisture in the air here. It's clear and blue in the early morning, but by noon the haze has turned the sky milky and by mid afternoon, like in the desert, the squalls are beginning to form. Unlike desert storms, though, which move across the earth's surface with the wind, these squalls don't go anywhere. Or they don't appear to. They are moving UP, not across. As the NE trades blow toward the equator from one side, the SE trades do the same from the opposing side, They meet approximately at the equator, within a couple of degrees of the equator, and the place where they meet moves backand forth generally in the vicinity of the equator, rapidly and frequently, as atmospheric pressure and wind speeds change on one side or the other. The atmosphere is a fluid. (Lee would tell you that, by the way!). This is the Intertropical Convergence Zone, the area where those tropical winds converge.

Two bodies of air moving toward one another in a steady and constant flow change direction and both move UP when they meet, up into the atmosphere, the only place they really have to go. As that warm moist air rises, it generates the perfect conditions for squalls, short, often violent, little storms that reflect the instability of that air that's moving up into the atmosphere. It's really an interesting phenomenon, and not all that complicated. The area of convergence moves around apparently unpredictably like the balance between two arm wrestlers. It would take a Kray computer and more to figure out precisely where the ITCZ is going to be, but in fact, the only practical way to sail through it is to gather the best data you can, set your course, then work within the limited ability of your sailboat to miss squally areas that pop up seemingly out of nowhere.

Be prepared for squalls.

We hit the first two squalls that came on our radar. Or, rather, they hit us. I was at the helm and expected them to proceed off to the west in the 'normal' way of weather. But I'd forgotten the whole point that makes the ITCZ different. The squalls are not moving any direction but up. So, we smacked right into the first one rather than passing behind it per my plan. I thought we'd miss the second one, it was dissipating as we approached its fringe, but a new squall developed right where we were headed, so Mersoleil got another free bath.

ITCZ squalls form in lines that look on radar like a train marching across the chartplotter. But they aren't marching across at all. They're ascending like little angels! And a new squall is forming behind them in the unstable low pressure air that remains at the surface as more trade winds pile up there. So what I saw on the radar was, rather than a train processing across the sea, a series of intense areas forming, then dissipating, then new ones forming only to dissipate and be replaced again. And again and again.

I learned by trial and error that I have to sail all the way around that area of greatest convection, the whole thing, even if it's twenty-five miles wide. The gap that I see while approaching it probably isn't really an opening at all. It's just a spot not involved in a squall at the moment. Give it time.

The broad areas of moderate to severe convection seem to hang around for a while, days not hours, which is why the sailboat captain might as well know about them far in advance and steer clear. Hence my desire for frequent weather updates in this location. We may not be able to escape a squall line that suddenly appears five or ten miles ahead, but we can surely choose to avoid the five degrees of longitude with the strongest activity if we know about it three days in advance!

The second squall line gave me an opportunity to try out my hypothesis and it served us pretty well.

The line was around twenty miles long in a curving line from east to west and there were about seven squalls active at any given moment. I watched them form, intensify, dissipate and disappear only to be replaced by a new squall in the same or a nearby spot. My preference would have been to pass the whole line around the west end, but this is a sailboat and we were sailing nearly dead downwind in a NE wind. I had a little leeway (literally) to turn to port, so I headed off in that direction aiming for the far east end of the squalls. I knew I couldn't make enough easting before we reached the latitude of the weather, but I did my best to keep the sails filled and reach the right place at the right time.

It worked! We sailed through a sizable gap between the last two squalls. I couldn't get far enough east to miss them all, but no new squall developed in the gap (that was a gift) and we felt only a few scattered raindrops blown in from the next closest convective activity and no gusts at all.

All this convective activity increases as the earth and air warm during the afternoon and then the dropping temperatures of the evening accentuate the drama with more intense squalls, putting lightning into bold relief to add even more punch. On this night we experienced no squalls. After all, we hadn't even reached the actual ITCZ yet, we were just approaching it. This was just the premiere!

And now, additional notes based on the experiences of day two crossing the ITCZ�.... theory needs significant modification.

I still like my neat "it only goes one way - UP" theory, but admit I was wrong. Partly. Today as we sailed through the latitudes of 5N and 4N we dealt with squalls that DID move laterally across the ground. Their behavior was quite unpredictable.

I got myself into quite a pickle early in the day when I headed around a little group of squalls hoping to pass them on the west side. Another squall or two developed still further west leaving me in the middle again. A few of the oldest squalls of the group failed to dissipate. In fact they got larger. Then I realized this group was moving, moving faster than a sailboat, which was terribly inconvenient. I had no choice but to run through the squall at a nearby point where it seemed a little narrower. This worked out just fine, but I was nonplussed that my theory developed just yesterday was already trash.

On the same watch I found us approaching another group of squalls. These too were in a cluster, not following one another like train cars. In this particular cluster there was a very promising opening and I sailed for that opening. Before I got there it had filled with a new squall and, again, the entire cluster had grown and intensified. I had to call Robbie up on deck to help me make some sail changes in a hurry and I decided ultimately that the least risky plan would be to turn around and run back in the direction from which we had come. The cluster was changing so rapidly it amazed me. And it was definitely moving, although with the constantly changing cells it was difficult to know in which direction. It was getting larger, too, which added immensely to the excitement.

We escaped both these squall systems with a good bath for Mersoleil and a generous slice of humble pie for me. Yesterday I thought I had the ITCZ squall activity al figured out.

Today I think I have the ITCZ squall activity figured out, too. Here's today's solution:

Weather in the ITCZ plays with your mind. If you actively make effort to outrun or otherwise escape a squall it will stalk you. If you try to understand its dynamics it will let you go on for a while and waste some time before it shows you for a fool.

My new approach to the ITCZ? Avoid the known problem areas, those identified by the National Weather Service. Once you've done that, get going and keep going. Most of the squalls are little more than an inconvenience. It's easier and faster to stick with your plan and keep moving in the intended direction, even when weather pops up from nowhere, than it is to try to outsmart these fickle systems.

Enjoy the rain, enjoy being in such a remarkable place on the surface of this earth. Sail on. And maintain your sanity!