Weather: with Simon Rowell, Rowell Yacht Services

CONTRIBUTED BY SIMON ROWELL, ROWELL YACHTING SERVICES

Paul Wyeth

Looking Above the Surface

We’ve all got up in the morning and stood by the water in hardly any breeze, while above us the clouds trundle along quite happily. At this point I usually wonder something along the lines of, “Is all that lovely breeze going to make it to surface anytime soon?”. There are many sources of weather forecasts that we’re used to using, but can we dig a little deeper and look into what’s going on above the surface to help us understand how the surface wind may change during the day? Well the answer is yes, and while it can be a bit daunting at first it’s always worthwhile to start looking at forecasts for heights above the surface and at those rather scary vertical charts, tephigrams.

Let’s start off with gusts. In this context we’re talking about clear air gusts, not those driven by clouds. Most weather forecast will give you 10m wind speed and then a “gust” reading – where does this come from? To answer this we need to understand how gusts actually work. The bottom part of the atmosphere, usually the bit underneath the clouds, is known as the boundary layer, and is the section most directly affected by surface heat and by the friction between the

Figure 1: the boundary layer with geostrophic wind above it (left), and with a faster parcel of geostrophic air captured and brought down to the surface (right).

surface and the wind flowing over it. Just above this flows a smoother and generally faster wind, often referred to as geostrophic wind (Figure 1, left).

The boundary layer itself is a chaotic place, with surface friction causing tiny turbulent eddies right at the surface, then effects like surface heating causing convection, with the warmer air effectively bubbling up, and generally the higher you get the larger these mixing circulations become. One of the great researchers of the early 20th century, L.F. Richardson, summed it up well: “Big whirls have little whirls that feed on their velocity, and little whirls have lesser whirls and so on to viscosity”.

Occasionally one of these eddies will break through the boundary layer, capture a parcel of faster moving air and drag this down to surface – this is the mechanism by which we get surface gusts on clear days. This also explains why some gusts seem to hit the water nearly vertically and spread out in a catspaw manner – because that’s exactly what is happening (Figure 1, right).

Thinking about this then, if we can work out what the wind speed is above the boundary layer, then that will give us a good idea of gust speed. When you’re looking at forecast data above the surface the height tends to be given not in units of distance but as a pressure, e.g., 925 mb (millibars), 700 mb, etc. The units of pressure may differ too, with some sites using mb, others hPa (hectopascals). The good news is that in a most uncharacteristic unit comparison 1 millibar exactly equals 1 hectopascal. The height I use to get an idea of gust speed is the 925mb layer, which is usually around 750m or 2500 feet. I find this gives a good idea of the gust speed, and is easy to get hold of on the many weather apps available. By looking at how this layer’s speed changes through the day you can get a very good idea on when you can push the top end of your sail plan and when you might want to be a little more conservative.

Moving on from gust speed let’s have a look at gust characteristics – are the pressure patches going to be quite large and long lasting, or are they going to be small and transient? This can have a bearing on your race tactics when you’re deciding whether to try and get into pressure patches or just take them as they come. This is of course entwined with the cloud patterns, but it’s worthwhile talking about anyway, as it’s an important part of the whole. This is where the tephigram, also known as the sounding or the skew-T diagram, comes in handy. The Met Office website has an excellent detailed explanation of these (factsheet 13 available from https:// www.metoffice.gov.uk/research/library-and-archive/publications/ factsheets), and we’ll go through the basics here.

With the availability of model data you can get forecast tephigrams from several sources. The one I use is Xygrib (available free at https:// opengribs.org/en/downloads), though other sources are of course available. These can be a bit confusing, but they’re handy for looking at the wind above the surface, where the clouds are and whether the atmosphere has separated into layers at all – the most obvious one of which is the inversion that we often see under a high pressure system, with the morning having a hazy or dusty lower layer.

Figure 2 shows 2 examples, a high pressure day and a deep low. The red lines show temperature, and in the high pressure example there’s a temperature inversion at 1500m (height scale in km up the right hand side), where the subsiding dry air of the high is warmer than the air below it in the boundary layer. This is what causes that

Figure 2: typical tephigrams for high (left) and low (right) pressure systems.

hazy and dusty layer we so often see. The blue line is the dew point temperature, which is the temperature at which moisture condenses out. Where the temperature is within a couple of degrees pf the dew point, that’s where we get clouds, so the height of the inversion gives you an idea where the scrappy clouds often seen in a high will be, and therefore the depth of the boundary layer. With the low pressure example the dew point and temperature lines overlay all the way up, which indicates deep cloud and likely lots of rain and squall activity – this was from Tokyo in July, and was a particularly strong low. This is the first glance information you can get from a tephigram – where the cloud is likely to be, and how deep the boundary layer is.

This is all very well, and it’s good to know about cloud heights, but what does this have to do with the surface wind? To answer this we need to look at the lower sections of the tephigram. This is where the wind barbs up the right hand side come in very handy.

Looking at the top tephigram first (Figure 3 top), the wind increases above the surface under the inversion. As the boundary layer is quite deep (1500m or so) some of this faster air will get circulated down

in quite large swirls, so in this case pressure patches are likely to be quite big, and probably worth heading for.

The bottom tephigram (Figure 3 bottom) has a much shallower boundary layer, only about 800m – this means that the size of the swirls bringing any stronger wind down won’t be as big, so the size of the pressure patches will be smaller, and they probably won’t last as long. This means that if you get them great, but they’re probably not worth hunting as by the time you get there they’ll be gone.

One thing in general about gusts – looking at the wind barbs it seems that often the wind above the surface comes from the right of the surface breeze (these are all N hemisphere examples, it’s the other way round S of the Equator), so you might think that the gusts will be from the right. This is not the case – due to the chaotic nature of the boundary layer the gust direction almost always ends up being spread evenly either side of the mean.

While looking at what’s going on above the surface can initially seem overly complex, with a bit of practice you can quickly and easily get an idea of the three dimensional makeup of your breeze for the day, and that can help you plan your strategy and tactics for the day’s racing. If you’re just out for a pleasant day on the water it’s also handy, as knowing what’s going on just above us is always useful when it comes to sail choice in those awkward “just at the top of the sail range” moments.