WEATHER WEATHER
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).
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