nquisitive Surfline user “Barry” writes:
“Ok so I’ve been looking at the details and I’ve already got stumped. As I understand it, wind starts from air moving from high pressure areas to low pressure areas and air pressure is measured in millibars. Here’s where I’m mixed up: I’ve been looking at charts of low pressure areas ( smaller circles with lower millibar levels) moving under Africa towards Australia. They’re sorta like peaks on a topographical map only just the opposite and the smaller circles represent the “low” instead of the peak. If the wind blows towards the low from the high, how does the wave energy move towards Indo or Western Australia. It seems like I’ve got the concept in reverse. Dang, there goes my career as a forecaster!
Thanks for some assistance.”
Surfline’s Kevin Wallis responds:
Great question! Ok, with this we need to take a couple things into consideration. You’re correct when you mention that wind flows from areas of high pressure to areas of low pressure, as the earth constantly tries to reach a state of equilibrium. The tendency for air to move from an area of high pressure to an area of low pressure is called the pressure gradient force (PGF). Now if the earth didn’t rotate about its axis the wind would flow directly from high to low.
But in reality the earth rotates, causing the wind to deflect to the right (in relation to a straight line) in the Northern Hemisphere and deflect to the left in the Southern Hemisphere. This is known as the coriolis force. Check out this resource for a good coriolis force demonstration.
So, when these two forces act against each other (and other factors such as friction come into play) the wind will not flow directly from areas of high pressure to areas of low pressure. For simplicity’s sake, we’ll just say that the wind will closely follow the path of the isboars, (equivalent to contour lines on a topographic map) such as below. The black arrows represent the basic flow of the wind:
READ: What Causes Storms?
With areas of low pressure, cyclonic flow will closely parallel the isobar lines with a slight bend toward the center of the low. The opposite is true with areas of high pressure, or areas of anticyclonic flow: the wind will closely parallel the isobar lines, with a slight bend out from the center of high pressure.
Furthermore, in the Southern Hemisphere, winds will rotate clockwise around an area of low pressure and counter clockwise around an area of high pressure. The opposite is true in the Northern Hemisphere: winds rotate counter clockwise around an area of low pressure and counter clockwise around an area of high pressure.
Now what I’m sure you’ll also notice on the above map is that it appears the wind is blowing directly from west to east and the resulting swell will pass well to the south of the Indonesian Islands, particularly from the storm south of Africa. But of course this is a flat map and the earth is round! So, have a look at the dark lines that converge at Bali and then extend out toward the West Indian Ocean. These are the great circle paths that correct for a flat map vs. round earth. When you see the isobar lines running parallel to the great circle lines, wind and eventually swell, are being aimed at your location.
Now have a look at the concentric rings (for lack of a better term) that intersect the great circle lines. Each one of these rings represent a distance of roughly 600 miles, which is about how far a swell with 17 second interval can travel in one day. So, strictly looking at this map and ignoring some other factors, there are new swells heading toward Bali/Indo, with arrival time 5 days from now, 7 days from now and 9 days from now.
Hope that explains things a bit more and makes sense. Feel free to contact us again with any further questions.
Kevin Wallis
Surfline Forecaster
