Who has seen the wind?
Neither I nor you:
But when the leaves hang trembling,
The wind is passing through.
Who has seen the wind?
Neither you nor I:
But when the trees bow down their heads,
The wind is passing by.
–Christina Rossetti (1830-1894)
Today was exceptionally windy. The weather data for the nearby town recorded a top speed of 65 km/h (40 mph) mid-afternoon. We lost power twice, just little hiccups as (presumably) a branch broke off a tree and bounced off the lines. Dan was outside with Raven when a dead tree beside our driveway snapped and its upper half fell just a half dozen meters from where they were. The tarps over our multiple wood piles were all flapping wildly (one even came loose and tried to make a break for it, but was trapped by the deck railings). Our platform feeder is now listing to one side, and a tube feeder that hung on a shallow hook was blown off its supporting branch. The trees in our yard were bending and waving wildly, and I was certain that we would have more casualties than the single dead tree by the driveway. Trees are amazingly resilient, and even the seemingly stiff trunks can bend with the flexibility of a supple green branch when subjected to enough force. It’s hard to capture a photo of the wind, but I tried to get one of the trees moving in it, at least, through long-exposure shots.
Wind is a mysterious force. We can’t see it, and we can’t touch it or hold it, but we can sure feel it as it whips about our hair and clothing and buffets our chests, and we can hear it as it rushes between branches and rustles grass and leaves. We know it’s a movement of air, but just what causes wind?
Most wind, at least the sort we’re usually concerned with in weather forecasts, is a movement of air from a region of high atmospheric pressure to an area of low pressure. These pressure zones always seem a bit mysterious themselves – just what are they? To get a good idea you have to think of the atmosphere at its molecular level. Although we see the air as an absence of things, really it’s full of molecules just like the land or water, they’re simply much less densely packed. You can “see” air when gasses that have some colour, such as water vapour, are involved. Water shows up as a whiteish gray. Think of fog, low to the ground, or steam as it rises off a warm lake on a cool morning – it’s not a uniform blanket of white, there are thick, opaque patches and thin, transparent areas. Now apply that to the atmosphere as a whole and you get an idea of the variability in density of air molecules that is found throughout it.
Molecules “want” to move from areas of high density to areas of low in order to form a uniform density throughout, through the process of “Brownian motion” (where molecules, which are always vibrating, randomly move about, bump into others and force them apart when they’re closely packed, and eventually fill in the spaces between widely spaced molecules, until there’s an equilibrium). Here’s another analogy: think of pushing a bowl down into a sink full of water. When the edges slip past the level of the surface the water rushes in to fill the empty space. As the bowl fills up there becomes less empty space and the initial rush of water slows and tapers off, and then eventually ceases. The atmosphere works on this principle, and wind is the result, as the air molecules from dense areas (high pressure) move to fill in the spaces in thin areas (low pressure). The direction of the wind is obviously a result of where the two pressure zones lie relative to each other, and the strength of the wind depends on just how dense (or not dense) each zone is, with stronger flow occurring when the difference is great, and weaker flow when the difference is small.
Today there was a high pressure zone to the west of us, and a low pressure zone to the east, as a cold weather front passed through. They don’t directly tell you what the barometric pressure is in each zone, but they do show the number of isobars (each representing an increment in difference), of which there were many. Today’s winds were the sort I tend to associate with storms, but there was no storm, just creaking trees and the snapping of tarp edges.