Walks at this time of year are always so invigorating. You can’t head out into the wild air, marvel at the spinning leaves, and have the wind take your breath away without returning home feeling renewed and refreshed, if a little disheveled. What is it that sets all this in motion? At the root of all this twirling and swirling is, of course, the wind. But what exactly is wind, and why does it move the way it does?
When it comes down to it, nothing much on earth would be happening if it weren’t for the power of the sun, particularly in the case of plants — the ultimate sun-power users. Heat from the sun is the fundamental source of energy that allows air masses to move from one location to another, that allows for the formation of the winds that characterize this lovely, temperamental season.
Wind is caused when one parcel of air is heated to a different degree than another, either directly by the sun, or by a sun-warmed body of water or mass of land. This differential heating leads to variation in pressure in these different locations, a phenomenon referred to as a pressure gradient. The air parcel which is heated to a greater degree suddenly must expand, pushing against (and being pushed back by) the ground surface and neighbouring parcels of air; the result is a build-up of pressure, and an air parcel which can now be called a high pressure system. In contrast, an air mass that is heated to a lesser degree (or which is cooled, say, by a loss of radiation) becomes, in relation, an air mass of low pressure. These are the H (high) and L (low) systems that the weather forecaster on the news loves to point out.
But why do high and low pressure systems matter in relation to weather, and to wind in particular? Due to those fun properties of physics that we all learned about in high school, an equalizing tendency leads to the movement of volumes from areas of high pressure to areas of low(er) pressure. A force created by the differential in pressure across space (the pressure gradient) sets up a pressure gradient force, which causes air to move from H to L. So this accounts for air movement in a straight line from point A (high pressure) to point B (low pressure).
But why do we see on the news those swirl-looking graphics that depict the circular movement of wind? This is where a bit of fun comes in. As we all know, the earth spins on its axis; this spin causes, in the Northern hemisphere, a deflection of wind to the right, a phenomenon known as the Coriolis effect. So those swirl graphics around systems of high and low pressure (clockwise in the case of highs and counterclockwise in the case of lows) are due to this property of deflection. Wind diverges away from a high-pressure system (due to the pressure gradient force) in a clockwise direction (due to the Coriolis effect), a set of circumstances known as an anti-cyclone. In contrast, wind converges toward a low-pressure system (again, due to the pressure gradient force) in a counterclockwise direction (again, as a result of the Coriolis effect), something known as a cyclone.
Friction and surface obstacles can also act on wind and influence its direction and patterns of movement; friction acts to pull wind back, to slow it down. So over smooth areas like the ocean, with little surface roughness compared to, say, a forested area, wind can reach great speeds. Obstacles, such as buildings, get in the way of direct air movement from one point to another, and cause localized patterns of swirling air movement referred to as eddies.
Yes, in the weather world there’s an awful lot of terminology. But, stored up for your next Trivial Pursuit game, this entire lingo might just come in handy.
At this time of year, strong variations in the heating of air masses over land compared to oceans, of parcels of air closer to the equator versus those closer to the poles, are typical. The result is the formation of pressure gradients (and therefore pressure gradient forces) of particular strength. Those tempestuous autumn days, the unforgettable visuals of trees you once thought of as solid being blown to and fro by the sheer force of moving air, the wind suddenly whipping up and nearly knocking you down — all of fall’s windy fun can, at its root be traced back to the power of that blinding light in the sky, to life’s best friend — the sun.