Wind Revisited
Before revealing the influential details, let’s quickly review how wind is created. Remember, air is set in motion by the Pressure Gradient Force (PGF). Since Mother Nature is always trying to stabilize, regions of excess move toward regions of deficit. One of these balancing acts results in air moving from high to low pressure systems in attempts to equalize everything. Basically, when pressures change, air starts moving. What’s so awesome for kiters is this PGF force acts on a horizontal plane, giving us our beloved wind.
Uneven heating and the Earth’s rotation are major causes for these pressure differences. Different wind types (General Circulation, Synoptic, and Mesoscale) exist depending on the size, altitude, and occurrence of pressure systems. For more knowledge on wind types and pressure changes, see Wind Part I in Kiteworld, issue #30 page 120, or read Winds Part I online here
The Core 4 Influencers:
Wind’s strength, stability, and direction are merely results of four accelerating and decelerating forces. They’re controlled by everyday natural rules, which were first devised by Sir Isaac Newton as the laws of motion. Now, his first law states stationary objects will stay put and moving objects will keep going as long as there’s no opposing force on the object. This law also says a moving object’s path will remain straight unless some force acts upon it. So, if wind is our object, what are the forces making it go, stop, and change direction?
Leading the Core 4, uneven heating around the globe, creates the PGF and is the primary wind maker, and destroyer, on all local and global scales. According to Newton’s Laws, this would be one force moving and stopping air flow. For kiters, this force is important between two neighboring geographic areas. The greater the temperature differences, the greater the air flow. When these areas start equalizing, winds greatly slow down.
Regarding straight lines, Newton would call the Coriolis Affect (AKA Earth’s rotation) the second force. It influences wind and other moving objects in very predictable ways. In the Northern Hemisphere, wind is deflected to the right of its path, while in the Southern Hemisphere it is deflected to the left. Coriolis force is absent at the equator, and its strength increases as one approaches either pole. It only acts on air when it has been sent into motion by the PGF. And, while this force influences wind direction, it has little affect on localized wind speeds.
Core 4 Continued
The third force affects both wind direction and speed. A bit technical, it’s known as Centripetal Acceleration, and only acts on air circulating around pressure centers. If you consider an object moving in a circle, the speed may remain constant, but the direction is changing constantly. Therefore, there is a net force being applied because there acceleration is needed to change direction.
A byproduct of Centripetal Acceleration is Centrifugal Force (pressure toward the outside of a turn). Simply, the faster the wind and tighter the travel curve, the larger the Centripetal Force as wind speeds accelarate towards the turn’s center keeping the air moving in a circle. Got it? In layman’s terms, Centripetal Force is responsible for helping generate high winds circulating in hurricanes and tornadoes.
If the world was flat and smooth, there would be little wind variation from place to place. But by adding topography (hills, valleys, trees, buildings, etc), a complex and highly variable wind regime exists. Of the Core 4, Mr. Newton would call this last force the most influential for kiters. Known as Surface Friction, it affects both wind speed and direction. Rough surfaces, like areas with trees and buildings, will produce more friction and turbulence than smooth surfaces such as lakes or open cropland. The greater friction means the wind speed near the ground is reduced.
Friction can only affect wind after air is in motion. While buildings, hills, valleys, trees and mountains typically create drag, each geographical feature influences wind flow in certain ways. We say typically, because in certain conditions, high terrain and valleys can actually increase wind speeds (more on this later). While Newton could rave on about frictional forces, they are limited to winds at the Earth's surface.
Localized Wind Laws
Of the Core 4 forces influencing wind, the PGF is the main driving power. The other three forces only come into play when the wind begins to blow. Regarding your local area, heat and topography have the most influence at any kite beach. They pretty much dictate winds by affecting local climate and wind patterns. To help you have more great sessions, here’s a few general laws you should apply before and during your ride.
Time Creates Heat Both time of year AND day drastically affect winds. Wind speed usually increases during the daylight hours as the sun heats the environment. In transitional months (April – May & October – November), you’ll likely find greater temperature differences resulting in greater pressure differences (more wind).
air triva April & November = WIND!
Wind energy plants report maximum poer output from wind generally occurs in the transition months of April & November. According to a University of Minnesota Study on Wind Power.
Clouds Kill Wind You’ll typically find wind speeds decreasing as clouds move in, killing the heat. If you’re in a thermal driven area and see a cloud line approaching, your winds will likely weaken significantly.
Convection is Dangerous Also known as thunderstorms, convection often produces strong, shifting winds as air rises and sinks quickly. You should always avoid riding since winds can suddenly change direction and/or speed without notice.
Fronts = Change These pressure systems moving in result in air mass changes. If there’s a quick rise or fall in pressure due to an approaching warm or cold front, strong winds may appear.
Trees & Buildings Are Bad These are the most common wind obstacles in potential kite launches. They disturb air both upwind and downwind by reducing wind speed and increasing turbulence.
Valleys can be Awesome When oriented parallel to wind flow, valleys may channel and enhance wind speeds by funneling air through a smaller area. This is often the case in narrow mountain passes or gaps facing the wind. BUT, being lower and sheltered, they can have lower wind speeds as cool heavy air drains down from higher terrain. This cool air layer results in the calm lowlands.
High Terrain Swings Both Ways Hills, Mountains, Plateaus and Bluffs, can drastically reduce wind speeds and add evil turbulence. However, Hills and mountains can also accelerate wind flow. An approaching air mass is often squeezed into a thinner layer so it speeds up as it crosses the summit. Isolated hills and mountains may accelerate the wind less than ridges because more of the air tends to flow around the sides. The downward, or "lee," side of high terrain features should be avoided because of the presence of high wind turbulence.
When Forces Balance
The moral of all this scientific kiter craziness: understanding the forces influencing wind is critical. Because winds are naturally weaker than their creators, these forces can balance. They make it possible for winds to sustain at nearly steady speeds for long time periods.They are souly responsible for making your session epic. Or, destroying winds all together. The following breiefly explains a few key balancing acts your kite will love. Studying them will help prepare you for Wind Part III: Observing Winds, in KiteWorld’s next issue.
Geostrophic Balance When the pressure systems exist for long periods, particularly for large-scale weather systems, a balance is achieved between the PGF and the coriolis force, called geostrophic balance. For the coriolis force to balance the PGF, the wind must blow sideshore, with high pressure to the right and low pressure to the left. That way, the horizontal pressure gradient is trying to accelerate the air to the left, and the coriolis force is trying to accelerate the air to the right. At just the right wind speed, the two forces cancel and the air blows steadily.
Gradient Balance Either surface friction or the Coriolis could balance the PGF. In either case, the stronger the wind, the stronger force. When wind continues to accelerate until friction comes into balance with the PGF, this is called Gradient Balance. The awesome part is that acceleration ceases and steady winds prevail!
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