How does friction affect wind? You may have heard about the power of friction but how does friction affect wind direction?
The wind is an important factor in the development of climate and ecosystems on Earth.
It occurs naturally when air molecules flow from a region of high pressure to a region of low pressure.
Friction creates resistance by acting as a counterforce to the motion of the wind. But exactly how does friction affect wind speed and direction?
When wind meets resistance from the earth or other barriers, it slows down and is deflected in a different direction, often going around or even over them.
How Does Friction Affect Wind?
Friction is a force where the motion of one object resists the motion of another. Friction influences the winds.
Winds are formed as areas of Earth’s surface heat up unevenly and have different air pressures.
But topographical features on the ground cause frictional drag between the surface and the air above causing air currents to change direction and speeds.
The global winds form a cycle with winds blowing from areas of high pressure to areas of low, resulting in weather patterns where the air currents meet at fronts.
These weather patterns can be characterized by the different winds they generate.
Fact: Friction levels between different types of terrain can affect wind velocity and direction.
The Impact of Friction on Wind Speed
Differences in ground surfaces affect how much friction is exerted.
Wind blowing over a flat surface like the ocean has nothing to negotiate around and slows it down.
This is unlike wind passing through hills which is forced to change course, losing energy to frictional drag whenever it meets an obstacle.
Slowing Down of Wind due to Friction
Earth has various wind zones, all within the boundary layer of the atmosphere that ends one or two miles above the surface depending on the terrain.
These wind zones include the following:
- The polar easterlies
- The westerlies
- Horse latitudes
- Trade winds
- Doldrums
Friction is believed to be an electromagnetic attraction between charged particles in the atmosphere and negative particles on the surface.
It acts to slow the wind down by pulling on atoms creating drag.
The effect creates turbulence as the winds slow and change direction when above, where the topography of the land has less impact, the winds circulate unimpeded although air pressure and temperatures still exert influence.
Decreased Wind Speed Closer to the Earth’s Surface
Winds move faster higher up because they are subjected to less friction.
Higher up, there are far fewer obstacles to impede the wind’s progress than lower down, nearer the surface.
The more obstacles there are, the greater the friction and the greater the disruption to the flow of the current, varying its speed and altitude as it collides with objects.
The turmoil forces air currents to react with one another resulting in storms in certain conditions.
Relationship between Wind Speed and Friction
Friction will be low over a frozen lake but high in a city or forest. The more friction, the less speed the wind generates, which is measured as wind shear.
Wind shear is measured both horizontally and vertically since air currents can be forced up or down.
Wind shear is instrumental in forming weather patterns, driving the movement of air from areas of high pressure to low.
Near the surface, friction reduces the wind speed, which reduces the Coriolis force, a force generated by the Earth’s rotation.
It has a weak to zero effect at the equator allowing winds there to move faster.
Fact: Hurricanes and tornadoes are two examples of extreme weather patterns caused by friction-driven wind spirals.
The Impact of Friction on Wind Direction
Above the boundary layer, the only forces affecting wind direction are the Coriolis forces forcing it right in the Northern Hemisphere, and differences in air pressure.
Below, nearer ground level, the winds encounter various irregularities and slow down.
As the speed decreases, the Coriolis effect decreases.
However, the air pressure difference, the pressure gradient, stays the same so the winds are driven more strongly in the direction of the low-pressure areas.
Fact: The effects of friction vary according to the terrain so that in hilly areas the winds may be deflected by as much as 300 from its original course.
Deflection of Wind Due to Surface Friction
The Coriolis force deflects the wind so they blow to the right, counterclockwise, in the northern hemisphere and the left in the southern.
Therefore, the wind flow around low and high-pressure systems circulates in opposite directions. The Coriolis effect influences global wind patterns.
With no Coriolis effect, the air would only flow from areas of high pressure to areas of low pressure and not around the globe.
You can see the effect by throwing a ball from a roundabout.
From the roundabout, the ball appears to take a curved line although it looks like a straight one to someone observing.
The Ekman Spiral and the Direction of Wind
The Ekman spiral is a theoretical model put forward by oceanographer Vagn Walfrid Ekman in 1902.
It attempts to account for the currents in bodies of water produced by the winds blowing over them.
Ekman proposed that when water molecules are moved by the wind blowing across the surface of the water, they drag the water molecules into the deeper layers with them.
Fact: The Ekman Spiral effect decreases at depth and stops completely at about 330 feet (100m).
The Impact of the Ekman Spiral
Deeper water is deflected by the Coriolis effect, with each successive layer moving right or left more slowly to generate a spiral of water movement.
We can observe the effect when a tub of water has the plug pulled out.
The speed of the spiraling water decreases with depth, shifting the current’s angle to the wind, which is 450 at the surface, until at Ekman depth, the current flows in the opposite direction.
Influence of Coriolis Effect and Friction on Wind Direction
In rare conditions, the Coriolis effect produces geostrophic winds that blow parallel to the isobars along the edges of high- and low-pressure systems.
They only happen when the Coriolis force and the pressure gradient are in balance.
As the air moves down the pressure gradient from high to low, depending on the steepness of the pressure gradient, it gains speed.
Here, the deflection is greater until the Coriolis force is equal to the pressure gradient force.
What Other Factors Affect Wind Direction and Speed?
Friction certainly has a role to play in changing wind speed and direction, but other factors may have an impact as well.
For instance:
Pressure Gradient Force
Nature dictates that regions of excess move toward regions of deficit so air flow always moves from high to low pressure in an attempt to equalize.
The amount of difference is termed the pressure gradient.
The steeper the gradient, the faster the wind speed as the current moves to reach equilibrium.
Temperature
Differences in temperatures also create gradients.
The greater the difference the stronger the wind as warm air rushes into the cold air area.
The wind direction and temperature are good weather predictors.
When winds blow from the south, warmer air is approaching, whilst from the north means cold air is coming.
Altitude
When air currents are close to the surface of the earth, they encounter obstacles and are slowed by friction and deflected by the Coriolis effect.
Winds that are high in the atmosphere over 2000 feet are free of effects of friction.
They travel much faster sometimes as a jet stream.
Topography
Topographical features like mountain ranges influence how the atmosphere circulates and so do oceans.
There is less friction over water and through the exchange of heat and water vapor and converting solar radiation to kinetic energy, winds are lighter and less dense.
Also, they are able to travel faster until they make landfall.
Takeaway
How does friction affect wind? Near the surface of the Earth, friction is a major factor in how fast and which way the wind blows.
Drag is the result of the resistance from the ground surface and topographical characteristics, which causes wind currents to change direction and lose energy.
Aside from friction, pressure gradient force, temperature, altitude, and topography can also affect wind speed and direction.
Knowing how friction affects wind is important for forecasting weather as well as the evolution of Earth’s climate and ecosystems.
