Define wind and also why that occursUse u, v, and also w to describe motionDescribe the five physical pressures that have the right to act top top a thoreau of airDraw pressure diagrams because that geostrophic wind, gradient wind, and wind in the atmospheric boundary layer (ABL)Compute the rate of a geostrophic wind, and also be able to qualitatively estimate the wind rate from isobars
Map of wind barbs and isobars (Public Domain).Introduction

Wind is the movement of air relative to the earth’s surface. Just like all relocating things, that is brought about by a force acting top top it. Pressure is a pull or press that alters the resting state, motion, or direction of an object. Pressure can additionally cause objects come accelerate.Human skin have the right to sense wind once an uncountable number of molecules collide v us together they circulation along in the air, and also sense the pressure transforms in the air flow.

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Main forces

There space five forces that influence the rate or direction that horizontal winds.

Pressure gradient forceAdvectionCentrifugal forceCoriolis forceTurbulent drag

Remember from chapter 1 that according to the Cartesian coordinates, x points east, y points north, and z clues upwards. To define wind, we usage wind components u, v, and also w which correspond to the x, y, and zdirections. These wind contents are used in equations of movement used come predict wind come designate direction in a three-dimensional plane. We’ll go v each of the 5 forces one by one to discuss how they impact wind rate and/or direction.

Pressure Gradient Force

A pressure gradient(PG) is a change in push over a distance.Therefore, the systems of the push gradient pressure are Pascals every meter (P m-1). The press gradient can be calculated merely as the readjust in pressure separated by the distance over which that readjust occurs. The size or strength of the press gradient determines the dimension or strength of the force that outcomes from it.


The pressure gradient force(PGF) is a force from high to low push over a distance. Without differences in pressure, there would certainly be no wind since there would certainly be nothing to advice airflow.In chapter 9 us learned about isobars, or lines of continuous pressure. When isobars are tightly packed, we understand that over there is a strong pressuregradient or huge change in push over a fairly short distance. A strong pressure gradient outcomes in a huge pressure gradient force and, together we’ll see, greater wind speeds. Relying on the message you read, you’ll see a variety of different creates of the push gradient force. Two equivalent representations are provided below.


The PGF together defined over is just a readjust in pressure split by the distance and also air density. This provides the force per unit mass, for this reason PGF/m. Above we specify PGF in the x-direction, but it deserve to act in any type of horizontal plane. The devices of the PGF will be in systems of acceleration over because pressure = mass * Acceleration, but since we’re separating the left-hand next by fixed the systems will be m s-2.The an unfavorable signs in the over equation is as result of the reality that the PGF acts indigenous high pressure to short pressure.

Another equivalent way to stand for the PGF is together follows.


Because Mass = Volume * Density, us then watch that Volume = Mass/Density. The 2nd equation is the same to the first, other than thatMass/Density is on the right hand side in the kind of Volume rather of gift split between left and also right political parties of the equates to sign. Also, the variable L is provided for length or distance instead ofΔx so the it isn’t for a specific direction. Similarly, the absolute value of the PGF is calculated, and also the direction is determined, through the place of High and also Low pressure systems. Typically in class we won’t calculation the PGF, just the PG. However, it’ll be widely supplied in force balance equations.


While advection isincluded in the list of forces, it is actually no a true force. Still, advection can an outcome in a change of wind speed in part locations. Wind relocating through a suggest carries certain momentum, i m sorry is defined as momentum per unit mass. Recall that momentum is mass time velocity. Certain momentum climate is just equal to the velocity or wind speed. Therefore, as wind moves by a point, the wind can move or advect sport in winds to the resolved location.

Centrifugal Force

The centrifugal force is an evident force that contains the impacts of inertia for winds relocating along a curved path. The directionality the the centrifugal force points outward native the facility of the curve. The centrifugal force is the contrary of the centripetal force. As we know, inertia is the physical tendency to continue to be unchanged. Therefore inertia causes an air parcel to “want” to move along a right line. Turning the waiting parcel follow me a bent path needs a centripetal pressure that traction inward come the facility of rotation. Together a result, a net imbalance the other forces occurs.

You have actually felt the centrifugal force numerous times in her life. The centrifugal pressure is conveniently felt as you take trip in a moving vehicle about a corner. The force that you feel pulling girlfriend outwards is the centrifugal force.

Coriolis Force

The Coriolis force (CF) is another apparent pressure that occurs due to the rotation of Earth. The Coriolis force is a deflecting force. That acts just on objects currently in motion. Thus it cannot develop wind, however it can readjust the wind direction by deflecting it. The Coriolis pressure acts perpendicular come the direction of motion, but whether the Coriolis force acts 90° come the best or left that the motion vector depends on the hemisphere top top Earth. In the northern Hemisphere, the Coriolis force acts90° come the appropriate of the motion vector while inthe southern Hemisphere, the pressure acts90° to the left of the activity vector. The equation below gives the Coriolis force


where m is because that the mass of the object in kg, uis the speed of the thing in m s-1, the symbolΦ denotes latitude in degrees, andthe angular rotation rate, Ω, is found from the rotation rate of Earth. Earth turns 2*pi radians over 24 hrs, therefore 2*pi/24 hrs givesΩ = 7.27E-5 radians·s–1. Based upon this, we can see the the Coriolis parameter will certainly be 0 at the equator once sin(0)=0 and also maximized at the poles as soon as sin(90)=1.

We can also see that the Coriolis force is strongly dependent on the speed of the object. If us assume the “object” is in reality wind, stronger winds will certainly be an ext strongly deflected by the Coriolis force.

Turbulent Drag

Turbulent traction occurs as soon as Earth’s surface or objects on it cause resistance to airflow and also reduce the wind speed. Any kind of object ~ above Earth’s surface can cause drag, such together grass, trees, and also buildings, i m sorry block and also decelerate wind. The bottom class of the troposphere about 0.3 come 3 kilometres thick is called the atmospheric border layer (ABL). Turbulence in the ABL mixes the incredibly slow movement of air close to the surface ar with the much faster movement of wait in the ABL and also slows the wind rate in the whole ABL.

Force Balances

The 5 forces from above affect aspects of horizontal wind speed and also direction, and result in a variety of common pressure balances found throughout earth atmosphere.

Geostrophic Balance

Geostrophic balance is maybe the most important force balance in the atmosphere and holds practically all the time, except for a couple of specific instances scenarios to be discussed later. As soon as in geostrophic balance, wind in the atmosphere has a balance in between the press gradient force and also the Coriolis force. In geostrophic balance, PGF = CF. The resulting wind is referred to as a geostrophic wind. Setup the equation because that CF and also PGF same to each other and solving because that u provides the adhering to equation for Ugeos.


Because geostrophic winds room dependent top top the press gradient, geostrophic winds are faster when isobars are very closely spaced.

A variety of assumptions are implicit come geostrophic balance. Geostrophic balance uses only under the complying with conditions: big temporal (>12 hrs) and large spatial (> a couple of km) scales; over the ABL once no surface friction is exhilaration on the air; winds space steadily relocating in a straight direction (no acceleration, negligible vertical velocity); finally, because the Coriolis pressure is essential for the balance, the cannot organize at the equator as soon as the CF is 0. The typical bounds are frequently given together >2° latitude.

The course of the geostrophic wind is parallel to the isobars. In the north Hemisphere, the wind direction is parallel to the straight isobars through the low press to the left side of wind. In the southerly Hemisphere, the direction is parallel to the directly isobars v the low press to the wind’s right. The image below shows the pressure balance present in a geostrophic wind in the north hemisphere.

Geostrophic wind pressure diagram in the north hemisphere (Image created by Shintaro Russell via

To get into geostrophic balance, moving air will undergo geostrophic adjustment. First, wait feels the pressure field (PGF) and begins moving from high to low pressure. Next, the Coriolis pressure (CF) deflects the object’s direction when it is in motion. Finally, the air find itself in a balance in between the PGF and the CF relocating parallel to the isobars rather of throughout them.

Gradient Wind

This next pressure balance uses when waiting is not relocating in a straight line.Gradient winds space winds flowing follow me curved isobars. Winds generally blow follow me isobars, also if they are curved, yet a different name is needed due to the fact that the force balance contains one more component. Contrasted to geostrophic winds, gradient winds function a balance between the Coriolis force, the pressure gradient force, and the centrifugal force. The centrifugal pressure arises since the waiting is flowing on a curved path. The centrifugal pressure acts in the same direction as the coriolis force, opposite the pressure gradient force.

Gradient wind force diagram (Image developed by Shintaro Russell via

Atmospheric boundary Layer

Balanced wind in the atmospheric boundary layer (ABL) occurs when there is a balance between the press gradient force, Coriolis force, and also the frictional drag force. Both wind shear turbulence and also convective turbulence reason drag, which outcomes in the ABL wind gift slower than geostrophic (subgeostrophic), and causes the wind to overcome isobars toward the low pressure.

Atmospheric border layer wind pressure diagram (Image developed by Shintaro Russell via

Again, the frictional drag force acts in the plane of motion and also slows down the wind speed. The press gradient pressure doesn’t change, but due to the fact that the wind speed is slower, the Coriolis pressure is weaker. When that wake up the wind can not balance the push gradient force, the is pulled more by the push gradient force, and also turns toward the short pressure.

Cyclostrophic Wind

Cyclostrophic wind occurs at smaller sized cyclonic scale (at the mesoscale) such together tornadoes, waterspouts, and also even the center of a tropical cyclone. Since the range is small, the Coriolis pressure does not play a role. Once a small cyclonic range such as a tornado first forms, both tangential winds and centrifugal pressure increase much faster than the Coriolis force because of the very strong pressure gradient force. Together a result, centrifugal pressure balances v the press gradient force, ignoring the negligible impacts of Coriolis force. Since the range is small and independent of the Coriolis force, the direction of cyclostrophic winds deserve to be either clockwise or counterclockwise in both hemispheres. Because that anticyclones or highs, however, they execute not typically have solid pressure gradients. Thus, winds approximately the high are too weak to be in cyclostrophic balance.

Cyclostrophic wind force diagram wherein the push gradient force is well balanced by the centrifugal pressure (Image produced by Shintaro Russell via

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All of the wind balances debated (geostrophic balance, gradient wind, ABL wind, and also cyclostrophic wind) take place in Earth’s atmosphere under differing conditions. The adhering to chapters will make this applications clearer and also you have the right to check ago here for reference.