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You are watching: Which of the following is the force that initially causes the wind to blow?

After completing this section, girlfriend should be able to describe the main pressure that create the wind (the pressure-gradient force). You should likewise be able to identify the direction the the push gradient force given a map that isobars, and also qualitatively called the stamin of the pressure gradient force to the rate of the wind


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The an initial step in evaluating the wind direction and speed in ~ a offered location is to identify all of the forces that pat a part in relocating the air. So, let"s start with the most an easy question: what force causes the wait to move horizontally in the an initial place? In other words, what causes the wind come blow? together you can have guessed, since we"ve been discussing atmospheric pressure, the factor that air move horizontally is pertained to pressure. Specifically, differences in pressure throughout the globe an outcome in a force, referred to as the "pressure gradient force" the sets wait in motion. Let"s explore.

The Pressure-Gradient Force

Recall that a low pressure mechanism is a "lightweight" (the wait column over the center of a low weighs much less than any kind of of the surrounding air columns) and a high pressure mechanism is a "heavyweight" (the air column over the facility of a high weighs more than any kind of of the neighboring air columns). But, as we also learned, the sea-level pressure difference between a fairly strong high-pressure system and also a solid low-pressure mechanism usually isn"t much much more than about five percent. Still, it"s this contrast in sea-level pressure (difference in tower weights) in between highs and lows the drives the wind.

To see what ns mean, let"s execute a an easy experiment. A Plexiglas container (pictured below) has two compartments separated by a removable partition. There"s much more water in the left compartment 보다 there is in the one ~ above the right, translating to a greater weight that water top top the left than on the right. Thus, there"s greater water push on the bottom of the left compartment 보다 on the bottom of the ideal compartment. If I eliminate the partition, there"s a flow of water from higher pressure to reduced pressure. In other words, the water, originally at rest while the partition remained in place, increased from rest when I removed the partition.


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When the obstacle is removed between the two water columns (frame 1) the water flows from the higher column to the reduced column since of the push gradient pressure (arrow, structure 2). The result (frame 4) is two columns v equal heights and also thus no pressure gradient.

Isaac Newton"s 2nd law of motion states that as soon as a net pressure is used to an object, that accelerates; therefore, over there must have been a net pressure acting on the water in bespeak to set it right into motion. In this case, the catalyst pressure was the pressure-gradient force, which action from greater pressure toward lower pressure.

If the amounts of water in each compartment differ by a smaller sized amount, climate the pressure-gradient pressure (PGF) is lot smaller because the weights that the water in both compartments begin out almost the same. V a smaller pressure-gradient force, the flow of water will be lot slower. Thus, we arrive in ~ the adhering to result: The magnitude of the pressure-gradient force (represented through the difference in water pressure throughout the partition in this experiment), dictates the speed of the circulation of water.

In a sense, the environment is like an "ocean the air," and also switching our conversation from water to air provides the very same result. Recall the the gradient of an atmospheric variable actions the distinction over a offered distance. So, if we"re talking about the pressure gradient, we"re measure the difference in push over a certain distance. Exactly how do us assess the size of the press gradient? You might recall the the sample of isobars tells united state how big or tiny the pressure gradient is:

If isobars space packed very tightly (close together), that way there"s a relatively large pressure adjust over a addressed distance, for this reason the press gradient is large.If isobars space packed loosely (far apart), that way there"s a relatively small pressure change over a addressed distance, so the push gradient is small.

I need to note that, mathematically, the worths of most sea-level press gradients it seems ~ small. Also tight packings that isobars (equating come a "strong" pressure gradient force) ultimately amount come a adjust of a tiny portion of a millibar per mile. Yet, as soon as isobars space tightly packed, wicked winds deserve to blow! because that example, inspect out the seal-level pressure gradient approximately the extremely strong 944-mb low-pressure device over the Bering Sea on November 9, 2011 (see 06Z surface analysis below). The pressure-gradient pressure caused winds to really whip, as you can tell native this YouTube video from Nome, Alaska (winds gusted end 50 miles per hour for number of hours).


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The 06Z surface evaluation over the Bering Sea and Alaska ~ above November 9, 2011. At the time, a deep low-pressure system, v a main pressure of 944 millibars, to be generating very strong winds east of the low"s center.

So, identifying locations with a fairly strong pressure-gradient pressure is as an easy as finding locations on a map of sea-level press with whereby isobars room packed closely together. But, in what direction does the pressure-gradient pressure act? ~ above maps the isobars, the pressure-gradient force is always directed perpendicularly to the isobars, and is command from high to low pressure.

To check out what ns mean, check out the idealized surface weather map below. Ultimately, the pressure-gradient pressure has both a magnitude and also a direction, therefore it"s drawn as a vector the points from high to low pressure. Note how the orientation the the vector changes depending on the orientation the the isobars, but the pressure-gradient force always points perpendicular to local isobars, from higher toward reduced pressure. The size of the pressure is shown by the length of the vector; note just how the vector is longer where the isobars room packed closer together, denote a stronger pressure-gradient force.


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The pressure-gradient force (PGF) is a vector i m sorry points from higher pressure to lower pressure while crossing isobars in ~ a ideal angle. That magnitude depends on the push gradient, i m sorry is a measure up of the spacing between isobars.

See more: Which One Of The Following Is Not True About Mechanical Weathering?


So, as soon as isobars are packed closer together, the wind have to blow quicker as the data native the extreme low over the Bering Sea top top November 9, 2011, indicate. The 12Z surface evaluation from April 30, 2011 provides another great example. First, keep in mind the 40-knot continual winds over east Montana and also western phibìc Dakota contrasted to the 10-15 knot continual winds over eastern North Dakota (where the isobars are packed an ext loosely). You must also notification that the winds aren"t blowing directly perpendicular come the isobars, from greater pressure to lower pressure. What"s up v that?

The wind would blow directly from higher to lower pressure if the pressure-gradient pressure was the only pressure acting on the air, but that"s no the case. We need to account for other forces also when trying to evaluate wind direction and speed. We"ll examine an additional one of those pressures up next. Review on.