Pressure Gradient

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A pressure gradient will form when there is a difference in air pressure between two points in the atmosphere. As a rule, fluids – including air – naturally move from areas of high to low pressure, in an attempt to equalize the imbalance. This flow of air is known as wind, and the force pushing the air from high to low pressure is known as the “pressure gradient force”.

This image shows an idealized flow around high and low-pressure centers. As air attempts to flow away from high pressure, and toward low pressure (blue arrows) in the Northern Hemisphere, it is curved to the right of its path by the Coriolis force (yellow arrows). Friction will oppose the pressure gradient force and Coriolis slightly. This will continue until the pressure gradient, friction, and Coriolis forces are completely balanced, which directs the wind to spiral slightly inward toward the low pressure/away from high pressure. While other, real-world factors can alter these flows, this is the overall resulting flow of wind.

The pressure gradient force drives the wind, and it is directly proportional to the difference in pressure over a given distance. The greater the pressure difference, the stronger the pressure gradient force and the faster the wind will blow. In the Northern Hemisphere however, the Coriolis force – a force generated due to the turning of the spherical Earth - pushes moving air to the right of its path. However, as this wind is moving over rough surface terrain, friction also reduces the speed at which air can move from one place to another. Observed wind patterns are the result of the equilibrium between these forces.

The result is a spiraling wind motion around the pressure centers, largely following isobars, but not perfectly. Winds spiral slightly inward around a low, and slightly outward around a high. The frictional force is present due to the wind being in contact with the earth’s surface, and as one moves up through the atmosphere, friction disappears, and winds will follow the isobar pattern more exactly.

Image Source: Environment and Climate Change Canada

The density of isobars (lines of equal pressure) represents the rate of change of pressure across an area. Lots of isobars close together is indicative of a large change in pressure over a small region, which generates a larger pressure gradient force and therefore stronger winds to attempt to equalize pressure.

High-pressure systems are generally broad and have a small pressure gradient. Conversely, low-pressure systems have smaller centres and therefore much larger pressure gradients (denser isobars), and generally stronger winds. The extreme case is a hurricane, with very low central pressure, a high-pressure gradient (tightly packed isobars), and dangerously strong winds.

This surface analysis is a demonstration of pressure gradient over Canada. Isobars are typically less densely packed around a high-pressure center, while they become closer together around a low-pressure center.

Image Source: Environment and Climate Change Canada

This is a simplified view of wind forced through valleys and how they interact. When several air streams meet and enter a narrow valley, air speed will increase, and may be associated with updrafts. Conversely, when air streams are split into several valleys, the wind speed will decrease, and can cause downdrafts.

When a pressure centre gets closer to another (a low moving toward a ridge, for example), the pressure gradient between them will increase significantly, increasing the winds dramatically.

In valleys and complex, mountainous terrain, however, pressure gradients can cause a very different effect. Winds in such complex terrain will typically follow the valley terrain and are more dependent on the direct pressure gradient forces (meaning that air will flow more directly from high to low pressure). This effect is strongest when pressure gradients lie exactly perpendicular to the valley and have large differences in central pressure.

To complicate matters further, in terrain with interconnected valleys, winds will constantly be separating, and crashing together in locations where valleys split and join. When wind splits into multiple valleys, the speed is reduced (as the force is divided) and can cause downdrafts. Conversely, when winds converge and move into a single valley, the wind speed increases (force of several streams combines), which can also cause updrafts. And naturally, as winds are traveling through terrain in a different direction than winds aloft (above terrain), there can also be significant wind shear moving into and out of valleys.

Image Source: Environment and Climate Change Canada

Lastly, when such wind is channeled through a narrow gap or passageway (such as through small gaps in mountain ranges, or where the Saguenay River meets the St. Lawrence River), it can be suddenly accelerated as it passes through, making a localized, but extremely dangerous change in wind speed. This phenomenon is known as a “gap wind”.

This image is a simplified depiction of gap winds.

A) Winds converge at a narrow opening in terrain, creating a local area of high pressure on the windward side.

B) Wind accelerates as it pushes through from the local high pressure at A, toward the relatively lower pressure region at C.

C) Accelerated winds escape the narrow opening and begin to slow, causing a very local, dangerous jet downstream of the opening in terrain.

Image Source: Environment and Climate Change Canada

Here is an example of dissipation of wind channeling. When pressure centers move, and are no longer on either side of terrain, forcing wind through valleys, the surface winds in the valley will shift to follow the prevailing winds caused by the circulation around the pressure centers, as opposed to being pushed directly from high to low pressure.

Dissipation

A pressure gradient will begin to weaken when either the low or high-pressure system loses its upper-level support and begins to dissipate (low-pressure centers fill in, and pressure rises; high-pressure begins to dissipate and central pressure falls). Weakening central pressures allows for the pressure pattern over a larger space to equalize and reduces the force of the push from high to low pressure.

A pressure gradient will move with the overall movement of the pressure centers. In this way, pressure gradients laid over terrain that cause wind channeling through valleys and lower terrain will dissipate when the pressure gradient moves away or shifts to no longer be perpendicular to the terrain. This example shows the dissipation of wind channeling. When pressure centers move, and are no longer on either side of terrain, forcing wind through valleys, the surface winds in the valley will shift to follow the prevailing winds caused by the circulation around the pressure centers, as opposed to being pushed directly from high to low-pressure.

Duration

Pressure gradients will last as long as there remains differences in pressure over the Earth’s surface but vary in extremity. So, while they never truly diminish, their strength and lifespan are directly tied to the strength and lifespan of the pressure systems causing them, as well as their location.

Pressure gradients encircling a pressure center can last up to several days at a time, until the centers begin to weaken and dissipate. Pressure gradients over terrain causing wind channeling can last several hours and up to a few days, depending on how long the high and low-pressure system remain on either side of the terrain in question.

Image Source: Environment and Climate Change Canada

There’s no particular climatology related to pressure gradients in general, since they can happen anywhere and span huge regions, however there are a few regions that are more likely to get wind channeling.

Deep valleys through which winds can be forced are prime locations for wind channeling to occur, such as narrow straits, fjords (BC coast, Newfoundland, etc.), long river valleys, or mountain valleys (Rocky Mountains, Long Range mountains, etc.).

Wind channeling through valleys can be set up either by a large pressure change from one end of topography to the other (eg. a H and L on opposite ends of a large river/water body, or on either side of a mountain range), or prevailing winds being forced into and through terrain.

There is sometimes a challenge in forecasting these persistent ridges, and the resulting blocking pattern often leads to very tight pressure gradients and very strong winds between approaching lows and quasi-stationary highs.

In the case of wind channeling, over large surfaces the effect can be fairly easily accounted for, but in mountainous regions where terrain is incredibly complex and data is extremely sparse, it can be difficult to adequately describe and verify the extent of the valley winds. While determining the directional shear is not so difficult when the orientation of a valley is known, the extremity of the speed difference can be very difficult to forecast, especially with limited observations.

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These two GFA panels, valid at 1200Z on April 5 forecast an intense low-pressure center over Wisconsin and a neighboring strong high-pressure center over central Quebec. Forecast pressure differences between these two systems, as shown in the science explained section, predicts the presence of a significant pressure gradient, shown both in the close proximity of isobars and wind barbs in the clouds and weather panel, indicating expected strong sustained winds and gusts.

By 1200Z on April 6, the low center is forecast to be over James Bay, while the high-center should have moved into northern Labrador. The strongest pressure gradient would then remain east, north, and west of the low, while over southern Quebec, isobars are expected to spread out, which would result in a much weaker pressure gradient. Resultant winds in that region would therefore likely be weaker than those closer to the low, where isobars are expected to be packed more closely together.

The GFA panels at 1200Z on April 5 forecast a strong pressure gradient lying perpendicular across the Saint-Lawrence River valley in Quebec, which is oriented NE-SW. For CYUL, nestled in the valley, this type of forecast translates into the possibility of strong northeasterly winds (as winds channel from high to low pressure through the valley) as noted at 2200Z. The low was expected to move to the northward, slackening the pressure gradient over the river valley and the airport, which would cause winds to weaken in response. By 0600Z on April 6, winds were expected to remain from the north-northeast but weaken to 10KT sustained, followed by a shift to the southwest with the passage of the cold front around 1200Z on the 6.

This RDPS numerical forecast data for April 5 depicts expected precipitation type and intensity from April 05th at 0000Z to April 6 at 1800Z, as well as the forecast trajectory of the low-pressure system. The model predicts a very strong pressure gradient across much of Ontario and Quebec, which coupled with the presence of a low-level jet (as indicated in the GFA), drives the forecast for strong and gusty surface winds shown in the GFA and TAF.

Image Source: Pivotal Weather

METARs show wind data at a singular point within the larger wind field. While looking at graphical isobars will provide insight into what is driving both wind magnitude and intensity, winds in the surface observations from April 5-6, 2023, capture the results of changes in the magnitude of the pressure gradient in the vicinity of CYUL. Through April 5th, the observations show strong and gusty northeasterly winds flowing from high to low pressure and accentuated by the large difference between the strength of the anticyclone northeast of the airport and the low-pressure system to the southwest. As the low progressively moves eastward, the pressure gradient both changes orientation and weakens over the CYUL area, which results in weakening winds. By 1030Z on April 6, 2023, winds have diminished to 5KT.

Image Source: OGIMET

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This CMC surface analysis shows observed conditions at 1200Z on April 5, including isobars and frontal systems. Specific to this example, the strong synoptic low centered over Wisconsin and strong anticyclone (high) over east-central Quebec set the stage for an intense pressure gradient as the changes in pressure between the low and high are significant. This is shown by the proximity of isobars west of the high and east/north of the low. As demonstrated in other sections, the increased pressure gradient supports strong surface winds, which will remain in place until the pressure gradient weakens.

Image Source: Environment and Climate Change Canada

The CMC surface analysis of conditions at 1200Z on April 6th shows the synoptic system still impacting Ontario and Quebec. However, the pressure gradient over southern Quebec has weakened considerably, shown by how much further apart isobars are from one another in the area, and is now oriented roughly parallel to the Saint-Lawrence River Valley. This change would translate to weakening winds expected at the surface in that region.

Image Source: Environment and Climate Change Canada

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