Meterology lessons

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N.B. I wrote the majority of the following posts during late 2016 and early 2017.

The Jet Stream (at least the one which affects the UK and countries on a similar latitude) is a band of fast-moving air located roughly 30-40,000 feet above the surface, around the tropopause (the divide between the troposphere and stratosphere, respectively the lowest and second-lowest layers of the atmosphere). Globally, there are actually four jets - each hemisphere (northern and southern) has a polar jet (the one that concerns us) located in the upper-mid latitudes, and a weaker subtropical jet much closer to the equator. For the avoidance of doubt, in this thread, any mention of the jet is a reference to the northern hemisphere polar jet. It flows from west to east, and its velocity and path varies depending on global factors that I'll mention at a later date.

Jets are caused by circulations of air rising from near the equator (warm air rises above colder air as its density is lower). This air drifts northwards at altitude, and, as it cools, it sinks towards the surface, then flows southwards again. What converts this into a westerly flow is the Coriolis Effect, which is an apparent force caused by the fact that the circumference of the earth is largest at the equator and drops to almost nothing at the poles. Quoting from the follwing site:

http://www.theweatherprediction.com/habyhints/27/


The Coriolis force is an apparent force. From an earth observer, it is an apparent curving of a wind flow. The earth spins counterclockwise when viewed from the North Pole and clockwise when viewed from the South Pole. Therefore, the Coriolis deflection is the opposite in the Northern Hemisphere as compared to the Southern Hemisphere. If you stand on the North Pole, your body will make a complete counterclockwise rotation in 24 hours. However, if you are on the equator, your body will not rotate but will rather face forward as you move with the earth. The Coriolis force is a maximum at the pole (perfect spin) and a minimum at the equator (no earth generated spin). The earth's linear velocity (distance per unit time) is a maximum at the equator and a minimum at the pole.
What is happening is that, from a viewpoint in space, air is rising near the equator with the same high angular velocity as the surface below thanks to the large circumference of the earth at that point. As it flows north, however, the surface below is, realtive to the parcel of air, moving more slowly - it has lower angular velocity. From the space observer's point of view, the air is flowing due north; it's just that the surface below is rotating more slowly as the circumference reduces. However, from the frame of reference of an observer on the ground (say in London), the flow appears to be a westerly, as that person is unable to compare their angular velocity with that of the surface in the area the air originated from. It's a difficult concept to explain, and requires some thought to grasp, but, once you 'get it', the reason for the westerly jet flow is suddenly obvious. To quote from the above site (which I think explains it as eloquently as anywhere else):


An air parcel in the Northern Hemisphere moving from the equator toward the pole will carry its higher angular velocity as it moves north. This will cause the air parcel to deflect to the right of its path of motion. If an air parcel moves north to south in the Northern Hemisphere, it will carry its lower angular velocity with it. Since it is moving into a region of higher angular velocity, the earth will spin underneath the air parcel, causing again, an apparent deflection to the right. The word deflection is used because it is relative to an earth observer. Someone watching the air parcel from space would not see the parcel deflect but would rather see the parcel moving straight and the earth rotating out from under the air parcel.
An important concept to grasp is that, in the northern hemisphere, this rightwards deflection applies to air moving parallel with the surface in any compass direction in the absence of other effects. This idea is fundamental to understanding the development and maturation of areas of high and low pressure. For now, the important concept is that, all else being equal, weather in the northern hemisphere tends to come from the west or south-west, hence why that is our prevailing wind direction.
 
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chrisbell
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If everyone who's interested has understood the first part of my description of the jet, I'll move on to the second part. From now on, unless I specify otherwise, I'm referring to the Northern Hemisphere.

Although there is no clear line, thanks to mixing-out, there is a band where the colder, lower-pressure airmass from the pole meets the warmer, moister, higher-pressure tropical airmass. This band is called the Polar Front, as it is a boundary between airmasses of differing characteristics (which is the definition of a front). In winter, as insolation (warming due to UV from the sun) is at its lowest, the pole cools, and the mean position of the polar front is further south (lower latitude). Conversely, in summer, the pole warms, and the mean position of the polar front is further north. As the polar front dertermines the location of the jet, it can be seen that, on average, the jet heads north in summer and south in winter. North of the polar front (at least in winter) lies the Polar Vortex, which was in the news a couple of winters back as it brought extreme cold to many parts of the United States. It is a semi-permanent anti-clockwise (cyconic) vortex of cold, dry air whose behaviour is influenced by multiple factors (not all of which I fully understand), and which is the main determining factor in which parts of the NH see cold weather in winter.

Owing to the westerly flow of the jet, high-altitude winds at the polar front flow in a mean west-east direction. When the temperature gradient acrosss the polar front is at its steepest (i.e. the contrast is greatest), the flow is more powerful, thanks to the Thermal Wind being at its highest. In turn, the thermal wind results from the balance between the opposing effects of the Coriolis Effect (which tries to turn flow to the right, if you recall!) and the Pressure Gradient Force, which owing to the higher pressure south of the polar front, wants to veer the flow to the left, or north (from higher pressure to lower pressure). As air gets drawn in to the polar front, it accelerates, due to the speed of flow in the polar front being greater than that either side of it, and, as air exits the polar front, it slows. This phenomenon changes the balance of the Coriolis Effect versus the Pressure Gradient Force at both the entry and exit points, resulting in the mean advection (movement in a horizontal plane) curving right into the flow from the south (Coriolis overwhelms Pressure Gradient Force due to increasing velocity of flow), and curving left out of the flow to the north on exit (Pressure Gradient Force overwhelms Coriolis due to decreasing velocity of flow). This, in turn, tends to cause areas of lower pressure to form within these Jet Streaks (the area between entry and exit), and tends to cause the resultant depressions/cyclonic flow/Low pressure areas to have an anti-clockwise rotation, or Positive Vorticity, to them. It should make sense that rotation in the other direction is called Negative Vorticity.
 
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chrisbell
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As an addendum, if the Coriolis Effect and Pressure Gradient Force were in perfect balance, the resultant westerly flow would be in what's referred-to as Geostrophic Balance. This never actually happens, but more intelligent people than me (i.e. professional meteorologists rather than amateur enthusiasts) use this theoretical straight flow, which is called the Geostrophic Wind, as a term on which many complicated calculations and algorithms are performed to work-out how vorticity, surface friction etc. will affect the flow. These calculations involve the Ageostrophic Wind, which is simply the difference between the actual wing and the Geostrophic wind, hence, the ageostrophic wind can be used to determine the development and positioning of low pressure systems.
 
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chrisbell
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Following-on from post 2, the thermal wind is a product of the temperature gradient either side of the polar front, and converts what would otherwise be a lazy drift of air from west to east at 30-35,000 feet, thanks to the Coriolis Effect curving warm air drifting north from the tropics, into the ribbon of air we call the jet stream. Its strength is regulated by the temperature gradient, and its direction, and the extent of vorticity (twisting or curving) is controlled by the balance between Coriolis and the pressure gradient force. Coriolis is, if you recall, trying to curve flows to the right, (in the case of an east-west jet axis, to the south), and the pressure gradient force between mean higher pressure south of the polar front vs. north of it, is trying to curve the flow to the left, or north in this case. The faster a flow of air moves, the more it is affected by Coriolis, as it magnifies the influence of the change in velocity of the earth's surface as you move towards or away from the Poles. Conversely, as an air mass slows, the presure gradient force overwhelms the reduced Coriolis Effect; the air gets pulled towards the Pole more strongly.

Now, if you combine all these factors (and this is where my ineptitude with physics hit me hard when I initially tried to understand this) the net result is that a fast-moving jet, powered by a steep temperature gradient, tends, like a fast-flowing river, to follow a relatively straight path, with only minor curves - a Zonal jet. However, if the jet is slower, with less energy input from the temperature gradient, it does what a slow-flowing river does, and meanders. This is a meriodional jet, because it flows north-east, then curves to the south-east or even the south, thereby following meridians (lines of longitude). Where the jet dips, the 'u' shape is a longwave trough, and, where it arcs, the 'n' shape is a longwave ridge. One ridge plus one trough is a single Rossby wave - if this is elongated enough that one ridge and trough is enough to circumnavigate the northern hemisphere, it's said to be a wave 1 pattern; 2 ridges and troughs per circumnavigation, and it's a wave 2 pattern etc. These Rossby wave numbers tend to be 6 or below, and the position of the ridges and troughs relative to the land masses underneath can be slow-moving or stationary. Higher wave-numbers are the result of related by moving patterns of smaller features (or Cyclonic Waves, which are areas of high and low pressure developing and fading as described in post 6 below) which curve the isobars within Rossby waves and cause a pattern of short waves to overlie the Rossby pattern.

The following chart shows a northern hemisphere view, with the Pole in the centre, at the 500 mB pressure level:



To quote the site (http://www.wxonline.info/topics/waves.html):
This image below shows a 500 mb chart for the Northern Hemisphere. The contours represent the topography of the 500 mb pressure surface, identifying valley/trough and ridge areas. When interpreted in terms of geostrophic/gradient flow, the contours show a westerly current that meanders between 35 degrees and 65 degrees north latitude.


If you plotted the height pattern along a latitude circle you would see a series of waves of varying amplitude and wavelength. These features can be described as follows:


  • Troughs are areas of relatively low height. If you move perpendicular to a trough line, height values increase.
  • Ridges are areas of relatively high height. If you move perpendicular to a ridge line, height values decrease.



Using these defintions, a trough (red line) can be seen running from a low center over Hudson Bay, southward to the Dakotas. A ridge (blue line) extends from the high center just west of Washington State, northward into the Yukon.

The actual middle to upper troposphereic flow is a combination of waves of varying wavelength. This can be seen in the 500 mb chart above. On this chart there appear to be four long waves around the hemisphere (wave number 4). There are long wave troughs over the eastern United States, Europe, Central Russia, and the central Pacific Ocean. The long wave trough over Central Russia is rather broad while the one over Europe is distorted by the ridge over western Russia. This example points up the difficulty of visually identifying long waves on a hemispheric chart, as was discussed above.


Superimposed on the long wave pattern as numerous short waves. Focusing on the long wave trough over the eastern United States, there is a short wave trough indicated by the red line from low center over Hudson Bar into the Dakotas. Another short wave trough is found from Indiana to Georgia, while a third trough is over Newfoundland. Each of these impulses is being steered by the long wave trough.
 
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chrisbell
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Beyond the long, Rossby waves with their associated high and low pressures, there are shortwaves or synoptic waves, as they are sometimes called, moving through the pattern. It's as though we have a card disc with up to 6 equally-spaced holes, representing the Rossby waves, and a second disc overlying it representing the shortwave pattern, having anything from 7-20 smaller holes which rotates over the first. Some of the time, the holes in the two discs don't align, creating a degree of 'cancelling-out' - this is like a shortwave moving out of a trough into a ridge and weakening, or an area of high pressure weakening as it moves into a trough. More often, though, holes will align in at least one place on the two discs, amplifying the effect, which represents the maturation of areas of high or low pressure in their respective ridge of trough.

To quote a retired forecater posting on Netweather:

I always find it really difficult trying to manually visualise for a given chart what the component waves might be. It might not be possible at all....however, you can think of it in a simple way just using two different waves:Imagine in your latitude belt you have just two waves. One of them is Wave 2 and the other is Wave 7. So very distinct waves. Wave 2 is perhaps stationary, very persistent whereas Wave 7 moves in the westerly flow, and by itself would just look like a standard mid-latitude zonal pattern with lows and highs.



Where the Wave 2 ridge and Wave 7 ridge are in phase, there would be "constructive interference" (from standard wave theory). The resulting ridge would therefore be more intense than under other circumstances. Likewise, where the troughs are in phase, you would probably see a deep low. In areas where the phases don't match up, you would see destructive interference. So for example, a Wave 7 ridge beneath a Wave 2 trough would perhaps result in a fairly weak area of high pressure. And a Wave 7 trough + Wave 2 ridge would probably see the resulting low pressure system not being deep or not forming a closed low at all.

Click to expand...
 
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chrisbell
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In post 2, I explained about positive vorticity and how that anticlockwise spin sets-up in a developing low pressure area. Air enters a relatively zonal jet from the south on a rightwards curving path and exits to the north on a leftwards curving path. As mentioned there, it tends to take place in a section of the jet with especially high windspeed, which is called a Jet Streak. Low pressure formation is reliant upon air accelerating into the stream, and decelerating on exit. As the velocity of the airflow increases, the Coriolis Effect increaes in magnitude, so the entry is on a right-hand curve and the bulk of the air enters the streak from the south. On exit, the velocity drops, weakening the influence of the Coriolis Effect and therefore leading to the Pressure Gradient Force becoming dominant, causing the air exit to the north.

Why does this happen, though? Well, as air enters a jet streak, the inward flow is squeezed together, a process called Confluence, causing it to accelerate in accordance with Bernoulli's Principle. On exit, it spreads out and slows, a process called Diffluence. It's this spreading apart of the air on exit that encourages the pressure drop that matures the low pressure area; as the density reduces due to the diffluence, the barometric pressure drops. Flow, both into and out of the jet, can be either confluent of diffluent, and it is the difference between these factors, along with the presence or absence of a jet streak, that determines whether low or high pressure is generated. In the case of low pressure formation, air enters the streak at a confluent ridge, with much more air entering from the southern side of the jet than the northern side, and it then exits at a diffluent trough, with much more air exiting on the northern side of the jet. Consequently, the flow accelerates into the stream, and decelerates out. We therefore say that low presure areas develop at a Right Entry Region in a confluent ridge, and mature in a Left Exit Region in a diffluent trough. High pressure areas emerge where there isn't a jet streak, as flow decelerates on entry to the stream, and accelerates as it leaves. Air from mainly the northern side of the jet enters a diffluent ridge, curving to the left as velocity drops and the Pressure Gradient Force becomes dominant, before it exits the jet an a confluent trough, where it accelerates out on a rightward, southerly curve (velocity increases so Coriolis takes over). We therefore say that high pressure areas develop in a left entry region of a diffluent ridge, and mature in the right exit region of a confluent trough. Owing to the fact that it's accelerated on exiting the jet, an area of high pressure will continue to curve to the right, so actually makes its way back from the underside of the trough to the base of the preceding ridge, which is where it is usually found. By contrast, as areas of low pressure slow on leaving the jet, they curve left and stop in the base of the longwave trough.
 
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chrisbell
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Right - next part - air flow in and around high and low pressure areas.

You should recall that flow around an area of low pressure is cyclonic (anti-clockwise, or postive vorticity), while flow around a high pressure cell is anticyclonic (clockwise, or negative vorticity). This, however, is only part of the actual flow, as it ignores vertical movement entirely, and, by itself, cannot explain how low pressure areas deepen despite the tendency of air from regions of higher pressure to flow into them. What actually directly causes air pressure to drop is that, in an area of low pressure, two things are happening. Firstly, the polar air (from north of the jet) gets caught-up in the circulation, and, as polar air is less dense, it sits lower in the atmosphere than the warmer air around it. This means that, at a given altitude, there is less air above than in a surrounding warmer air mass, and, with less mass above, pressure is reduced. By contrast, in an area of higher pressure, warm air is trapped in the circulation, causing expansion of the air column; more air is found above any given altitude, so pressure is raised as explained here:

http://scied.ucar.edu/shortcontent/highs-and-lows-air-pressure



Imagine the atmosphere consisting of an infinite number of slices in the vertical. As heat is pumped into each slice, by whatever method (advection from somewhere else; heating from below etc.), that slice expands (gains energy), and as well as expansion sideways, and downwards (against a net expansion upwards by the layer below), there is a general expansion upwards. The definition of atmospheric pressure is the force per unit area due to the weight of the atmosphere above any defined point. Thus, for any level in the atmosphere, if heat is supplied to a column of these infinite number of slices, the whole column expands effectively upwards, and because more of the atmosphere is now above any one point, the pressure at that point increases.(There must be a net expansion upwards, because the earth's surface forms an effective block to net downward expansion.)
Returning to low pressure areas, the reduced height of each layer of the atmosphere has no effect on surface pressure as the entire atmosphere is, by definition, above the surface, but, here, the second causal factor comes into the equation - the air in a low pressure area rises as it rotates, thereby reducing the pressure at the surface. As this happens, air moves in near the surface from areas of high pressure, as nature always tries to fill a (partial) vacuum. In turn, this air has dropped in an anticyclonic spiral through a region of high pressure, before spreading-out near the ground. The following illustrations show this concept:



From this, it can be seen that areas of low pressure arise when the flow out at altitude exceeds in inward flow nearer ground level, and fill (when pressure equalises with its surroundings) when inward flow exceesds outward. It also becomes evident that this supports the acceleration on air into an area of low pressure I discussed previously, as it rushes in to fil the partial vaccum generated by upwards spiralling of air. Thus, we can see Positive Vorticity Advection from an area of high pressure into the base of a depression, and Negative Vorticity Advection at altitude from the top of that depression into an anticyclone. The following diagram demonstrates the position of both PVA and NVA with relation to longwave ridges and troughs:



(Source: http://www.geologywales.co.uk/storms/upthere.htm)
 
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chrisbell
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I've already explained air movement into and out of the jet stream, jet streaks and their part in the formation of low pressure areas (cyclogenesis), how areas of high pressure form in the jet, and, most recently, I covered the vertical movement of air within areas of high and low pressure, which explains how they maintain their core pressure for longer than might otherwise be expected.

As you're probably aware, anticyclones (high pressures) do not generate weather fronts, though a weak front may pass through the fringes of them. Areas of low pressure, however, do generate weather fronts, as anyone who has left home in sun and returned that evening in heavy rain can attest! These fronts are formed as the jet stream runs along the polar front which I described earlier in this thread - this is sometimes called the Polar Front Jet, and it separates air of arctic origin to the north from air of tropical origin to the south. I've already covered how areas of low pressure develop due to air accelerating into a jet streak from the right (southern side) of the jet stream, and the fact that such streaks are found between a ridge and trough in the longwave jet pattern as it circumnavigates the planet in the mid-to-high latitudes. If we assume that this section of jet in isolation is straight (which you may recall is a simplified version of reality, wherein the flow is never completely straight), you might be able to understand why the acceleration of air into the jet, followed by deceleration out of it imparts a torque (turning moment) to the flow, rather as one runner elbowing another may force their previously straight running path to the side. Using diagrams from the following site which explains the Norwegian Cyclone Model, I'm going to demonstrate the effect of this at a localised level (bear in mind that this is just within a short straight-ish section of the jet between a longwave ridge and a trough):



https://web.archive.org/web/20170331053315/http://oceanservice.noaa.gov/education/yos/resource/JetStream/synoptic/cyclone.htm



Stage 1 - Initial condition







These show that simplified, straight flow, with anticlonic (clockwise) flow south of the jet associated with higher pressure and air sourced from the tropics, and cyclonic (anticlockwise), polar vortex flow to the north, associated with mean average lower pressure and a polar-sourced airmass. This forms what's called a Stationary Front (in this case, the polar jet front), and it is indicated by an alternating red and blue line with red semicircles facing into the cold air mass, and blue triangles facing into the warm air mass. This is the standard way of indicating a stationary front on a synoptic chart, including (on the occasions when they occur at our latitude) on BBC forecast charts.

Stage 2 - Initiation







Here, the jet is being curved as the disturbance in the upper troposphere (the lowermost section of our atmosphere where most weather occurs) passes through this section of the jet, with the flow into and out of the jet streak which caused the disturbance imparting a torque. We can see that, in the centre of the disturbance and towards the right-hand end (eastern), warm air is Advecting (moving in a plane parallel to the Earth's surface) into the colder air to the north - this section of the stationary polar front has become a Warm Front, as indicated by the red line with the semicircles facing the direction of travel. By contrast, to the left-hand (western) end, cold air is heading south in the form of a Cold Front, as indicated by the blue triangles, again indicating the direction of travel. Both the Warm and Cold Air Advections (warm and cold air flows into the developing Low) are curving towards the front. In addition, the upper diagram shows the position of heavy (dark green) and light (ligher green) precipitation (rain, hail, slleet, snow etc.) associated with the developing depression.

Stage 3 - Intensification









At this point, the depression is developing within the jet streak, ready to leave the stream in the 'left exit region' of the 'longwave diffluent trough' (see previous lessons for explanations of these terms) it's heading towards. Both fronts are becoming more defined, and the area under the developing low pressure system that is experiencing precipitation is increasing. Note that the cold front is swinging from the hinge point at the centre of the low rapidly anti-clockwise to the south-east due to the relative ease it experiences as its dense, cold air undercuts the warm air ahead of it (visible in the right-hand diagram), while the warm front is moving more slowly, due to its lower density warm air having to climb over, or override, the colder air ahead of it. This means that, as the depression matures, the Warm Sector (the area of milder air south of the centre of the low between the warm front and the cold front behind) is narrowing, with the warm air mixing slightly with the cold air at each frontal boundary, but otherwise being lifted by the cold front wedging under it.

Stage 4 - Maturity








As the cold front catches-up with the warm front (a process which initially emerges close to the centre of the row before heading down the warm front), the fronts merge, creating an Occlusion/Occluded Front, as indicated by the purple line with alternate semicircles and triangles. Any mixing of the airmasses that was going to happen is complete as the occlusion forms; the rest of the warm air that was in the warm sector is lifted into the upper troposphere above the colder air at the surface, as there's nowhere else for it to go. Note that the gap between the fronts further south is narrowing, reducing the gap for, and therefore amount of, warm air advecting in. By contrast, the amount of cold air advecting in from the east and north, curving anticlockwise to the north of the low centre and then swinging south-east to sit behind the cold front, stays the same. At this stage, the mature low pressure system has exited the jet streak, and its central pressure is as low as it's going to get; the reduced warm ar advection into the ever-shrinking warm sector, while cold air advection behind the cold front remains constant is going to result in the low filling-in and weakening from this point onwards. Note also that the precipitation is concentrated around the centre of the low, along the occlusion, and behind the cold front. Often, the warm front may bring drizzle or just low cloud, with heavy rain or snow not arriving until the occlusion or cold front arrives at a given location.

Stage 5 - Weakening







Finally, the occlusion is almost complete. Due to the warm sector now being small, warm air advection is greatly impeded, and the depression is being filled, with the central pressure rising. Note that the trailing cold front is actually a warm front at the westernmost end, as it is heading north at this point - the front is waggling, or 'waving', as meteorologists seem to put it. If you hear a forecaster mention "trailing fronts" they are referring to these lengthy, waving tails which can affect areas hundreds of miles behind the weakening depression; these are also the fronts which can trail through the edge of an area of high pressure, bringing cloud and light, patchy precipitation to the areas underneath them.
 
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chrisbell
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This lesson covers Warm, Cold and Dry Conveyor Belts, and is almost an addendum to the previous lesson on the Norwegian Cyclone Model, so I'd advise that you re-read that first.

As can probably be inferred from the Norwegian Model, the development of an area of low pressure and its associated fronts, is predicated on warm and cold air inflows (the Conveyor Belts) to maintain the warm and cold sectors. There is also a Dry Coneyor Belt behind the cold front, which is responsible for the cloud clearing after it passes, though the cooler airmass is often showery as it is inherently unstable (instability of an air mass is something I intend to cover in a later series of lessons).

I'm using a diagram from the following site: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter12/warm_cold_conveyor.html as the response to using diagrams in earlier posts was positive - certainly, diagrams can be much clearer than lengthy explanations. The War, Cold and Dry Conveyor Belts are indicated by the orange, blue and gold arrows respectively.






As can be clearly seen, warm air close to the surface enters the warm sector (the area between the warm and cold fronts), where it rises and heads to the north. As I've previously explained, the warm sector reduces in size as a low matures, due to the cold front catching-up with the warm front and occluding; the warm inflow air is therefore partially mixed-out and partially lifted above the colder air undercutting it as the cold front sweeps in. At the earlier stage illustrated above, however, the warm front is quite sizeable, and, with a gently rising moist, warm airmass passing through the system from the south, the typical weather experienced between fronts is cloudy (mainly stratiform types) but not usually highly convective, in other words, there tend to be few or no heavy showers, and precipitation is often light and drizzly. However, with cold air sweeping in from the east and north and swinging around into the area behind the cold front, there is pronounced undercutting of this cooler airmass under the warm moist air ahead of it This forces the warm air to rise rapidly, rather as hammering a wedge into a thick plank of wood or section of slate forces it apart. This causes precipitation associated with the cold front to be much more intense than at the warm front, which often contains little or no appreciable precipitation at all. If this lifting is sufficiently rapid, and the warm air ahead sufficiently moist and hot, an area of thunderstorms may develop in the precipitation associated with the cold front, alternatively, in winter heavy snow may result.

Finally, behind the system, slightly milder dry air (Dry Conveyor) flows in from higher in the atmosphere to fill the partial vacuum created as the dense, warm, moist air of the warm sector is replaced by less dense cold air from the cold Conveyor. This tends to reduce shower/electrical storm activity further behind the cold front, and effectively provides either the dry respite between lows or a foretaste of any settled anticyclones (high pressure cells) which may arrive from upstream. The following image is a satellite shot over the south-eastern US showing the cloud-free 'dry slot' associated with this inflow behind a low pressure system:


 
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chrisbell
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Not all low pressure systems show the stages described above, especially so-called 'weather bombs' which are characterised by rapid deepening in the western Atlantic and which sometimes feature a powerful section of jet streak at low altitude called a Sting Jet. Rather than describe these in detail, I'll link the following site for those who are interested in what is only an addendum to the main concept (the Norwegian Model). Also, please ignore, for now, the section on the NAO, as I intend to cover this at some later point:


https://rgsweather.com/2014/12/14/bombs-away-the-fall-and-rise-of-racy-depressions-december-2014/
 
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