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Dulging into the weather world

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Gust Front Outflow

[font="Calibri"][size="3"]Just wanted to talk about a day of an evening storm chasing which included an interesting gust front ahead of what was an interesting day of thunderstorms at the time.[/size][/font]

[font="Calibri"][size="3"]It was one of the most impressive sights that I’ve seen to the visual eye for a long time and certainly gives you the edge when something like this comes at you, it is something you don’t see every day and I hope that more will come in the future.[/size][/font]

[font="Calibri"][size="3"]This type of gust front came about from some thunderstorms that moved up from France and then across parts of the south-west UK and at the time and moved north and east though missing the south-east the time these photos were taken. [/size][/font]

[font="Calibri"][size="3"]At the time this gust front was huge in appearance and must have been at least few miles wide and reports came in before it passed over, of strong outflow winds in excess of 40mph or so and even a “gustnado” was reported across the Channel and no surprise when you see this shelf cloud coming at you.[/size][/font]

[font="Calibri"][size="3"]The first photo is the shelf cloud approaching in from the south-west and advancing towards the observer and you can see the trees in the distance being pulled in one direction, this was the result of the winds beyond the shelf cloud.[/size][/font]

[font="Calibri"][size="3"]Later in the sequence you can see the shelf cloud move through and then another outflow boundary although weaker and less defined moving through, the air was reasonable stable at lower levels you can teelof the smoothness of the shelf cloud and little precipitation fell and just the outflow winds passed by.[/size][/font]


Stormfanatic

Stormfanatic

 

The Gust Front Up Close

[font="Calibri"][size="3"]All information and photographs have been done by myself and a lot of time has been spent doing this so please enjoy reading.

You might have heard the term gust front, but exactly what is it and how do you know if one is coming your way.[/size][/font]

[font="Calibri"][size="3"]A gust front is an outflow boundary that forms from a heavy, convective shower or thunderstorm. This gust front separates warmer, calmer conditions ahead of this front to gusty, colder conditions located behind. These gust fronts are most common amongst thunderstorms, of which the name will be used form now on.[/size][/font]

[font="Calibri"][size="3"]They come from cumulonimbus clouds, formed by descending cooler air that comes from the parent cumulonimbus.[/size][/font]

[font="Calibri"][size="3"]In further detail the gust front divides the warm environmental air ahead of the storm to the cooler, thunderstorm, descending air which is located behind the front; in essence this boundary is like that to a surface cold front passage where cold air from the storm is replacing the warm air, ahead. It is along this boundary is where the battle commences between the warm and cold air, these boundaries can come from localised heavy showers and thunderstorms, of which they can also be attached to weather fronts, like a cold front or from a trough . These gust fronts can be seen regurarly with thunderstorms and give a good tell-tell sign of what is coming; the downdraught from the storm creates a gust of wind which occurs when the cooler air passes over thus the name gust front gets its name.[/size][/font]

[font="Calibri"][size="3"]The beginning stages of a gust front formation start when a cumulonimbus cloud fully forms into the mature stage just after glaciation has occurred where the storm reaches freezing level within the tops of the cumulonimbus cloud and as it reaches further into the mature stage, the water droplets within the cloud can’t sustain themselves no longer and precipitation starts to fall, as more precipitation falls toward the surface the cold rush of air from the cumulonimbus known as the downdraught spreads out and hits the ground and lifts warm air from below to a point where it condensates to a level where a low cloud forms and eventually forms a boundary, commonly know as an outflow boundary, the cold air that pulls out from the storm creates a wind known as the gust front, this wind is often gusty and will often feel colder once the front passes over.[/size][/font]

[font="Calibri"][size="3"]Once the cold rush of air from the storm takes over and begins to dominate and cuts off the supply of warmer air a low cloud forms from underneath the cumulonimbus, it takes on a smooth-like appearance, with it being smooth on top but almost ragged-like underneath, this cloud type is known as the arcus cloud, which is located ahead of the advancing storm.[/size][/font]

[font="Calibri"][size="3"]There are two types of main arcus cloud one is the roll cloud which is detached from the parent cumulonimbus cloud and the other is the shelf cloud which attaches itself underside of the parent cumulonimbus cloud and represents the new gust front. The shelf cloud is like a wedge or bank of low cloud that attaches itself underside of the cumulonimbus. If the air ahead of the storm is stable with little shear, (wind change in speed and direction at different levels of the atmosphere), the shelf cloud will often be very smooth in nature even under foot but in more unstable air it may well appear more ragged and wind-torn reflecting higher shear and turbulence, especially where the intersection of warm and cold air is taking place, this can been seen from low rugged scud-like clouds that hang underneath from the shelf cloud.[/size][/font]

[font="Calibri"][size="3"]The shelf cloud in essence looks the same on the most part but the shape and size depends on the ingredients up the atmosphere. In simpler terms the shelf cloud looks like a low, hanging smooth cloud which is turbulent underneath and smooth on top. Underneath the shelf cloud is where a lot of turbulence takes place, with the clouds looking torn and ragged, depending on certain conditions.[/size][/font]

[font="Calibri"][size="3"]The outflow boundary becomes more evident and obvious as we enter the dissipation stage where the outflow of the storm takes over and eventually dies as the warm, inflow air gets cuts off. By this point the gust front moves further away and the storm dies. From time to time as the storm dies and the gust front pushes further away, new storms may develop along it if ingredients favour for further storm development, but this would need to be inhibited from other factors such as fronts, convergence lines like sea breezes or topography of land better known as a orographic lifting.[/size][/font]

[font="Calibri"][size="3"]The shelf cloud is like a small cold front that passes over toward the surface as the cloud nears, the wind drops, temperature rises but as it passes over the temperature drops and wind picks up rapidly with precipitation imminent. It at first attaches itself underside of the cumulonimbus cloud like a dome. As the gust front with the shelf cloud passes over a “whale’s mouth” like effect may occur as the gust front moves further away where the ragged clouds underneath become revealed. By this point you are under the cooler air that has passed over with precipitation not too far away and often very imminent.[/size][/font]

[font="Calibri"][size="3"]The gust front forms ahead of a thunderstorm in the direction of the storms movement. The front may be clear to see if there s good visibility or skies are relatively clear ahead of the storm. If the shelf cloud is very low and dark and has good structure then severe weather is likely on the way, where occasions thunder can be heard from a distance and lightning can been seen if visibility is good and the storm contains lightning.[/size][/font]

[font="Calibri"][size="3"]Because gust fronts are types of outflow not inflow, tornadoes very rarely form along gust fronts, though this is not impossible. The cold air undercuts the warmer air at the surface, and in extreme and intense storms, the gust front lifts dust and dirt form the ground and these may rotate due to the turbulence and power from the downdraught of the storm and very short-lived rotational vortexes may form and these known are gustnadoes also if there is severe wind shear (winds change in altitude) then the chances of severe weather are very likely to increase.[/size][/font]

[size="3"][font="Calibri"]Damage and the danger from gust fronts can vary from nothing at all, this being a light wind and light precipitation to more severe threats like heavy, damaging precipitation, severe wind squalls and lightning. Most of the impressive gust fronts seen come from the more severe and more damaging thunderstorms.[/font][/size]

[font="Calibri"][size="3"]Most of these are common in the areas like the Great Plains in the United States and in some places of Europe. They are most seen from multi-cell storms (multiple thunderstorms) and super-cell thunderstorms (rotational thunderstorms) or from squall lines (linear thunderstorms). These gust fronts are more likely to be seen during summer, spring and autumn where thunderstorms are more prevalent, though they do occur all year round.[/size][/font]

[font="Calibri"][size="3"]To catch a gust front requires good timing as well as a bit of luck. The main importance is trying and get ahead of the storm before it dissipates and ideally location is paramount, the chances of finding a gust front increase when the storm is late mature stage or just entering the dissipation stage as mentioned before but as noted you need some luck on the way and they can be breathtaking sight, the important thing to remember is when the main downdraughts take over and thus the arcus clouds forms. [/size][/font]

[font="Calibri"][size="3"]In the UK these gust fronts can been seen well in clear, cool air masses, like maritime and polar air masses where visibility is very good and deep convection can been seen from a distance. Sometimes, however in other air masses low cloud, like scud or even banks of stratus may obscure the view of shelf clouds approach and appearance and become obscured from view.[/size][/font]

[font="Calibri"][size="3"]For further diagrams on gust fronts here are is a good link: [/size][/font][url="http://www.britannica.com/EBchecked/topic-art/594363/19393/Evolution-of-a-gust-front-During-a-thunderstorm-a-large"][color="#0000cc"]www.britannica.com/EBchecked/topic-art/594363...[/color][/url] You can see that it forms ahead of the advancing storm and so getting ahead is a good way to find one, if you’re lucky enough.

The images below shows very well the position of the gust front as well as the different stages.

Below are examples

Drawn not to exact:

The blue arrows reflect the colder air the downdraught of the storm with the red arrows the warmer air, the inflow where the air is being lifted from the colder air below and the yellow arrow shows the storms movement.






Stormfanatic

Stormfanatic

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