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Vorticity0123

A short guide to stability

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Below is a guide to stability (based on a post I made in the model output discussion thread of 26 January 2015). The post has been edited slightly to make it more generally applicable.

 

Stability of the atmosphere

 

(Un)stability has to do with the 'tendency' of a parcel of air to rise from a certain position (in altitude) or to stay at the same position. This tendency is related to the temperature a parcel has compared to its environment.

 

Imagine a parcel starts to rise from a certain altitude (say, 1000 meters). The parcel then cools adiabatically (meaning it does not 'mix' with its environment) up to a certain height. If a parcel then finds itself being cooler than its environment (stable conditions), it will drop back to its original position (remember that a certain volume of cold air is in general heavier than an equal volume of warm air). However, if the parcel is warmer than its surroundings (unstable conditions), it will continue to lift to even higher altitudes until it reaches a height when the parcel becomes saturated. This height is the height where clouds start to form. Thereafter, the parcel will still continue to rise up to where it finds itself in an environment that is warmer than the parcel itself. (Note that the cooling process during ascent of a parcel is different when the parcel is saturated, but goes too far to treat this in detail). The parcel then stabilizes, and this can (under great simplifications) indicate the height of a cloud.

What this comes down to is that when the air is unstable, showers are easier to form based on the parcel analogy described above.

 

A good measure of stability is the change of temperature with height. If the temperature drops sharply with height, the atmosphere can be considered unstable (referring back to the parcel analogy). When the temperatures decreases only weakly with height or even increases with height, the atmosphere is stable (from the parcel analogy: a parcel will find itself colder than its environment after ascent, meaning it will drop back to its original position).

 

To illustrate this, below is a series of images showing the parcel analogy:

 

post-20885-0-66576500-1422727048_thumb.p

Stable situation

 

post-20885-0-51472100-1422727052_thumb.p

Unstable situation

 

In the images above, the x-axis indicates the temperature, while the vertical axis (y-axis) denotes height.

For both graphs, the red line indicates the change in temperature over height of the environment of a certain parcel (technically spoken: lapse rate). Note that the environmental temperature drops much more with height in the unstable situation than in the stable situation.

The black dot indicates a parcel on a random level. The arrow pointing to the upper-left stands for the adiabatic rising (and the accompanied cooling) of this parcel. For both images, this parcel cools at a same rate (so the black arrow has the same slope to the left on both images).

 

As can be seen in the stable situation, the parcel becomes colder than its environment after rising. Therefore, it is being forced downward again. On the other hand, in the unstable situation, the parcel becomes warmer (and thus lighter) than its environment, indicating the parcel will continue to rise.

 

Temperature difference representation between surface and aloft

 

Coupling the part given above back to the presence of low pressure at higher heights and stability, one can realize that the difference in temperature between the surface and aloft (I'll be using the 500 hPa level, being about 6 km, as a reference for now) must be very large in order to have an unstable atmosphere. If the atmosphere can be more or less unstable when the temperature at the surface stays the same, the temperature at 500 hPa has to vary accordingly. In other words, changes in stability can be explained by variations in temperature at 500 hPa level.

 

Simplifying a bit, one can assume as a general rule that low pressure activity at higher altitudes is accompanied by lower temperatures at that same level. (more in-depth explanation can be found here). This means that, in general, low pressure at higher altitudes indicates the atmosphere is more unstable than when high pressure is present at higher altitudes (and thus showers are by approximation more likely to form when low pressure is present at higher altitudes)

 

Seasonality in stability

 

An important difference between summer and winter regarding stability is that the surface is usually colder during winter than summer. This means that the upper air has to be colder in winter to acquire instability than during summer.

 

An example of an unstable atmosphere in winter

 

The weather that we are about to observe this Thursday up to the weekend is a very nice example to illustrate the relation between stability and the presence of low pressure at higher altitudes. Therefore, given below is the pressure forecast of the GFS for Thursday 30 January, 18Z:

 

Rtavn00120150130.png

GFS surface level pressure and 500 hPa heights (colours), Friday 29 January, 00Z.

 

The image is from the Wetterzentrale archive, so it is not completely identical to the original (which is not available anymore). Also, the chart is of 6 hours later than the analysis described below. Still, the differences are small enough to preserve a reliable analysis.

 

It is important to focus solely on the 500 hPa heights, indicated by colours. As a rough guide, purple/blue colours indicate low heights (lower pressure activity at 500 hPa height) while yellow/red colours indicate high heights (high pressure presence at 500 hPa height).

 

Note that there is a very deep trough (low pressure area) present at 500 hPa height over Western Scandinavia and Northeastern UK. Referring to the explanations, low pressure at 500 hPa should coincide with lower 500 hPa temperatures. Much higher heights (relatively higher pressure) are present to the southwest, west and north of the UK. Therefore, the 500 hPa temperatures for the same timeframe (from the GFS forecast) are given below:

 

15012918_2712.gif

GFS 500 hPa temperatures, 12Z T+54 (the given timeframe is Thursday 18 UTC)

 

Note that there is a large swathe of very cold 500 hPa temperatures present to the east of the UK (down to -38*C). This is associated with the very deep trough present to the east and over the UK. Much warmer 500 hPa temperatures can be found to the south and west of the UK, while the 500 hPa temps are also slightly warmer to the north of the UK.

 

The surface temperatures do not vary much in the neighbourhood of the UK at this timeframe (except for land/sea effects). The surface temperature chart for this Thursday can be found here.

 

Thinking of the parcel analogy given in the beginning of this post, it becomes evident that showers are more likely to develop over or to the east of the UK than to the north, west or south (assuming equal surface temperatures).

 

Northerlies and stability

 

Regarding wind, as of Thursday 28 January (18Z) there were northerlies present over and to the north of the UK, while to the east of the UK there was barely any wind. (you can find the wind forecast from the GFS here). However, as we can see above, the air to the north of the UK was less cold than over the UK itself. This means that if northerlies are stronger to to the north of the UK are stronger than south of the UK, the air over the UK could still be more unstable (due to the lower upper temperatures).

 

Summary

 

To summarize the relationship: low pressure at high heights is coincident with cold upper air, yielding a bigger temperature difference between the surface and aloft. This yields a more unstable atmosphere.

 

It has to be kept in mind, though, that this relationship is simplified, so it does not have to match the actual conditions in any case.

 

 

If one would like some explanation about this via Skew-T diagrams, it can be added afterward :wink: . A good read about Skew-T diagrams, which could also serve to visualize stability, is given below:

 

https://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

 

Sources:

http://www.wetterzentrale.de/topkarten/fsavneur.html

http://www.keesfloor.nl/weerkunde/10neerslag/10neerslag.htm

https://forum.netweather.tv/topic/27989-how-to-try-and-forecast-snow/

https://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=ukuk&MODELL=gfs&MODELLTYP=1&BASE=-&VAR=z500&HH=48&ZOOM=0&ARCHIV=0&RES=0&WMO=&PERIOD=

Edited by Vorticity0123
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