So, can we make it possible for the ordinary layman, outside of the scientific community, to gain a working understanding of exactly how the atmosphere actually functions? Without being patronising, can it be explained in a non too technical way? At least we can try !
The first thing to bear in mind is that the bit we call ‘The Weather’ is just a very thin layer at the bottom of a very deep ocean. The behaviour of that layer is subject to a huge number of variables and influences that can cause it to do one thing while the bulk of the atmosphere is doing the opposite and we need to understand that basing an analysis on purely surface related measurements can be highly misleading.
If we are to understand the behaviour of the climate – which, in essence, is the overall behaviour of the atmosphere – then we need to understand what is happening throughout the whole of the atmospheric ocean, not just the bottom layer, and therein lies the big problem – we just don’t have the data !
There are, and probably always will be , arguments as to how much influence is exerted by factors such as the Pacific Oscillation, ‘El Nino’, Arctic Amplification, the upper atmosphere’s Quasi-Biennial Oscillation, even volcanic activity and yes, all of these things do play their part, but these are interacting, inter-relating effects rather than actual causes.
It is a bit like arguing about why an orchestra is playing a particular harmony, is it the strings, the brass, or the woodwind? Or is it, just perhaps, the person on the podium? The one who does not seem to be making much of the sound but did actually compose and now conducts the symphony?
Self evidently, the vast majority of the heat energy in the atmosphere originates from the Sun, anything originating from geothermal sources is very small by comparison – although it is important to note that geothermal/tectonic activity beneath the arctic ice sheet does alter the thermal and chemical structure of the Arctic Ocean and has been observed to cause localised – and perhaps broader based – melting of the ice sheet and that this can have repercussions within the atmosphere. That from human activity is almost too small to measure except at a very local level.
It seems common to assume that the sun is a friendly, calm, steady source of energy. In reality, whilst the overall average TSI(*) does not vary greatly, especially when measured at Earth surface levels, the sun is anything but steady. It throbs and it surges, it spits and sparks and flashes. And if we get in the way, we take the blows – and the results show up in the atmosphere.
We have records of sun spots going back over two hundred years, from this we know that the sun displays a cycle of approximately 11 years in period. There are also indications that other cycles may be in play – cycles of 200 years or even longer.
Unfortunately there are problems with attempting historical analysis and comparison of sun/earth atmospheric interaction using sun spot data alone, the principal of these is the effect of Coronal Mass Ejections (CMEs). The result of a CME impact, or more importantly a series of such, is to cause a short term ‘heave’, or expansion, in the upper atmosphere structure changing pressure and/or thermal gradients. These can occur as much as 2-4 weeks after the impact by the time the incoming energy is dissipated throughout the body of the atmosphere and it may, in practice, be very difficult to tie cause and effect together. It also appears that even a relatively light series of impacts or disturbances, if sustained, can delay or accelerate seasonal movement of those gradients.
The existence of sunspots does not necessarily imply the occurrence of a CME or, more specifically, a terrestrial impact. Although we have reasonably long records of sunspot activity, we can not say the same for CME activity. It follows, therefore, that sunspot impacted atmospheric variability data will correlate only loosely without CME data. This is complicated even further by the highly variable nature of individual interactions; with the added problem that any resulting reaction and any consequent surface activity depends entirely upon the pre-existing atmospheric conditions at the time. Sorry if that sounds a bit complicated but it is made even worse by the effects of magnetic filament flares and flashes – especially X-ray flares associated with CME activity – not to mention the solar wind, all of which inject energy into the atmosphere in an extremely variable and unpredictable manner.
In addition, we have the complication that incoming energy from sources such as cosmic rays may affect cloud cover; a rise in comic rays may increase cloud, increasing the planetary albedo levels (the ‘reflectivity’ of the planet) a decrease reduces cloud allowing more solar radiation to reach the surface. An increase in explosive solar activity sweeps away cosmic rays and this is thought to allow an increase in the amount of base line (TSI) solar radiation heating the atmosphere and reaching the planetary surface.
There is, fortunately, a rather important ‘However’ in this respect, in that we do have available a number of key indices which allow a general assessment of the level of solar originated energy impacting the atmosphere; perhaps the most relevant of these being the ‘Planetary Ap Index’. This gives a very useful assessment of the level of broad scale solar induced geomagnetic energy impacting the Earth, and it is relevant to note that the graph of this index correlates very closely with observed surface weather patterns and phenomenon – perhaps even closer in short timescale terms than the coarser sunspot data.
If we are to carry out any meaningful measurements within the atmosphere, then we need to establish standard points or levels at which key measurements may be taken and related to each other over time and during different atmospheric states. At the surface, the standard ‘mean’ pressure is taken to be one atmosphere, 1 bar or 1000 millibars (hectopascals) and measurements are related to that. At upper levels, the common standard point is half of surface mean pressure or 500 millibars (see : Definitions)
The ‘Key’ point in the upper atmosphere, the point or level in the upper atmospheric gradient that most influences surface weather behaviour – often referred to as the ‘steering’ level – is delineated most closely by the 550 to 560 hPa (mb) isobar lines in the gradient between the higher pressure at the equator and the lower pressure at the poles. (It should be noted that this is the opposite of what happens at surface levels where the equator maintains low pressure while the poles retain a high, an effect driven mainly by temperatures at ground level.) It is at this point in the gradient, the point at which the gradient tends to steepen-almost a ‘crease’ in the atmosphere- that the primary ‘Jet Stream’ tends to form and it is along this isobar line that low pressure cyclonic cells and storms tend to travel.
This key isobar moves north and south with the seasons and is twisted by the thermal differences between land and sea. The pressure gradient in that area can be steep or shallow and both the precise position at any particular time of year as well as the gradient appears to be heavily influenced by short term solar impacts and overall geomagnetic activity.
Thus, if it is seasonally positioned at a relatively low geographical latitude, and then comes under the influence of a significant solar impact event, it can be nudged to a slightly higher latitude, the gradient can be steepened and both the intensity and path of low pressure surface storms influenced.
If short term explosive solar activity is low, then this line can drift to unusually low geographical latitudes – for any given time of the year – with the consequent effect on the path of cyclonic systems and thus indirectly on the surface weather – including temperature.
Reduced overall solar activity – both baseline and explosive – would imply lower atmospheric energy levels and a lower geographical latitude for the 550 hPa line. However, when this happens in autumn , the cyclonic low pressure rotations tend to pick up warmer air and throw it to higher latitudes, giving unseasonably warm surface temperatures – implying that atmospheric energy levels are higher when in reality the overall energy level is lower. A solar induced ‘push-back’ of the line at this time of the year would give repetitive storm impacts in the effected regions with consequential flood risk.
Conversely, when it happens in spring, the dominant temperature structures are of winter and the effect is to drag cold air from the north unusually far south. Sluggish advancement in late spring or early summer can cause a build up in heat energy giving abnormally high temperature readings on the equatorial side of the line, while storms and rainfall to the polar side will be intensified by the additional energy levels.
The ‘twisting’ effect can also mean that weather patterns over continental land masses display the opposite tendency, exacerbating colder conditions over the eastern side of continental land masses.
In short, the complexity of deep atmosphere behaviour makes analysis extremely difficult and a much deeper understanding of both solar impact activity and upper level reaction to it is required.
‘Steering Level’ – Seasonal Movement.
Fig:-1, below, is typical of the deep atmosphere chart (Europe) during northern winter. The upper level isobars (black) show the gradient between the equatorial pressure, (normally around 600 hPa ) and the polar region (normally around 450 hPa) at the altitude under consideration. The ‘key’ level of around 550 hPa may lie around Mediterranean latitudes at this time of year.
The cyclonic low pressure centres (T) being ‘steered’ by the key level can be seen in the white, surface level isobars
Fig:-2, below, is typical of the same chart during northern summer. It will be noted that the key level under consideration is now up around Icelandic, Norwegian latitudes. During any given year, the positions reached at summer and winter extremes, and both the position and the profile of the isobars at any intermediate time of year will vary dependant upon conditions prevailing within that year.
Fig: – 2
The two charts shown have been selected purely because, at that particular point in time, the situation was fairly simple, the charts were relatively uncomplicated and provided a state of affairs that – while being real and unmodified – were examples that were easy for the non-professional person to comprehend. Usually they are significantly more complicated.
Ap Index Chart.
Ap* Index Related to Annual Sunspot Number.
*TSI = Total Solar Irradience.
hPa = Hectopascal : 1 hPa = 1mb.
MSLP = Mean Sea Level Pressure.
This is the surface pressure reduced to sea level. Solid lines are isobars (every 5 hPa), that is lines of equal MSLP. These charts show surface pressure patterns – areas of high and low pressure which are associated with different weather types. Usually low pressure systems (cyclones or depressions) bring unsettled weather whilst high pressure systems (anticyclones) are associated with settled weather. In the northern hemisphere the air rotates anti-clockwise around the low pressure centres and clockwise around the high pressure centres (the opposite applies in the southern hemisphere). Wind speed is roughly proportional to the distance between isobars: so closely packed isobars mean strong winds, and vice versa.
500hPa Geopotential Height
The geopotential height of the 500 hPa pressure surface shows approximately how far one has to go up in the atmosphere before the pressure drops to 500 hPa (i.e. 500 millibars). On average this level is around 5.5 km above sea level, and it is often referred to as a steering level, because the weather systems beneath, near to the Earth’s surface, roughly move in the same direction as the winds at the 500 hPa level. Height contours are labelled in tens of metres (=decametres, =”dam”) with an interval of 6 dam. The contours effectively show the main tropospheric waves that “control” our weather – low heights indicate troughs and cyclones in the middle troposphere whilst high heights indicate ridges and anticyclones.
850 hPa Temperature
Colour shading indicates temperature at the 850 hPa level in degrees Celsius (oC), in 4oC colour bands. This is the temperature approximately 1.5 km above sea level, usually just above the boundary layer. At this level the diurnal (daily) cycle in temperature is generally negligible. Therefore temperature at 850 hPa can be used to indicate frontal zones (i.e. areas of large temperature gradient, where the isotherms are more closely packed together), and naturally also to distinguish between warm and cold air masses. Sometimes temperature at 850 hPa can be used to roughly assess the maximum temperature at sea level by adding 10 to 15oC, and for higher ground one can interpolate. However there are situations when this method does not apply, particularly in winter.