Weather and the Hydrogen Bond.........

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Meterologists amongst others like to think of ascending air by considering theoretical little parcels- nice little 100g 100ml or 100 mmHg etc which are nice for calculations and actual lab experiments. For a given weight of air it will have a parcel size (volume) at the surface and as it rises through the atmosphere it will expand with the ambient pressure gradient. So like a little helium weather balloon, 100g of slighly warm air will rise and expand to a height where it is no longer bouyant - cannot go on floating on surrounding air- then it just in our wee theory dissipates or the baloon pops, given all things are equal.

However this is complicated by moistness in air. Air will absorb different amounts of water depending on it’s pressure and temperature- the higher the two the wetter air can be. When air fills up with water molecules it is said that it becomes absolutely saturated.It is at 100% - it’s relative humidity RH is equal 100%. In other words for a given amount of water molecules around it has absorbed as much as it will hold in “solution” if you like..... analogous to being like salt in water. ( very warm and dense air can actually contain more than it's theoretical relative humidity without condensation, but that is not important for us)

Air in our atmosphere will hold it's maximum humid content at 30' C apparently..... at a decided air pressure- surprisingly for such a stifling day in the tropics, the actual weight content is only 3% or 3 g per 100g air.

Thereafter excess water vapour in the atmosphere will condense. Also that parcel of air will have a ‘dew point’ a temperature and pressure where it cannot hold it’s water molecules in solution- the property of water to loosely bind to itself exceeds the ability of the air to hold the individual molecules below a microscopic drop size. Usually the small droplets form a cloud or fog but to actually "precipitate" into drops big enough to fall as rain or to crystalise into snow flakes, the droplets need to have a seed crystal so to speak, which is usually a dust particle. Near flight paths into air ports you can sometimes tune your radio in to a repeated information “autonomic” message which includes “due point xx Metres “ this is the height at which the days air characteristics and the temperature gradient in the atmosphere will allow condensation of water droplets- that is to say this defines the cloud base!! Phew….at least I didn’t throw any equations at you!

There is now the diabolical diabatic conundrum. When the theoretical air packet absorbs or gives up heat energy without changing pressure notably, then it will not convect significantly – if warm, moist air over a cold surface or cold, dry air over a warm sea or lake interacts with those surfaces in a way to create dew point then the result is fog! This process need not necessarily happen with particularily low temperatures- there just has to be a very large gradient. A couple of times when sailing on warm days and once when cycling down the bonnie banks of Loch Lomond, the super humid air was in a sheet just around about dew point- spots of water would form on your clothes and hit you sporadically. Suddenly the diabatic process went adiabatic- the air blanket tried to convect, reached dew point as a mass and a huge thunder like shower resulted – to last just a ten minutes and to leave only small whispy clouds ascending in it’s wake! Luckily temperatures were in the high twenties and my bike shorts dried out within a fairly comfortbale time.

But if an already moist air parcel does not give up energy or absorb energy from it’s surroundings, but rather converts the energy from heat to kinetic (movement energy) then it will convect. It will rise and expand and go upwards in the sky like a hydrogen filled balloon. This is an Adiabatic process because there is no net effect on the surrounding environment (think like asymmetric NOT symmetrical) – surfaces and air surrounding a moving parcel are not altered in temperature- just those parcels or columns, or bodies of moving air. At some altitude sufficiently moist air will condense and this releases further energy because of the heat of vapourisation – a fairly unique feature of water whereby all the new hydrogen bonds between water release a net heat energy in normal atmospheric conditions. What we now see is the leading edge of a cloud which is either mechanically convected (blown air up a mountain ) or thermally convected ( by the action of the sun or a particularily warm surface, or convergence-convected where two air masses collide and force air upwards.

I guess the whole issue of thinking packets versus actual streams of air is a bit heisenberg uncertainty...you can't know all of the factors in a rising column of air at the same time and you can't measure them without disturbing the prcess.

Weather forecasters send up balloons and use infra red information from earth stations and satellites to establish the temperature gradient in a mass of air, below the isopause. Also they can measure the pressure gradient. But temperature is usually predominant in determining the limit to convection –in therory from these practical observations a knowledge of RH% at the surface, they calculate the height to which a given air parcel can convect by nature of it’s buoyancy, before it is so cold that it will no longer act adiabatically.

Sometimes the limit to convection can be very low or alternatively a temperature inversion can create a notable ceiling above which any warm air, or smoke will not rise at it rises to the point or height at the same temperature and pressure as it’s surroundings. The parcel of air is therefore not any longer buoyant. If water behaved like some other substances condensing then there would maybe be large, flat clouds covering any air mass with convecting parcels. In fact when some chemical spills or concentrated pollution occurs then just this happens.- a flat cloud developes at that chemicals “dew point” and this can be either dangerous or unsighltly.

What happens near the limit to convection for an air parcel is that the pressure of the “expanding balloon” of the parcel reduces as does the temperature – think of a bike pump getting hot and a spray can being cold at the nozzle. BUT what happens thereafter is peculiar to water- the dew point is reached internally to the parcel and water begins to form droplets- it’s innate attraction to itself by van der vaals force called hydrogen bonding overcomes the ability of the gasses in the air to break between these and hold water largely as mono-molecular in “solution”. Now when the regular structures of microscopic droplets form many billions of hydrogen bonds are formed in each tiny drop. It takes energy to break these bonds- like when water boils- but when they are made again there is a release of energy. So the air parcel absorbs this energy and becomes buoyant again.

This explains firstly why clouds most often have a leading edge with an upwards wind component and why some clouds are higher and more volumous than others. Metereologists will know that for example on a sunny day or with underlying warm air, a relatively cold, steep temperature gradient may allow for the probable formation of cumulounimbus- thunder clouds. So this is one area of interest to the yachtsman because thunder can mark a 180’ wind shift!

In the day, thunder clouds are easy to spot but at night yachtsmen should be wary of forecasts for lighting and the associated wind shifts. Also a significant drop in water temperature or sudden very cold wind, could mean a diabatic process is about to happen and the yacht will be fog bound! So all this theory so far already has it’s practical aspects. Sometimes the weather forecasters get it wrong for a given sailing area, like the Firth of Clyde or Sydney harbour for example and you can see that there is warm air at the surface, and very high clouds are forming with dense water droplets. Eventually these may form the characteristic anvil top and you will know lightening is on the way. If you are right underneath them then

Other clouds form mechanically- like the long white cloud which gives New Zealand it’s Maori name and these can be near permanent when moist air “parcels” are being forced upwards by a horizontal wind ie. A surface wind created by isobaric or gradient wind at altitude.

Clouds are formed in two general ways – the adiabatic thermal and the lifted fog bank which is perhaps both a mechanical and thermal process reliant on wind blowing the sheet over warmer land or into colder air or altitude. Either process can result in a typically dreary morning under frontal weather zones with a low cloud base (dew point) and high amount of the sky , given in 8ths, covered.

In low pressure with a significant gradient breeze then these water droplets and their associated air parcels or vertical columns of air will be driven along. Clouds can reveal how strong the gradient wind is by skuing the tops of clouds, or some clouds may even reach the jet stream – an important coincidence in development of lows and storms (polar and temperate region cyclones).

As a completely overcast sky in a low pressure, the clouds are driven along by the gradient wind by friction at their upper reaches. At the surface we may feel only a light wind, which may blow in a completely different direction of more than 45’ to the gradient breeze above. Through the course of a summer sailing day, the sun warms through the clouds from above and to the ground below. So there are gaps developing between them. The opposite effect happening at the leading edge of a cloud aids and abets the wind above coming downwards.

As a cloud is either formed by convection or a cloud sheet is broken up and rebuilt by convection into cloud masses, there develop trailing edges – here the condensation effect is happening – water is able to absorb enough energy to break it’s droplet prison and in doing so it cools our imaginary flock of wall-øess ballons- there developes a descending column or eddy or air. This can then attract the attentions of the higher, laminar flow – ie the gradient wind. In effect a gust, or a wind direction shift, in open water is the result of the trailing edge of a cloud guiding down the gradient wind to the surface. On the way down it encounters friction and backs somewhat, but is always veered because it retains more of the gradient breeze than the wind which at surface tries to make a short cut to the centre of the low- ie it is backed towards the centre .

Now some nice neat diagrams show that this ‘bringing down’ of the breeze happens somewhere quite far back from the trailing edge “down draughts” and these are nice and regular circular features. Well, in normal white-puffy-cloud days with say force 2 to 4 winds this is probably true, But I have experienced squalls right at the trailing edge of jagged, dense, rolling/morphing and fast moving clouds.

On the Race Course:How much the wind is veered and strengthen in the gaps and how far behind clouds is pretty much up to the actual beat to windward and often changes between each of the three beats on a classic W-L race!

However, you may go out looking for a pattern from the sky at an unfamiliar venue or from an unusual wind direction at your usual club nights only to find that there is an unrelated gust pattern and those gusts are even backed rather than veered. What you then have is another effect such as a valley, a major thermal over land-often say a hot city - or a mountain creating a surface laminar flow which races in from another direction and to a higher strength or even to a cancelling stegnth-direction so as to really confuse the pattern!

However puffy white cloud days happen a lot in northern coastal Europe, and lead to visual clues and timings between gusts and the backed lulls ….So too do days when a dense cloud layer is broken up by evapouration, friction and convection.

Sometimes larger cloud masses come in from the sea where they have previously been broken up by convection into large cells – huge clouds by area they cover - and assume a fairly regular distribution ie distance between them and size. These cells are moving roughly say at 70’ degrees to the isobaric winds and they drive over in a remarkably constant timing. Giving you the ability to spot when the clearer, windier and veered weather “gust” will come- it may just come down as a shift if this part of the low is not very “active”. Sometimes at night you can just feel a little colder suddenly and righly enough in holding a beat or controlling a spinnaker, the wind veers thereafter. A shiver down the spine is a call for observance before it is for a nice warm cuppa!

IN both cases, as now informed by the discussed above about when the down daruaght meets the surface, it may be some good distance behind a cloud under a clear area and because it’s hard to look backwards and upwards you can use a stop watch to determine how far behind a cloud they are as you sail under the trailing edge. On many days in exposed areas you can go out and take timings before the start ..but be wary because convective forces build to a peak around mid to late afternoon in our temperate low pressure systems over the summer sailing months…so a nicely timed out plan can go to pieces when convection alters the spacing of gusts or shifts.

For the Course: While of racing round an Olympic course in low pressure conditions then recongising what is happening with the cloud height, separation and the temperature out over the day can give more applied applications to the adiabatic theory.

High Pressure and Rising Pressure

High pressure manifests itself as either an anticyclone- viz a vi a large area of clockwise (nordlige halvkule) rotating air or as a ridge between low pressure areas. Calling it a ridge or thinking of the nice, high, unoppressive sky in an 'anticyclone' as being an upwards phenomenon is largely a misconception- air in high pressure is being compressd and descending. In fact the highest recorded natural pressures at the surface are in the desserts of the former USSR in winter. High pressure in winter produces cold and stable weather in our northern european areas.


Because the air is dense and descending it can adsorb water. Even in winter this is mannifest in the wake of low pressure by flat, thin clouds or the change from medium height cumulus to strato-cumulous clouds.

For the Race Course and Regatta: Flattening of clouds is a sign most of the time of less wind as far as the race course goes. In addition given both a rising barometer or fair weather forecast this means for the racer that the following day will either be light winds if any before the onset of any possible sea breeze, which will begin on any local exposed surfaces, move to a larger geographical centre of thermal convection and then veer right as it builds to a peak about 4 or maybe 5pm.

Visibility is usually reduced in summer conditions as the dense, damp air falls and also traps pollutants and micro paricles. My mum always used to say that when the other side of the loch looked too close then it was about to rain. This was true and caused by the exact ooppostie effect- air convecting all moisture and pollutants upwards into colder air and lower pressures making the air very clear and objects very destinct and discernable. The brain tricks you thereafter!