This is a work in progress - Please don't take it seriously until it's finished. I've already fixed two errors.

I thought I ought to write something about weather because, as I researched things a bit for diving and now hang gliding, it is something I need to know about.
What I discovered is that most things that people want to tell you about weather just don't help. They tell you what they think you ought to know and miss out the 'what on earth is happening' bit that I for one need at the beginning.
So I'm going to start at the beginning and ramble round the subject. The scale of things is the important factor.

Let us start with a statement:
   "We are all used to air, sunlight and water so we already know about weather from its basic components."
This is so wrong in so many ways it is frightening.

We know about air? Weightless stuff isn't it? Looking at a weather map today I can see a small block of cold air moving south across England. It is small because it's only about the size of Wales so, say 240 kilometres square and about 20 kilometres thick. Air weighs about 1.27gms/litre so that's 1152 thousand cubic kilometres, 11e5 (11 with 5 zeros after it) cubic kilometres, that's 11e14 cubic meters, 11e17 litres, OK I have to factor that by 70% as it gets thinner on its way up to 20 kilometres high, so about 1e18 grams, 1e15 kilograms, 1e12 tonnes. Write 1e12 out properly as a one million million tonnes. It probably has several million tonnes of water in those clouds too. Believe me that is just a small weather system.

And solar powered... Well this isn't like the flat panel you bought on EBay that lies on your car dashboard and totally fails to charge your car battery, that is a few inches square. This is the size of Wales again. 240 by 240 kilometres (57e3) and remember that for the atmosphere there is no such thing as a cloudy day as it is on top. The sun puts about 1.3Kw/square meter into the upper atmosphere but about 30% bounces straight off and up in England we don't have the sun beating straight down on us so factor out another 25% but even so 600watts per square meter is not bad when we have 57e9 square meters. That is 3.4e13 watts which is over one fifth of the entire Earth's population's appetite of electricity which is 16e13 watts. This is just driving one small weather system.

Right. Now you are getting a feel for things and I haven't even come to water yet. We have one more snag. If I take a room with a curtain down the middle and then I put cold air on one side and hot air on the other and then draw back the curtain we very rapidly have a reasonably evenly warm room. For ten feet style distances this works well but once we start talking in terms of the distances we use in geography air masses do not mix. The edges might fuzz out a bit but the rule is that the distinctions tend to persist until the temperatures even out.

OK so now you're beginning to see why a good tropical storm, real climate rather than the wimpy little weather we get in the UK, can convert you holiday sun destination into a disaster movie set in a couple of hours. A few million million tons of weather does what it wants to and we can make use of it or keep out of its way.

So what are the rules so we can understand the monster?

Well the first, very important, rule is that cold air sinks. When air becomes colder it becomes more dense. OK so it's only a fraction of a percent more dense but when you have millions of millions of tons of the stuff small fractions still work out as really big numbers. If some air is colder than other air it is going down and somewhere else air that is warmer is going up.

Once air is moving the very scale of the size of weather systems means that they cover a lot of planet and the turning of the planet turns them. We tend to think of the Earth rotating evenly but we all know the equator is rushing eastwards at about 1670mph (a bit over twice the speed of sound) while the poles are doing a pirouette going nowhere. Think of a block of air moving north in the northern hemisphere. It has a northerly component to its velocity but to be going due north to our eyes as we stand on the moving planet beneath it must have an easterly component too to keep up with us. We know it weighs millions of millions of tons and it's not bolted down or on rails so if the surface of the earth chooses not to be moving so fast below it as it gets further north it will just keep going and the easterly component starts to be seen. Similarly air apparently going due south will not have so much easterly component and as it comes south the increasing speed of the earth below it will make it apparently start to move west. What does this add up to? Well air that would have flowed straight into a low pressure area from all sides turns east when coming from the south and west when coming from the north so it spirals in anti-clockwise. In the southern hemisphere stand the whole argument on its head and air spirals into a low clockwise.

(Trying to extrapolate this to why the bath water goes down the plug hole clockwise or anticlockwise is a waste of time unless your bath is the size of the North Sea and the plug hole is about as big as Birmingham. Small fractions of less than nothing are very very small numbers.)

Weather chart I shamelessly stole on the web Right. Now we are getting the picture. We have blocks of warmer and colder air. They rotate and rise and fall and the borders between them tend to persist. Now we can look at a weather map and start to make some sense of it.

First spot the map underneath with its lines of latitude and longitude. It's a bit twisted because I've selected Europe and omitted the bulk of the Atlantic. Germany is dead centre and England to the left of that a bit.

Now over that we have isobars. These are the light grey lines and some have gaps in them with numbers in them. These are like the contour lines on a map that track round at points of equal height so these track equal air pressure. You can see they tend to run round places and the map makers carefully mark the centres of each maximum or minimum with an X and a value that is the pressure at that point. If the isobars are close together the pressure will change rapidly if you travel across them.

The pressures on this map range from 969 to 1029 milliBars. I think the maximum range ever recorded is 870 (Typhoon Tip 1979) to 1085.7 but those are serious extremes. What you can see is that south of Iceland the pressure is changing rapidly by distance (lots of close isobars) so there is lots of push generating wind (air being pushed out of the high pressure area and heading for the low pressure one) so it's windy in the Denmark Strait while out in the Mediterranean there are virtually no isobars so the pressure is pretty constant so the yotties are all becalmed and having a beer.

The darker lines with the lumps on them are the dividing lines between the blocks of air. We talk about warm fronts and cold fronts but that just says which way the divide is going. The triangles are a cold front (colder air moving into an area that was previously occupied by warmer air) and the rounded ones a warm front. Remember there aren't two different types of air, warm and cold, so you can have successive cold fronts following one another, each air mass colder than the one before it. Also in the tropics a 'cold' front may be a transition from 'horribly hot' to merely 'uncomfortably hot'. It is just cooler.

Now the guys drawing the maps try to put the blips on the fronts to show which way they are moving and if it's stopped they alternate sides. Add the complication that an old front can get so twisted up it is going in different directions at different points along its length so at some places it is a warm front and in others a cold front. Provided you remember that a front is just the boundary between two different blocks of air characterised by temperature it makes a lot more sense.

So just north of Ireland there is a nice warm front (round bits) with a cold front (triangles) following it. The way things work normally means the cold front will go faster and catch up. Now we probably still have a temperature difference so there is still a front but all the weather that was associated with the two fronts is now piled up on itself so we get an occluded front with both symbols. Occluded fronts tend to start to fade out as they are confused. The fronts to the west of Iceland are forming a nice new occluded front. The one trailing down over Scandinavia still has a lot of weather in it but the one that has drifted over to the Balkans and Turkey is coming to pieces and they are drawing the symbols because there is some weather left but the temperature difference has gone so it is now an ex-front.

There is a slight question about where is the warm air to the west of Ireland going as the cold front catches up with the warm front and the answer is actually pretty obvious. It is warm (lighter) air surrounded by cooler (heavier) air so it goes up. When we discuss the three-dimensional shapes of fronts this will make more sense.

Also air isn't strictly one temperature. A front can just end because the cold air at that end was a bit warmer and the warm air a bit cooler and the distinction went away. Don't expect a front line all the way round as the map makers only put in the significant bits which to them is the bits with weather in them.

So what are the lows and highs?

Well a centre of low pressure is just what we measure on the ground or a from boat if it's out in mid-Atlantic like some of these. What is happening? Well the air is warm and it is rising. This tends to leave less air behind so the cooler air moves in to take its place. That's actually spiralling in as we already discussed. Remember: Low pressure, rising air, anti-clockwise. If you look at the front pattern on the mid-Atlantic low you can see how the spiralling winds are already wrapping the fronts into curves.

A high pressure area is cooler air sinking and hence it is spiralling outwards, this time clockwise. This gives us the simple trick that if you stand with your back to the wind the high pressure is roughly on your right and the low on the left. This falls apart when we start to consider local effects but it's a handy rule of thumb and once you are well above the ground works well.

OK so that's air... What sort of weather does this cause?

Well by weather we are normally talking about water as clouds, as rain, as snow and stuff or quite invisibly as water vapour.

Humidity A few ground rules again: There is almost always water in the air but it is called 'water vapour' and this is water in its gas state so you can't see it. It's like sugar in your tea. Unless you're five and put in three big spoonfuls there are no crystals left. Clouds are not water vapour. Only when the water forms into droplets in its liquid state, even very small droplets does it appear. To put some numbers to that look at the graph on the left. This is the partial pressure (think of it as a fraction) of water that can exist in air as vapour at a given temperature. I've plotted from 0 to 40°C against 0 to 8kPa.

Temperature at altitude The first thing you notice is that warm air can hold a lot of this 'invisible', gas state water so we can immediately deduce that as wet air cools it will rapidly reach a point where the amount of water it contains exceeds its new carrying capacity and it stops being water vapour and becomes plain ordinary water as droplets. Store this fact in your mind with the label 'Dew Point'. For any lump of air with water vapour in it the Dew Point is the temperature where some of it stops being water vapour and starts to become ordinary, honest to goodness water. It may be the dew on your lawn or the cloud you exhale on a frosty day but it is the tiny droplets of water that you see.

Move to the second graph on the right. Take some air, starting at 35°C and make it just rise without any heat going in or out. The blue line is the pressure, starting at the usual 101.3KPa (1013mBar) and falling as we go up to 10000 feet to 70kPa (see the scale on the left hand side). The red line is the temperature scaled on the right. As the air expands its temperature drops. It started at 35°C which is a nice hot day but by the time it has got up to 10000 feet it was down to 4°C. Put the two graphs together and you can see why air with invisible vapour in it at ground level is releasing water droplets and becoming a cloud as it rises. Please remember that this is not warm damp air mixing with colder air as it rises, this is just the simple physics of reducing the pressure. As we have already discussed air bodies tend not to mix.

What I have here is a nice picture of a Cumulus cloud sitting on top of a thermal. A thermal is warm air rising through the air above and, naturally, getting colder as it does so. This means that at some point it is going to hit its dew point and start dumping water vapour into droplets and become visible.

On a nice sunny day you often get lots of these little bubbles that appear from nowhere and, once the hot spot that spawned the thermal ceases to drive things, they fade away again. They tend to be known in old folk terms as 'fair weather clouds' in that they didn't imply it was going to be windy or rain.

If they are big butch Cumulus they don't only create a bit of fluffy cloud but it gets wet enough up there to start dropping water on your head. Clouds that big tend to look dark because they are thick enough to stop the sun getting through and we call them Cumulo-Nimbus. Nimbus is just a Meteorologist word for 'this one's gonna rain on you'. The big version of Cumulo-Nimbus is a thunderstorm. The rising air is rushing up so lots of rain and some novel electrostatics. This is little more than making sparks by pulling your nylon jumper over your head and making crackling noises (try it in a blacked out room) except on geographical distance scales again. Cu-Nims tend to have a flat top as the rising air just runs out of rise and spreads out at some level or runs into high altitude winds that give it the 'anvil' look.

Big Cu-Nims are not nice clouds. They contain a lot of energy and viscous updrafts so large aeroplanes like to steer well clear of them and hobby fliers look at the sky and decide to stay home and mow the lawn before it rains. It would be quite easy to find updrafts below one of these that not only would outrun the sink rate of your glider but even that of your emergency parachute.

Right now a point about air rising...
For air to rise it needs to be lighter than the air it displaces. We have worked out that big lumps of air are heavy but that goes for all air. A few hundred tons of air coming across the countryside (wind) running into a few more hundred tons of air not going anywhere stops. You will see simplistic drawings in books of air running in and hitting a hillside and flowing up and over it. Well it has to be warmer air to do this because if it is cooler and hence heavier it won't overflow the hill any more than the incoming tide in the English Channel overflows Brighton. It just fills up to its level and stops.

This can mean that you stand on the top of a hill on a chilly morning looking out over a cool misty valley with a breeze blowing in your face and the mist is just sitting there. Obviously no ridge lift. The valley before you is just a still pool of colder air and the wind in your face is blowing across the top of it.

Perhaps best way to put it is that Wind doesn't make air move, Wind is air moving. Thinking of wind pushing air is such an easy trap to fall into and it's just so horribly wrong.

Let's look at a Cold Front in a bit more detail.

Cold Front This is cold air moving into an area that was once warm air. Since the cold air tends to sink relative to the warm air it tends to pile up and push along the ground.

Please excuse the scrappy sketch. This is roughly what it looks like in section. Think of a rounded bulldozer blade scraping along the ground lifting the warmer air in front of it up and displacing it. OK, we know what rising air means. The warmer air probably has a reasonable content of invisible water vapour and suddenly it gets pushed up so the water vapour finds itself being chucked out as droplets so all the way along the front we have lots of newly generated clouds. Showers if you're lucky and thunderstorms if you aren't. This is aggressive weather because it is so abrupt. As the warm air is being lifted away the ground level pressure tends to decrease as the front approaches, hits a low when it arrives and then increases as all that dense cold air sits on top of you.

You can see these things coming. It's a wall of nasty, probably black, clouds stomping across the countryside and when it gets to you it not only rains on you but it is also colder and on the front itself are gusty winds. After it has past it is a gloomy day as all your nice warm air has been lifted away and is now a roof of cloud over the colder block. Places that have serious climate get violent upsets at this point and tornado watchers chase cold fronts hopefully, expecting a show.

I'll get a picture later.

And now a warm front.

This is a very different scenario. Because the warm air tends to rise in the presence of colder air the encroaching warmer air tends to ride over the cold air and generally squash it out of the way. The slope of the boundary is not something I can draw as it is 1 in 100 or 1 in 200 (a hundred miles horizontally for one mile vertically) so you aren't getting a picture as it will just mislead you (lots of books draw something that looks about 1 in 4). If I drew it to scale all the way across your 1000 pixel monitor it would rise 5 to 10 pixels, about the height of a letter o. It would be a very boring drawing.

With a slope like that you start getting the effects of the incoming warm front high above you when the actual ground level transition is still over 500 miles away with Cirrus clouds at 26,000 feet ie. 5 miles high (remember the 100 to 1 slope). Then as it gets nearer you get lower and lower clouds. I'll do cloud names later and try and explain why we care about the differences. However the long 'overlap' isn't new rising air so the clouds tend to be reasonably consistent. Frankly a warm, or occluded, front coming in is just a gloomy day but usually dry.

Now that funny Occluded front thing

Now you know how a warm front's weather is spaced out in front of the front and a cold front's is following it you can understand how, when the ground level effects of a cold front catch up with the ground level effects of a warm front, both sorts of weather persist. There is still a long run of warm air stretching for hundreds of miles in front and a wall of cold air now up behind it so you get the effects of both sort of weather.

Actually as warm fronts aren't anything like as good at pushing other air masses out of the way cold fronts tend to catch up with them and the occluded effect is pretty normal. High altitude cloud followed by a wall of the wet stuff.

Thinking vertically
Temperature at altitude Right. Look back at my graph of temperature verses pressure as altitude. The formula to work this out is a cool application of the first law of thermodynamics but those lines look suspiciously straight.
We call this the Dry Adiabatic Lapse Rate. Dry as in air with no free water in it so not cloud or mist. It works out at 9.76°C/kilometer or 3°C/thousand feet.

Once the air has free water in it it stops being a straight line. It is horribly temperature dependant because as water transitions from vapour to droplets it give up its latent heat of vaporisation but what the heck - a good enough guess is 5°C/kilometer or 1.3°C/thousand feet. This is the Saturated Adiabatic Lapse Rate. 1.3°C is wrong but it will do.

Just be clear now. This does not tell you how the temperature drops with altitude. This tells you what would happen to a piece of air that rises or falls without extra heat or water entering or leaving it. If you just hang from a balloon and go up you are in different bits of air so the temperature and humidity can do just whatever they want.

Also a 'Lapse Rate' is just the way something changes with altitude. We tend to use it to talk about temperatures but the term can be used for anything. Don't get caught out assuming...

Dew Point
Humidity So back to the other graph. The humidity verses temperature one. Again an entertaining piece of maths can be simplified to the idea of a Dew Point. You don't care what humidity is in grams of water per cubic meter of air you just care when that piece of air is going to stop being clear air lapsing at 3° per thousand and become cloud lapsing at 1.3° per thousand. This is the Dew Point.

I love the term Dew Point. Somehow it conjures up images of an impeccably dressed Victorian amateur Scientist wandering out into his garden on a misty morning, watching his lawn, his greenhouse and some big pots with warmer and colder water inside them and meticulously recording temperatures in his journal. The Dew Point is just that. For a given lump of air it is the temperature at which dew begins to form. That is the 'is it now cold enough for the air to no longer support all the water vapour it contains?' point.

Wet-Dry bulb thermometer There are absolute measures of humidity and relative humidity but the dew point is the number we want for weather calculations. You can measure the dew point directly by having a wet thermometer. A wet thermometer is fed with water that is evaporating from it, and hence cooling it, until it gets down to the dew point when water can't evaporate any more so it doesn't get any cooler and it just stays there. Read the dew point. OK you can't just let it sit there in its own cloud of fug, you need to make sure the air about it is regularly changed but dew point thermometers normally come with a handle so you can wave them about (seriously!). If you want the relative humidity, effectively how much water there is in the air on a scale of 1 to 100% where 100% is hitting dew forming you take the temperature on a dry thermometer too and use the simple approximation that for every 1°C difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH=100% when the dew point equals the dry bulb temperature.

My picture is a device to do this. Two thermometers, one with a little cotton sock on it fed with water from the bottle at the end and with a handle so you can swing it round like an old rattle. They tend to come with a little chart so you can convert the readings into relative humidity if that's the number you want.

OK, so I just took it outside and did it. It took about 60 seconds of swinging to get readings that weren't changing any more and it read 18°C(dry) and 13°C(wet) hence a 13°C dew point. The chart tells me that that is 49% relative humidity. However if my 18°C garden was a hot spot and all my neighbours were, for some reason, cooler then my nice warm air would start to rise. With a 3°C per thousand feet lapse rate it would drop to 13°C at about 1700 feet so I would have my own personal little cloud seventeen hundred feet over my house sitting on a column of rising air.