Weather
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 as I researched things a bit
as with 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 you need at the
beginning.
So I'm going to start at the beginning.
The scale of things is the clue.
Let us start with the 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? 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 200 kilometres square and about 20 kilometres thick. Air
weighs about 1.27gms/litre so that's 800 thousand cubic kilometres, 8e5 (8 with
5 zeros after it) cubic kilometres, 8e14 cubic meters, 8e17 litres, so about
1e18 grams, 1e15 kilograms, 1e12 tonnes. Write 1e12 out 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 the flat panel that lies on your car
dashboard and fails charges your battery, that is a few inches square. This is
the size of Wales again. 200 by 200 kilometres (4e4) 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/sq meter is not bad when we
have 4e10 square meters. That is 2.4e13 watts which is about one seventh of the
entire Earth's population's appetite of electricity which is 16e13 watts. Just
for 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 and put cold air on one side and
hot air on the other we very rapidly have a reasonably even warm room. For ten
feet style distances this works well. Once we start talking in terms of the
distances you 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 know why a good tropical storm, real climate rather than the
wimpy little weather we get in the UK, can convert you sun holiday destination
into a disaster movie set in a couple of hours. The 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 air is getting colder it is
going down and if air is getting warmer it 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. 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 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 apparantly 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 small numbers.).
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 910 to 1085 but those are serious extremes. What you can
see is that south of Iceland the pressure is changing rapidly (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 they are becalmed.
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 warmer) 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 Scandanavia 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 spiraling 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.
A few ground rules again: There is almost always water vapour in the air but
when it is vapour you can't see it. Clouds are not water vapour. Only when the
water forms into droplets, 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.
The first thing you notice is that warm air can hold a lot of 'invisible' water
so we can immediately deduce that as air cools it will rapidly reach a point
where the amount of water it has exceeds its carrying capacity and it stops
being water vapour and becomes plain ordinary water as droplets.
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 hot day but by 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 this is just the simple
physics of reducing the pressure. As we have already discussed air bodies tend
not to mix.
What I need here is a nice picture of a cumulous cloud sitting on top of a
thermal.
Let's look at a Cold Front in a bit more detail.
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 and tornado watchers chase cold fronts hopefully.
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. The slope of the boundary is not something I can draw as it is 1 in 100 or
1 in 200 (a hundred miles horizontaly 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 front high
above you when the actual ground level transition is still over 500 miles away
with Cirus 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
consistant. Frankly a warm, or occluded, front coming in is just a gloomy
day but usually dry.
Now that funny Occluded front
Now you know how a warm front is spaced out and a cold front is abrupt 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 streching 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.
