Download Weather Talk Fire - U3A Site Builder Home Page

*
SESSION
4
FIRE (THE SUN)
Solar Heat
Temperature Gradients
VIS AID 66/67
For many thousands of years, humans have used the sun's energy for all sorts of reasons.
Archimedes used it when he was living in Syracuse and advised the local home guard to put
up a bank of mirrors to beat off an enemy fleet. He arranged for mirrors to direct the
sunlight onto the wooden ships of the enemy navy, many of which caught fire and sank.
Hot Stuff. Alexander the Great also used the same method via burnished shields to blind
attacking cavalry.
The effect of the sun in maintaining all kinds of life has long been recognised and it really
is the main engine which drives the weather conditions that the earth experiences. So,
when I talk about Fire in a Weather context I'm really talking about the SUN.
69
The Sun is the star at the centre of our Solar System. It is approx. 93 million miles
from the Earth, though this varies dependent on the annual cycle. It is about 330k times
heavier than Earth although it mostly consists of Hydrogen...
Its surface temperature is approximately 5500oC.
The Sun’s energy is mainly created by nuclear fusion of hydrogen nuclei into helium.
It was worshipped by many civilisations as a god and scientists really hadn’t much clue
about what it was, how it worked etc as late as the 19th century and even today there are
many aspects of the sun that are not fully understood.
1
The earth circles the sun, at an angle. This is one of the primary reasons for and causes of
our weather because it heats the earth's surface unevenly. This, of course, is because the
earth’s surface is so varied and the retention and loss of heat by these surfaces varies so
much. This causes air to move around, to a faster or slower degree, up & down, left or
right. Add the effect of the earth’s rotation and there will be air pressure variation,
expansion & contraction, variation in heat or cold. All this movement gives us what we get
WEATHER.
The various movements of the main constituents of weather i.e. Air & Water can be due to
various physical reasons such as Convection, Advection and Radiation.
Convection. This happens when two materials at different temperatures meet e.g. desert
sand and cool air, such as happens on a warm, sunny day at the seaside. The sand heats up
quicker than the sea so the air over the sea moves towards the land. This is because cold
always moves towards warm. It results in the cooler air moving towards the warmer sand
and then rising. This change in location is due to CONVECTION
ADVECTION.
This is the transfer of heat, usually horizontal, such as when moist
warm air moves over cooler surfaces and the lower levels of the air lose heat to some
degree, possibly reaching DEW POINT. This provides water moisture in the lower levels of
air and fog occurs.
RADIATION
This is the simple transfer of heat from one heat source to a colder
area. We all use Radiation, to keep ourselves warm in the house. E.G. a loss of heat from a
mass of air will happen on a still clear nights when heat is radiated from the ground,
cooling the lower levels of the air above. The air temperature can then drop to the DEW
POINT, forming mist or fog.
As the earth is a sphere and is tilted in relation to the sun, the heat falls unevenly.
The North & South Poles have the least heat and the Equator the most. The tilt of the
earth and rotation gives us the seasons, which again has different weather according to
the part of the earth facing the sun at any particular time.
EARTH/SUN System
VIS AID 70
The earth is covered mostly by water, which takes longer to absorb heat than the land.
Conversely, it holds heat longer. This is very useful to the U.K. because the temperatures
2
that we get for instance in October are relatively warm due to the influence of the Gulf
Stream. Other places on the earth at similar latitudes to ours but without the benefit of
nearby water areas can get considerably colder.
Like the movement of air masses, ocean currents occur because of this temperature
difference. It also results in the air moving around in the never-ending attempt to equalise
temperatures, resulting in winds of varying force and characteristics.
HADLEY CELLS
These are the low altitude circulations of air between found between approx. 30oN & S.
Air rising over the equator starts to move towards the Poles starts to sink due to loss of
heat at approx. 30oN or S. It then starts to reheat, returning to the equator. This
movement or circulation is constant and is responsible for the Tropical trade winds. Also
as it sinks in moving towards the Poles, it produces high pressure regions over the
subtropical oceans and the world’s hot deserts, which results in drier climates
FERREL CELLS
These are the middle latitude cells. In these cells, air meets at low altitudes to ascend
along the boundaries between the cool Polar air and the warm sub-tropical air, usually at
around 60oC & 70oC. It often happens around the U.K.’s latitude. Together with the air
masses from the Atlantic slinging their various fronts at us, this gives us our
unsettled weather.
POLAR CELLS.
The smallest & weakest of the cells, extending from towards the Poles from 60oC & 70o N
& S. Air at the highest latitudes sinks and flows towards the lower latitudes at the
surface.
The continued effect of these three air circulatory systems is also affected by the
CORIOLIS EFFECT and overall results in global circulation of winds, currents and air.
Another source of heat, which greatly affects the weather, is the underground heat
experienced when the earth's tectonic plates move or other subterranean stresses occur
which also cause tremendous heat to explode through faults in the earth's crust.
You will remember when we talked about Krakatoa last session how the eruption in 1883
caused a tremendous effect on the world’s weather. The resulting gaseous cloud circled
3
the earth for about 2 years causing extremely unusual weather conditions, the cloud
causing poor sunlight transmission. Harvest failures, colder winters than normal and poor
summers caused considerable hardships to the earth’s inhabitants.
Another volcano, TAMBORI, in Indonesia threw up great clouds of sulphurous gases when
it erupted in 1815. These gases were around for some time. In 1819, the year that Keats
wrote his ‘Ode to Autumn’, there was a very heavy snowfall in London and the Thames
froze down to Kew. Ice floes were seen in the Estuary and Tunbridge Wells experienced a
temperature of -22 degrees C (about - 9degrees F). Though it wasn’t confirmed, the cause
was probably the results of TAMBORI’S eruption. The terrible undersea earthquake,
which caused the Tsunami in 2004, created high winds, desperate weather conditions and
tremendous loss of life & damage.
INVERSION
VIS AID 69
This occurs when for some reason a layer of warm air exists above a lower layer of air
which is colder. As we know this is not normal.
WHY NOT?
It is a reversal of the normal dropping of the air temperature with altitude and is a layer
of warmer air over colder air.
It frequently happens in Winter and can be caused when radiation from the earth is
greater than that from the sun. They are usually the result of other weather conditions in
an area. Usually it is because a warm less dense mass of air moves over a colder denser
mass such as could happen on a clear night when air near the ground loses heat rapidly
Very often the inversion will become stable for some time and have effects on the earth
which can be dangerous and unhealthy. It can also happen in mountainous areas and in
coastal areas subject to cold currents or cold sea upwellings. The air atop mountains is
colder than that in the valleys below and gravitational forces often cause masses of the
cold air to slide down the mountainside and slide beneath the warmer air below.
Unfortunately, these inversions mostly happen during periods of quiet weather with little
or no wind, resulting in stability of the air masses and the inversions. They are the main
causes of Smog, as all the air pollutants mix with the cold layer which has no upward
movement. A reasonably strong wind will cause the air masses to mix and therefore
4
disperse them. If the sun can get through the blanket of the various layers as well, this
generally will hasten the process.
Very often, thunderstorms and tornados eventually occur because of the energy stored in
the cloud layers. They interfere with the normal process of convection. The results of
inversions are often extreme weather conditions which can be very unpleasant and in some
cases dangerous. In winter they can often be the cause of freezing rain. What happens is
that if snow is falling from the higher cold layer, when passing through the warmer layer it
will probably melt. If & when it hits the lower colder layer it becomes supercooled.
Supercooling happens when a liqid e.g. water passes through or is affected by an air mass
which is below the freezing temperature of the liquid it may not freeze. Upon hitting any
sloid item, it will then freeze instantly onto it. Freezing rain makes roads acquire a glasslike surface, on trees and power or phone lines it becomes long stalactites which can be
extremely dangerous when a wind blows and rattles the branches or the lines. I was nearly
brained once when visiting the control building near the TV & Radio mast at Belmont in
Lancashire. These icicles formed on the masts were like 2-3 ft. long daggers and could
have caused severe damage. Visitors now have to wear hard hats even if there is no
apparent problem when doing inspections of the guy lines and mast at this station.
The last aspect of heat I want to mention is the effect of man's activities. There's been a
great deal of palaver about climate change increasing temperature and so on over the last
few years recently and all the experts are churning out all sorts of statistics to prove
whatever case they want to make. As a result there was various investigations into the
latest set of forecasts and the statistics used to support the forecasts most of which
proved to be somewhat unreliable.
It isn’t that the climate isn’t getting warmer, it is just that it seems to be much slower
than originally predicted in recent years. Recently there has been another scientific
report that although sun spot activity has been higher, it has not materially affected the
Earth’s temperature and current climate change is happening very slowly.
There have been many tremendous climate changes to the earth's climate over the 4
billion or so years that the earth has existed. We get hot dry summers fairly frequently
(about every 5 years) with their accompanying droughts. The encroachment on the earth's
5
forests, bogs and other areas tends to affect the weather; large cities are getting larger
with their increasing release of heat from buildings. There is considerable evidence that
the tremendous amounts of carbon dioxide released by burning of fossils fuels is also
tending to make the overlaying atmosphere warmer, with its corresponding effect on the
weather. The ozone layer is apparently also getting a bit of a bashing but commenting on
that is not in my brief.
There isn't much more I have to say about the effect of the sun's heat. It really is the
main engine that affects the weather; it causes movements of air masses,the winds and
water currents with their accompanying effect on rainfall etc. so we will leave it there and
move on to the 4th major factor that affects the weather next session.
WATER
One last thing. The year 2014 was among the warmest on record. Norton Radstock like the
whole of the U.K. was 10C – 1.5 0 C above average. We have had the Spanish Plume which is
a mass of high temperature air coming North from Spain.It is sometimes accopnaid was
accompanied by dramatic lightning in some places and a temp. Of 30oC+. Lovely!
Climate warming protagonists are still forecasting warm wet winters and hotter summers
in future. We have had a warm wet winter but the 2016 summer has been a little peculiar.
Keep your eyes open for 2017.
REF Weather Talk Fire 4
6