The science of weather

The science of weather, how the sky works

The sun, the water on the earth and the rotation of the earth, the plants and animals, the wind and clouds are all factors of the constant changing of weather. But how do they work together and how do they influence the weather?

First we have to look at the main factor of this system, the engine of the weather.

The sun

All energy on earth comes originally from the sun.

The sun is a big ball of hydrogen, which is fusing to helium. That releases a huge amount of energy in form of electro magnetic waves. Under a temperature of 15`000`000 degrees Celsius two protons, which are connected, emit a positron and a neutrino to produce a proton and a neutron. When they “meet” another proton, a positron is emitted and tritium is produced. When two of these particles hit, two protons are released and a helium nucleus is left. In this way 700`000`000 tons of hydrogen are burnt to helium per second.

0.7% of that, about 5 million tons are lost, this difference is given off in form of positrons, neutrinos and gamma radiation.

This radiation needs 170`000 years to reach the surface of the sun. In this time it looses partly their energy by hitting atoms and being emitted again¹. Then it needs only 8 minutes and 20 seconds to reach the earth. Important for men is the visible light, but all radiation is all important for the weather because it heats our atmosphere up. The ozone layer of the earth absorbs the most gamma radiation, x-rays and ultra violet light that causes diseases to the human body. The rest of the radiation reaches the earth. A part is then reflected back in the universe by the ozone layer, the earth’s protection shield. It is already partly destroyed above Australia, were the number of skin cancer increases rapidly. But it won’t be explained further in this essay, because it doesn’t influence the weather.

A part of the rays is reflected back to the earth by the gases in the atmosphere, the natural greenhouse effect. Nowadays, when the amount of carbon dioxide in the atmosphere increases there is the unnatural greenhouse effect; that means that more of the energy, which is reflected back to space, is again reflected to the earth by the carbon dioxide and heats the earth more up then usual. So the whole clime of the planet changes.

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The solar constant is the amount of energy that reaches the earth’s atmosphere, that is33.5kW per day and m². But the radiation of the sun doesn’t hit the earth equal on every point, because our plant is, of course, round. That is why the poles are much colder then the regions around the equator.

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And there the developing of the weather begins. The earth is warmed up different at every point, hot air wants to rise and cold air wants to sink. Differences in pressure develop.

Pressure

Pressure is force on an object and measured in Newton per meter squared, which is equal to one Pascal. The normal air pressure is 1013 Pascal or one bar. The pressure is caused by the weight of the air column above it. That’s why the pressure high in the air or on a mountain is lower than on sea level. The main reason for a change of pressure is that the density and thus the weight of a certain volume of air changes with its temperature the relationships between volume, pressure and temperature (measured in Kelvin) are

1. The higher the temperature the bigger the volume and the less the temperature the less the volume of a certain amount of air (or any other gas). That means that if the temperature of 1.0 litre of air is increased from 250 Kelvin to 300 Kelvin, its volume will increase to 1.2 litre (in the Kelvin scale every value is 273.16 higher than on the Celsius scale. 0 Kelvin is the lowest possible temperature)
2. The higher the pressure the smaller the volume and the lower the pressure the higher the volume, if the temperature is constant.
3. The higher the temperature, the higher the pressure, under the condition that the air can not expand its volume.

That are the reasons why the pressure everywhere on the world are in a continuous change. The air is not heated equally and the pressure is changing. The density of air under low pressure is lower than under high pressure. The light air rises and leaves and leaves an empty space and the air is moving from regions of high pressure to regions of lower pressure. The air is moving. wind appears

The wind

The wind is every kind of moving air and is important for the weather, as it spreads the clouds and vapours.

Another reason for wind is the rotation of the earth. The earth is turning around, but its air isn’t fully moving with it. That is why we have a constant wind from the west. And of course the changing of night and day, every time when the air is warmed unequally there is a difference of pressure and wind occurs.

A special wind occurs at the coast. During the day the land warms up faster than the sea. The cold air from the sea slides on land, because the air over the land rises. The wind comes from the sea. In the night the air over the sea is warmer, because the sea doesn’t loose it’s heat as fast as the land. The wind comes from the land.

Clouds, big lakes and the sea, mountains and landscapes like deserts and forests can change the movement of the air as well. The wind mixes everything up and moves particles around.

In general there is the polar front between the polar air masses and the tropical air masses, and the subtropical front between the northern and southern air masses along the equator. They cause most impressive kinds of wind, the cyclone and the hurricane. The cyclone develops at the polar front as seen on the diagram below:

1. The polar front is undisturbed, the hot and the cold air move parallel to each other. The actual front is diagonal like a wedge, inclined 1 to 100, because the cold air is denser.
2. On the front there is much friction, this causes a wave to the colder side in the front. This waves grows very fast and the pressure at its crest falls rapidly
3. The wave consists now out of a cold front from point B to C, where the cold air slides under the warm air, the warm front, where warm air slides over cold air from C to A and the warm sector, the triangle between.

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The warm air rise fast and turning around the point of lowest pressure, the eye of the cyclone. The whole cyclone, which developing takes about ten days, moves now westwards. The violence of such a storm is enormous and the wind speed can be much higher than grade ten on the Beaufort scale (see below).

Tropical cyclones, which are known as “hurricanes”, ”typhoons” and “willy-willies”, develop in a similar way, but only in summer in the northern hemisphere, because the difference in temperature between north and south is then higher. In the southern hemisphere they develop only in the southern summer, December to January, out of the same reasons. [2, page105ff]

Tornadoes or “twisters” develop in a so called “supercell thunderstorm”. That are storms with a rapid updraft of more than 240 km/h. Amid the cell the pressure falls and the from the side coming air turns slowly around the point of lowest pressure. A funnel is getting visible. The wind speed near the funnel can reach up to 450km/h, that would be over 15 on the Beaufort scale.

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[5, page 144f]

The Beaufort scale

There has long been no scale for the speed of wind, but in 1806, Commander Beaufort, Captain of Her Majesty's Sloop Blossom, defined the strength of wind in the Beaufort scale; a scale in which the speed of wind can be determined without any equipment or similar. It describes the behaviour of the surface of the sea, branches of trees, smoke and other things, depending on the wind that blows in the moment. When for example smoke doesn’t rise vertically, but branches do not move, the wind speed on the Beaufort scale is one.

A modern variation can be seen below.

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³

The water

Water is often described as the most important substance on the earth. Water is probably the most important reason why we exist.

But why is that the case? Water is liquid at normal earth temperature; it is strongly polar and has a high latent heat capacity and specific heat capacity. That is very important because

1. If water was not liquid at normal temperature, but a gas or solid, there would be hardly any circulation of substances like salts, minerals etc.
2. If water was not polar, it wouldn’t solute salts but only oil and wax which are unpolar.
3. Without a high latent heat capacity, water would evaporate to easily and all oceans were already dried out
4. A high specific heat capacity means that water needs lots of energy to be heated and gives a lot of energy while cooling down. Otherwise the temperature of water would change all the time and water living animals wouldn’t be able to survive.

When the water of seas and lakes evaporates, it is as a gas in the air. Water vapour also comes from the respiration of animals and the transpiration of plants. When there is enough water in the air, it rains and the water comes down to the earth again.

The amount of water in the air is measured in gram per thousand litres or in percent of the maximum of water that can be solved in the air. If the amount increases over 100% clouds develop.

The basic condition for rain are now tiny particles, like dust, on which the water can condense.

This value cannot only change by more evaporation, but also by a change of pressure and temperature. That’s why it often rains on the western side of a mountain but seldom on its eastern side. The air which comes towards a mountain rises. The higher you come the lower the pressure and temperature is. The air cannot hold as much vapour as usual, it starts to rain. When the air is coming down again on the other side the air is drier and can hold more vapour, it doesn’t rain. And because the air travels in our regions to the west, it is dry on the eastern side of mountains.

It also starts to rain more often in the evening than in the morning, because the air is cooling down.

Clouds and cloud forms

Depending on the form of the form of a cloud you can see whether it is going to rain or not. And if it is going to be a normal rain, snow or a thunderstorm. Clouds are classified into a system that uses Latin words to describe the appearance of clouds as seen by an observer on the ground. The table below summarises the four principal components of this classification system.

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Further classification identifies clouds by height of cloud base. For example, cloud names containing the prefix "cirr-", as in cirrus clouds, are located at high levels while cloud names with the prefix "alto-", as in altostratus, are found at middle levels.

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A very deep cloud, that is on ground level is called mist. If the sight in the mist is under 2000m it is called haze, under 100m fog. It develops especially in winter when the air can’t hold much water.⁴

Precipitation

As already told above, the air can hold only a certain amount of water. If there is to much water in the air it cannot be hold any more, in condenses on dust and similar particles, which are called aerosols. But this precipitation is not always rain. The possibilities are:

1. Normal rain, drops of water condense on dust in the air and fall down.
2. Freezing rain, the drops are extreme cold but still liquid because during their fall they passed a very cool area, they freeze immediately after hitting the earth. If the droplets are smaller than half a mm it is called freezing drizzle.
3. Hail, hard balls of ice above 5mm. They are usually produced in cumulonimbus clouds when the temperature is far under freezing point. The water that condenses on the aerosols and freezes. While falling, more water condenses and freezes on it. If there is a strong updraft in the cloud, the hail stones must first become very big, before they fall own. Sizes up to golf ball size are not unusual.

[5, page 77]

Thunder and Lightning

When in or altocumulus violent winds are blowing up and down, than a static charge develops from the friction of the particles. The top of the cloud charges positively, the lower part negatively. When the difference is getting very high the natural resistance of air, 3000 V per cm, can be reached and air, can conduct electricity, although it is usually an insulator. There are two ways how that can happen.

1. A fork lightning, a branched flash of light that appears when the cloud discharges with the ground.
2. Sheet lightning, a bright flash in parts of the cloud occurs when the top and the bottom part of the cloud react.

The following thunder is slower than the lightning, the speed of sound is about 330 m/s fast, thus the number of seconds between lightning and thunder over three is equal to the distance to the flash in kilometres.

[5, page 93f ]

The rainbow

A very impressive view is the coloured half circle of a rainbow. It is nothing else than an image of the sun. When the rays of the sun hit a raindrop, the rays enter the drop and are refracted I the primary colours. They are total internal reflected on the back of the drop and emerge on the sun side. The observer, standing with his back to the sun, sees a bright coloured bow on the horizon, though it doesn’t matter how far the drops are away. As shown on the diagram below the red rays are reflected a bit more downwards than the purple one (the other colours are between red and purple but are not drawn to hold the diagram clearly understandable). The observer sees from the high drops the red rays and the purple ones from the lower drops. The bow is in reality a full circle, but half of it is hidden under the horizon. From an aircraft the full circle can be seen. It is with an angle of 42° around the so called anti-solar point, a imagine point on the opposite side of sun in relation to the observer (where the shadow of your head is or should be). On the diagram the value of angle A is 42.7°, of B 40.8°.

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Sometimes, under certain circumstances a second bow can be seen above the first bow. It is the secondary bow, which colours are turned upside down and occurs when the rays are twice total internal reflected. It’s angle is 51° round the anti-solar point.

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[2, page 95f]

Forecasting

In the early twenties a Norwegian scientist, Vilhelm Bjerknes, postulated, that with the laws of thermodynamics and fluid mechanics, it should be possible to forecast the weather. Unfortunately for him these calculation are so complex, that they would take years without nowadays technique.

The main elements of weather forecasting are

1. Data gathering and analysis
2. Prediction
3. Dissemination of information

Today there are 10,000 permanent ground stations and about 5000 weather ships. They record every change of pressure, wind, temperature and precipitation. They are mostly unmanned and transfer their information automatically. The new technologies of science are

1. Balloons, which are sent to the sky in hundreds every day, but give only a short and narrow view of the upper atmospheres conditions.
2. Geostationary satellites are in a height of about 35 km. They are in a fixed point over the earth. They can provide images of about a third of the earth’s surface.
3. Polar-orbiting satellites are in a height of about 8000 km. They move on a fixed orbit around the planet, but they donut rotate with the earth, so that their images move to the west. Because they fly deeper, their view is narrower but sharper.

Nowadays the technique on these satellites is very high developed, like infrared sensors for the measurement of the temperature of distant objects and many other equipment.

That is of course very expensive, and that’s why in 1878 the International Meteorological Organisation (IMO) was founded. It evolved to the World Meteorological Organisation (WMO) which is now a part of the UN, is based in Geneva, Switzerland and links the worlds meteorological departments together. [5, page 12ff]

Further relationships between the sun’s activity and the weather

There is of course a relationship between the sun and the weather, but there are a few more things to say than these mentioned before.

In the atmosphere of the earth “noctilucent clouds” can be observed. They are in a height of 90 kilometres and that is why the sun shines on them a short while after dusk (noctilucent means shining in the night). They contain lots of heavy elements which can only come from the sun. They must have travelled from the sun in form of sun wind, a stream of ions and other particles. These particles in the clouds are aerosols and can cause rain as mentioned before. So a heavy eruption of the sun can under some circumstances cause rain.

The a strong solar wind is indicated by an aurora borealis, the crown of the north, a bright light on the northern sky.

The solar wind is especially high I times of high sunspot activity. A sunspot is a regional high activity of the sun in its outer atmosphere. It throws huge amounts of particles in space. It could be observed that there is a eleven year cycle of sun spot activity. But there is also a eleven year cycle in the number of lightning in a certain area. When the numbers where taken together an amazing graph could be drawn.

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The values must be related to each other. After that discovery other cycles where studied in relation to the sunspots and similar results are found in the growing period (the time in which the temperature in 125 cm above the ground doesn’t fall under 5.6°C) and the strength of the monsoon in India. It was also thought about the possibility that the eleven year cycle may be mostly created by the gravitation of the planets, because the Jupiter goes round the sun once in eleven years.

It was also observed that if the earth and the Venus are on the same side of the sun the number of sunspots increases and decreases when they are on the same side. So the star’s constellations can influence the sun and thus change the weather on the earth. That is astrology! There are far more influences of planets gravity, the sun, particle streams out of space and our weather, but they are far to complicated to be mentioned here.

[6, page 36ff]

Acid rain and the influence of the man on the weather

In the last hundred years the man influenced the weather and the whole planet more than ever before. The amount of CO2 in the atmosphere increased rapidly and causes the artificial greenhouse effect, thousands of tons of chlorofluorocarbons were blown in the air in the last years. These substances are extremely stable, they stay in the atmosphere for to long a time. The can brake the ozone (O3) down and increase consequently the amount of dangerous ultraviolet radiation that reaches the earth and causes skin cancer. In coal is a small amount of sulphur, when the coal is burnt the sulphur oxide is in the atmosphere and can react with water to sulphuric acid. That is called acid rain. It harms man, animals and especially plants. The pH value indicates the acidity of a substance. It goes from 0 to 14. 1 is the acidity of the gastric acid, 7 is neutral, a strong base, like concentrated nitric base can be 13. However the pH value of rain is in the average 5.6. A slight acidity is usual but a value under 5.6 can indicate a strong pollution. [5, page 20]

There is also acid mist, sulphur that is contained in the air or in mist. It is very dangerous, because if it is breathed in, the sulphur oxide reacts to sulphuric acid in the lung!

LA NINA and EL NINO

These to Mexican names belong to periods of very strange weather “behaviour”. La Nina is the direct opposite of El Nino and is not as harmful as her big brother. Every 5 years, with big differences from time to time, El Nino strikes again, the water temperatures in a wide circle around Mexico are getting unusual high while streams turn around and the wind blows in the wrong direction. It all seems to depend on the very high pressure.

The boiling point of water decreases with decreasing pressure. My theory is that because of the high pressure, there doesn’t evaporate as much water as usual. The evaporation needs a lot of energy, which is usually taken from the water and cools it down. That s why the water gets hotter. With increasing temperature the water’s density goes down. Streams which usually are in a middle or high level in the sea are now heavier than the surrounding liquid and ”fall” down. The wind is influenced in a similar way. When the pressure increases and the temperature rises, the differences between temperature above the sea and above the water is changed, the wind blows in the wrong direction. But all that is only a thought of mine and with today’s technique it won’t take long until of the last mysteries of the weather are solved.

I want to thank my mummy because she has born me, my father because he caused that and the nurse who caught my father trying to replace me in the hospital.

I want to thank Moritz V.R. for leaving his laptop in his unlocked wardrobe I want to thank Alan G. for reminding me to work, Kevin H. for his sister and Chris S. for hitting me in my bal.. erm stomach with a CD.

I want to thank Peter D. for being caught in the act on the toilet. I want to thank Sebastian

P. for his small-headism and Mervyn D. for his molarism. AND JAW ,TOO

[...]

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¹: http://www.wissen.de/ sonne, Sonnentour

²: Meteorology and climatology for sixth forms and beyond by Ernest S. Gates, 3rd edition, George G. Harrap&Co. LTD

³: http://www.franksingleton.clara.net/beaufort_variations.html

⁴: http://ww2010.atmos.uiuc.edu/(Gl)/guides/mt

⁵: Factfinder Guide, Weather by Ian Westwell, Grange Books PLC

⁶: Forecasts famines and freezes, Climate and man’s future by John Gribbin Wildwood House LTD