My Science School

            Many people suffer from the "winter blues". Feeling tired, run down and a bit sad is a natural response to the long, dark days, cold weather and the effects of colds and flu. A few people experience exaggerated symptoms, which doctors have recognized as a medical condition known as Seasonal Affective Disorder, or SAD. A lack of daylight can cause sufferers of SAD to become very depressed and have problems sleeping and eating.

            Seasonal affective disorder (SAD) is a type of depression that's related to changes in seasons — SAD begins and ends at about the same times every year. If you're like most people with SAD, your symptoms start in the fall and continue into the winter months, sapping your energy and making you feel moody. Less often, SAD causes depression in the spring or early summer.

            Treatment for SAD may include light therapy (phototherapy), medications and psychotherapy.

Don't brush off that yearly feeling as simply a case of the "winter blues" or a seasonal funk that you have to tough out on your own. Take steps to keep your mood and motivation steady throughout the year.

Symptoms

            In most cases, seasonal affective disorder symptoms appear during late fall or early winter and go away during the sunnier days of spring and summer. Less commonly, people with the opposite pattern have symptoms that begin in spring or summer. In either case, symptoms may start out mild and become more severe as the season progresses.

Signs and symptoms of SAD may include:

• Feeling depressed most of the day, nearly every day


• Losing interest in activities you once enjoyed


• Having low energy


• Having problems with sleeping


• Experiencing changes in your appetite or weight


• Feeling sluggish or agitated


• Having difficulty concentrating


• Feeling hopeless, worthless or guilty


• Having frequent thoughts of death or suicide

Fall and winter SAD

Symptoms specific to winter-onset SAD, sometimes called winter depression, may include:

• Oversleeping


• Appetite changes, especially a craving for foods high in carbohydrates


• Weight gain


• Tiredness or low energy

Spring and summer SAD

Symptoms specific to summer-onset seasonal affective disorder, sometimes called summer depression, may include:

• Trouble sleeping (insomnia)


• Poor appetite


• Weight loss


• Agitation or anxiety

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            In the parts of the world that are close to the poles, the way the Earth tilts means that the summer months in those regions are marked by constant daylight. Parts of Scandinavia, for instance, are known as the “land of the midnight Sun”. In mid-winter, these areas experience the opposite — total darkness for 24 hours a day.

            The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight.

            Around the summer solstice (approximately 21 June in the Northern Hemisphere and 23 December in the Southern Hemisphere), the Sun is visible for the full 24 hours, given fair weather. The number of days per year with potential midnight sun increases the closer towards either pole one goes. Although approximately defined by the polar circles, in practice the midnight sun can be seen as much as 55 miles (90 km) outside the polar circle, and the exact latitudes of the farthest reaches of midnight sun depend on topography and vary slightly year-to-year.

            Because there are no permanent human settlements south of the Antarctic Circle, apart from research stations, the countries and territories whose populations experience the midnight sun are limited to those crossed by the Arctic Circle: the Canadian Yukon, Nunavut, and Northwest Territories; the nations of Iceland, Finland, Norway, Sweden, Denmark (Greenland), Russia; and the State of Alaska in the United States. A quarter of Finland's territory lies north of the Arctic Circle, and at the country's northernmost point the sun does not set at all for 60 days during summer. In Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles, where the sun can be continuously visible for half the year. The North Pole has midnight sun for 6 months from late March to late September.

            The opposite phenomenon, polar night, occurs in winter, when the Sun stays below the horizon throughout the day.

            Since the axial tilt of the Earth is considerable (approximately 23 degrees 27 minutes), the Sun does not set at high latitudes in local summer. The Sun remains continuously visible for one day during the summer solstice at the polar circle, for several weeks only 100 km (62 mi) closer to the pole, and for six months at the pole. At extreme latitudes, the midnight sun is usually referred to as polar day.

            At the poles themselves, the Sun rises and sets only once each year on the equinox. During the six months that the Sun is above the horizon, it spends the days continuously moving in circles around the observer, gradually spiralling higher and reaching its highest circuit of the sky at the summer solstice.

            Because of atmospheric refraction, and also because the Sun is a disc rather than a point, the midnight sun may be experienced at latitudes slightly south of the Arctic Circle or north of the Antarctic Circle, though not exceeding one degree (depending on local conditions). For example, Iceland is known for its midnight sun, even though most of it (Grímsey is the exception) is slightly south of the Arctic Circle. For the same reasons, the period of sunlight at the poles is slightly longer than six months. Even the northern extremities of Scotland (and places at similar latitudes, such as St. Petersburg) experience twilight throughout the night in the northern sky at around the summer solstice.

            Observers at heights appreciably above sea level can experience extended periods of midnight sun as a result of the "dip" of the horizon viewed from altitude.

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          Spring brings warmer weather. Flowers come into bloom, trees regain their leaves and blossom. Some sunshine will be accompanied by cool breezes and light showers of rain.

          In summer, the days are long and the land receives a lot of sunshine. Temperatures are high and trees and plants are green and leafy. Thunderstorms will bring rain.

          Temperatures drop during autumn, as the days begin to get shorter. Some places may experience violent storms at this time of year. Leaves go brown and fall from the trees.

          In winter, the days are short and the skies may be filled with dark, grey cloud. Many trees are bare, and the ground is often covered with frost, snow or ice.

          In geography, the temperate or tepid climates of Earth occur in the middle latitudes, which span between the tropics and the polar regions of Earth. In most climate classifications, temperate climates refer to the climate zone between 35 and 50 north and south latitudes (between the subarctic and subtropical climates).

          These zones generally have wider temperature ranges throughout the year and more distinct seasonal changes compared to tropical climates, where such variations are often small. They typically feature four distinct seasons, Summer the warmest, Autumn the transitioning season to Winter, the colder season, and Spring the transitioning season from winter back into summer. In the northern hemisphere, the year starts with winter, transitions in the first half year through spring into summer, which is in mid-year, then at the second half year through autumn into winter at year-end. In the southern hemisphere, the seasons are swapped, with summer between years and winter in mid-year.

          The temperate zones (latitudes from 23.5° to the polar circles at about 66.5°, north and south) are where the widest seasonal changes occur, with most climates found in it having some influence from both the tropics and the poles. The subtropics (latitudes from about 23.5° to 35°, north and south) have temperate climates that have the least seasonal change and the warmest in winter, while at the other end, Boreal climates located from 55 to 65 north latitude have the most seasonal changes and long and severe winters.

          In temperate climates, not only do latitudinal positions influence temperature changes, but sea currents, prevailing wind direction, continentally (how large a landmass is), and altitude also shape temperate climates.

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          As the Earth orbits the Sun, different parts of the planet face towards or away from it, receiving varying amounts of heat. The Earth is tilted at an angle and always tilts the same way. This means that when the Earth is on one side of the Sun, the Northern Hemisphere leans towards the Sun and experiences summer. At the same time, the Southern Hemisphere is leaning away from the Sun and is having winter weather. Six months later, the Earth is on the other side of the Sun and the situation is reversed. Spring begins in a hemisphere at the moment at which it starts to lean towards the Sun; Autumn starts when it begins to lean away from it.

           The seasons have nothing to do with how far the Earth is from the Sun.  If this were the case, it would be hotter in the northern hemisphere during January as opposed to July.  Instead, the seasons are caused by the Earth being tilted on its axis by an average of 23.5 degrees (Earth’s tilt on its axis actually varies from near 22 degrees to 24.5 degrees).  Here's how it works:

          The Earth has an elliptical orbit around our Sun.  This being said, the Earth is at its closest point distance wise to the Sun in January (called the Perihelion) and the furthest in July (the Aphelion).  But this distance change is not great enough to cause any substantial difference in our climate.  This is why the Earth’s 23.5 degree tilt is all important in changing our seasons.  Near June 21st, the summer solstice, the Earth is tilted such that the Sun is positioned directly over the Tropic of Cancer at 23.5 degrees north latitude.  This situates the northern hemisphere in a more direct path of the Sun's energy.  What this means is less sunlight gets scattered before reaching the ground because it has less distance to travel through the atmosphere.  In addition, the high sun angle produces long days.  The opposite is true in the southern hemisphere, where the low sun angle produces short days.  Furthermore, a large amount of the Sun's energy is scattered before reaching the ground because the energy has to travel through more of the atmosphere.  Therefore near June 21st, the southern hemisphere is having its winter solstice because it “leans” away from the Sun.

          Advancing 90 days, the Earth is at the autumnal equinox on or about September 21st.  As the Earth revolves around the Sun, it gets positioned such that the Sun is directly over the equator.   Basically, the Sun’s energy is in balance between the northern and southern hemispheres.  The same holds true on the spring equinox near March 21st, as the Sun is once again directly over the equator. 

          Lastly, on the winter solstice near December 21st, the Sun is positioned directly over the Tropic of Capricorn at 23.5 degrees south latitude.  The southern hemisphere is therefore receiving the direct sunlight, with little scattering of the sun's rays and a high sun angle producing long days.  The northern hemisphere is tipped away from the Sun, producing short days and a low sun angle.

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          Because the air is warmed by heat rising from the ground, the air temperature at the top of a mountain will always be lower than it is at the bottom.

          The lowering of temperature as you reach higher altitudes is due to the change in atmospheric pressure. You may be aware that the air around us is constantly exerting pressure on us due to there being lots of air above us weighing down on us. It sounds a bit strange to say that air weighs something but it does, we just don’t feel it because it is what we’re used to, just like gravity is constantly pulling us down.

          There is a direct relationship between temperature and pressure. If you increase the pressure of a system then the temperature will get higher. This is why bicycle pumps can get hot after use. Decrease the pressure and the temperature goes down as can be experienced letting the air out of a balloon very quickly, the balloon gets cold.

As you go higher up, the pressure gets less and less due to there being less air above you weighing down on you, therefore the temperature goes down too.

         If heat rises, then why is it so cold at the top of a mountain? Heat does indeed rise. More specifically, a mass of air that is warmer than the air around it expands, becomes less dense, and will therefore float atop the cooler air. This is true at any altitude, and if this were the only factor at play, one would expect the atmosphere to get uniformly hotter with altitude, like the second floor of a house.

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          Rising currents of warm air are called thermals. They are useful to glider pilots, who use them to help lift their craft into the air. Thermals can form over “hot spots” on the ground, such as a freshly ploughed field. Some large birds make use of thermals to circle in the air.

          The warmer air nearer to the surface expands, becoming less dense than the surrounding air. The lighter air rises and cools due to its expansion in the lower pressure at higher altitudes. It stops rising when it has cooled to the same temperature as the surrounding air.

          Associated with a thermal is a downward flow surrounding the thermal column. The downward-moving exterior is caused by colder air being displaced at the top of the thermal.

          The size and strength of thermals are influenced by the properties of the lower atmosphere (the troposphere). Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and can rise like a hot air balloon. The air is then referred to as unstable. If there is a warm layer of air higher up, an inversion can prevent thermals from rising high and the air is said to be stable.

 

 

          Thermals are often indicated by the presence of visible cumulus clouds at the top of the thermal. When a steady wind is present, thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by soaring and glider pilots. Cumulus clouds are formed by the rising air in a thermal as it ascends and cools, until the water vapor in the air begins to condense into visible droplets. The condensing water releases latent heat energy allowing the air to rise higher. Very unstable air can reach the level of free convection (LFC) and, thus rise to great heights condensing large quantities of water and so forming showers or even thunderstorms. The latter are dangerous to any aircraft.

          Thermals are one of the many sources of lift used by soaring birds and gliders to soar.

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         Where cool air lies above a warm area of land, the air will be heated. As the air warms up, it expands, becomes less dense (its molecules become less tightly packed) and it begins to rise. The surrounding cooler air replaces the rising warm air. As the warm air rises, it cools down and its density increases. These currents of warm and cold air are called convection currents.

           A convection current is a process which involves the movement of energy from one place to another. It is also called to as convection heat transfer. What is the reason that makes you feel hotter when placing hands above a campfire or when sitting next to it? Or, why is the movement of liquid so rapid when water is boiled in a pot? These things happen as a result of the Convection Currents.

          The convection currents tend to move a fluid or gas particles from one place to another. These are created as a result of the differences occurring within the densities and temperature of a specific gas or a fluid. Convection is one among the forms of heat transfers, of which the other two are radiation and conduction. Convection process only happens in the fluids i.e. in liquids and gases. This happens due to the reason that molecules within liquids or gases are free to move.

          The heat energy can be transferred by the process of convection by the difference occurring in temperature between the two parts of the fluid. Due to this temperature difference, the hot fluids tend to rise, whereas cold fluids tend to sink. This creates a current within the fluid called as Convection current.

          The mantle within the earth’s surface flows due to convection currents. These currents are mainly caused by a very hot material present in the deepest part of the mantle which rises upwards, then cools, sinks, again and again, repeating the same process of heating and rising.

          Hence Convection Current is defined as “a process of continuous heating up of liquids or gases by the process called as Convection. “

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            THE RELATIVE “shininess” of the Earth's surface in a certain area will affect the local temperature — this is called albedo. Icy, snowy areas reflect most of the radiation of the Sun and remain cold. Forests and areas of bare soil absorb the radiation and tend to stay warm.

            Albedo can be defined as a way of quantifying how much radiation is reflected from the surface. It is a comparison between the reflection radiations from the surface to the amount of radiation that hits it. This term also refers to the quantity of radiation generated by electromagnetic rays which consequently reflects away.

Seasonal Effects on Albedo

Summer

            To understand albedo better, we look at two scenarios. One, if you walk barefoot on the black soil during summer, you will feel a lot of heat and can even get burnt because the surface is absorbing and retaining more heat. Another person walking on white soil during the same season will not be burnt. This is basically because white surface tends to reflect more heat and absorb very little of it. Equally, if you touch a black car in summer it will feel much hotter than touching a white car. This is because black absorbs and retains heat while white car surface will reflect back the solar rays.

Winter

            During this season, it is generally wet with either water or ice. Water reflects approximately 6% of the light and absorbs the rest. Ice, on the other hand, reflects 50% to 60% of the incoming solar heat, thereby remaining cooler. A snow-covered area reflects a lot of radiation, which is why skiers having a risk of getting sunburns while on the slopes. Albedo diminishes when the snow-covered places start to warm up.

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           Energy from the Sun arrives on the Earth in the form of radiation. Some of the radiation is absorbed or reflected back into space by the Earth’s atmosphere and clouds, but most of it reaches the surface, where it heats up the land and sea. As the Earth heats up, some of this heat is also reflected back into space.

          Conduction is one of the three main ways that heat energy moves from place to place. The other two ways heat moves around are radiation and convection. Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules. Conduction occurs more readily in solids and liquids, where the particles are closer to together, than in gases, where particles are further apart. The rate of energy transfer by conduction is higher when there is a large temperature difference between the substances that are in contact.

             Think of a frying pan set over an open camp stove. The fire's heat causes molecules in the pan to vibrate faster, making it hotter. These vibrating molecules collide with their neighboring molecules, making them also vibrate faster. As these molecules collide, thermal energy is transferred via conduction to the rest of the pan. If you've ever touched the metal handle of a hot pan without a potholder, you have first-hand experience with heat conduction!

             Some solids, such as metals, are good heat conductors. Not surprisingly, many pots and pans have insulated handles. Air (a mixture of gases) and water are poor conductors of thermal energy. They are called insulators.

Conduction in the Atmosphere

            Conduction, radiation and convection all play a role in moving heat between Earth's surface and the atmosphere. Since air is a poor conductor, most energy transfer by conduction occurs right near Earth's surface. Conduction directly affects air temperature only a few centimeters into the atmosphere.

            During the day, sunlight heats the ground, which in turn heats the air directly above it via conduction. At night, the ground cools and the heat flows from the warmer air directly above to the cooler ground via conduction.

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          A Sundial shows the time of day by casting a shadow across its face. With the needle — the gnomon — of the sundial pointing north—south, the shadow indicates the time as the Sun passes through the sky from sunrise to sunset.

          When the earth rotates about its axis, the sun appears to “move” across the sky, causing objects to cast shadows. A sundial contains a gnomon, or a thin rod, that casts a shadow onto a platform etched with different times. As the sun changes relative positions over the course of a day, the rod’s shadows change as well, thus reflecting the change in time.

          If a sundial works based upon a rod’s shadow, then why can’t a simple stick in the ground work? As a result of the tilt of the earth’s axis, the visible movement of the sun changes daily.

          This can be accounted for in several ways. In a normal horizontal sundial, the base platform is kept steady, while the gnomon is moved to reflect the changes due to the earth’s axis tilt.

          Another method achieves the same effect by aligning the platform with the latitude and the gnomon perpendicular; mathematically, this is just the projection of the gnomon onto the platform.

          Sundials must be corrected across the span of a time zone. Every zone has a “reference longitude,” and with every degree of longitude away from the reference, the sundial is off by an additional 4 minutes.

          Thus, equation of time correction is employed in order to maintain a uniform time across the zone.

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          The number of hours of sunshine in a day is recorded on an instrument called a parheliometer. A solid glass ball focuses the Sun’s rays on to a strip of card. The intensified rays leave scorch marks on the card, moving along as the Sun moves through the sky. The longer the marks, the longer the period of sunshine.

         A sunshine recorder is a device that records the amount of sunshine at a given location or region at any time. The results provide information about the weather and climate as well as the temperature of a geographical area. This information is useful in meteorology, science, agriculture, tourism and other fields. It has also been called a heliograph.

          There are two basic types of sunshine recorders. One type uses the sun itself as a time-scale for the sunshine readings. The other type uses some form of clock for the time scale.

          Older recorders required a human observer to interpret the results; recorded results might differ among observers. Modern sunshine recorders use electronics and computers for precise data that do not depend on a human interpreter. Newer recorders can also measure the global and diffuse radiation.

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          Sometimes, intense amounts of electromagnetic energy are released from the Sun in the form of solar wind, or flares. The Earth is protected from solar wind — essentially an extremely hot gas — by its magnetic field, which stretches out into space. The particles of solar winds are known to affect satellites and even cause power blackouts on Earth. Scientists are still investigating the possible long-term effects of this activity on the Earth’s climate.

          Mt. Washington, New Hampshire is the windiest location in the United States, with an average wind speed of 35 mph. At this speed, trees start to sway and it becomes difficult to walk. Barrow Island, Australia has the highest wind speed ever recorded on Earth at 253 mph. This is strong enough to blow the roofs off of most buildings and uproot trees and shrubs. That is a pretty strong wind. But this is a drop in the bucket compared to the wind on the Sun.

          Solar wind is more than 4000 times as strong as the wind speed recorded on Barrow Island. Additionally, it reaches temperatures of around 1 million degrees Celsius, almost 15,000 times the hottest recorded temperature on Earth.

         The solar wind refers to the steady stream of highly charged particles that continually blow off the Sun in all directions. It is caused by the solar corona expanding into space. The corona is the outer atmosphere of the Sun. You can see it as a glowing halo around the Sun during a solar eclipse.

          The corona is so hot that the Sun's gravity cannot hold it in. Instead, it streams off the Sun as protons and electrons shooting through space at speeds of around 400 km/s (about 1 million miles per hour). At that speed, you could travel from New York to Los Angeles in 10 seconds!

          The solar wind causes the Sun to lose more than 1 million tons of mass per second. That may seem like a really big number, but consider this: The Earth's mass is about 6.5 sextillion tons. If you write that out it would be 6,500,000,000,000,000,000,000 tons. The Sun's mass is 333,000 times that of Earth. If you think about it like that, 1 million tons per second isn't actually that much.

          The solar wind escapes from coronal holes, which are generally found at the Sun's poles. A coronal hole is an area in the corona that is thinner and less dense than the surrounding areas. It appears as a dark spot on the Sun's surface since it is also a cooler temperature than the surrounding corona.

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          Some Scientists believe that sunspot activity may have an effect on the Earth’s weather. Sunspots seem to occur in cycles of 11 years. Research has shown that major periods of drought have occurred roughly every 22 years, or two sunspot cycles. We have yet to discover the exact relationship between the two.

          A new study in the journal Science by a team of international of researchers led by the National Center for Atmospheric Research have found that the sunspot cycle has a big effect on the earth's weather. The puzzle has been how fluctuations in the sun's energy of about 0.1 percent over the course of the 11-year sunspot cycle could affect the weather? The press release describing the new study explains:

          The team first confirmed a theory that the slight increase in solar energy during the peak production of sunspots is absorbed by stratospheric ozone. The energy warms the air in the stratosphere over the tropics, where sunlight is most intense, while also stimulating the production of additional ozone there that absorbs even more solar energy. Since the stratosphere warms unevenly, with the most pronounced warming occurring at lower latitudes, stratospheric winds are altered and, through a chain of interconnected processes, end up strengthening tropical precipitation.

          At the same time, the increased sunlight at solar maximum causes a slight warming of ocean surface waters across the subtropical Pacific, where Sun-blocking clouds are normally scarce. That small amount of extra heat leads to more evaporation, producing additional water vapor. In turn, the moisture is carried by trade winds to the normally rainy areas of the western tropical Pacific, fueling heavier rains and reinforcing the effects of the stratospheric mechanism.

          The top-down influence of the stratosphere and the bottom-up influence of the ocean work together to intensify this loop and strengthen the trade winds. As more sunshine hits drier areas, these changes reinforce each other, leading to less clouds in the subtropics, allowing even more sunlight to reach the surface, and producing a positive feedback loop that further magnifies the climate response.

          These stratospheric and ocean responses during solar maximum keep the equatorial eastern Pacific even cooler and drier than usual, producing conditions similar to a La Nina event. However, the cooling of about 1-2 degrees Fahrenheit is focused farther east than in a typical La Nina, is only about half as strong, and is associated with different wind patterns in the stratosphere.

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          Most people enjoy the sunshine, and the ultraviolet (UV) rays produced by the Sun help us to produce certain vitamins in our bodies. However, too much exposure is very harmful and can lead to serious diseases such as skin cancer. Always protect your-self with sunscreen and try to keep covered up for most of the time that you spend in the sunshine.

          The main risk factor for sunburn, premature skin aging, skin damage, and skin cancer is exposure to UV light from the sun. More than 90 percent of skin cancers are caused by sun exposure. Using tanning beds and tanning lamps also increases the risk for skin damage and skin cancer.

          The risk for skin damage and skin cancer is related to the number of sunburns a person experiences throughout his or her lifetime. The following physical characteristics also increase the risk for sunburn, skin damage, and skin cancer:

• Blond or red hair


• Blue or green eyes


• Fair skin


• Freckles


• Moles (also called nevi)

          The risk for skin damage and skin cancer is higher in people with lighter skin. However, people who have darker skin also must protect their skin from the sun to reduce lifetime exposure to harmful UV rays and help prevent skin damage and skin cancer. Lifetime exposure to the sun, which is associated with an increased risk for skin cancer, often is higher in older people and in men.

          Certain medications (e.g., antibiotics, antidepressants, acne medications [retinoids]) can increase sun sensitivity. Patients should speak with a physician about medications that can make the skin more sensitive to the sun.

          Having a family member with skin cancer increases the risk for the disease in adults and also in children. It is important to learn what to look for and how to monitor the skin for significant changes (e.g., asymmetrical mole, sores that do not heal normally).

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          Light from the Sun is made up of several different colours, each of which has its own wavelength. The wavelength of the blue part of the Sun’s light is shorter than the size of an oxygen atom. When the blue light waves hit the oxygen atoms in the Earth’s atmosphere, they are scattered, making the sky appear blue. The light waves of other colours (with greater wavelengths than blue) are also affected, but blue waves are scattered more than most.

          To understand why the sky is blue, we need to consider the nature of sunlight and how it interacts with the gas molecules that make up our atmosphere. Sunlight, which appears white to the human eye, is a mixture of all the colors of the rainbow. For many purposes, sunlight can be thought of as an electromagnetic wave that causes the charged particles (electrons and protons) inside air molecules to oscillate up and down as the sunlight passes through the atmosphere. When this happens, the oscillating charges produce electromagnetic radiation at the same frequency as the incoming sunlight, but spread over all different directions. This redirecting of incoming sunlight by air molecules is called scattering.

          The blue component of the spectrum of visible light has shorter wavelengths and higher frequencies than the red component. Thus, as sunlight of all colors passes through air, the blue part causes charged particles to oscillate faster than does the red part. The faster the oscillation, the more scattered light is produced, so blue is scattered more strongly than red. For particles such as air molecules that are much smaller than the wavelengths of visible light the difference is dramatic. The acceleration of the charged particles is proportional to the square of the frequency, and the intensity of scattered light is proportional to the square of this acceleration. Scattered light intensity is therefore proportional to the fourth power of frequency. The result is that blue light is scattered into other directions almost 10 times as efficiently as red light.

          When we look at an arbitrary point in the sky, away from the sun, we see only the light that was redirected by the atmosphere into our line of sight. Because that occurs much more often for blue light than for red, the sky appears blue. Violet light is actually scattered even a bit more strongly than blue. More of the sunlight entering the atmosphere is blue than violet, however, and our eyes are somewhat more sensitive to blue light than to violet light, so the sky appears blue.