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Sunlight shining through clouds in Dunstanburgh, Northumberland, England

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.

When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter.[1]

Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer. Sunlight takes about 8.3 minutes to reach the Earth.

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux, which includes infrared, visible, and ultraviolet light. Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth's surface.

Sunlight is a key factor in photosynthesis, a process crucially important for life on Earth.

Contents

Calculation

To calculate the amount of sunlight reaching the ground, both the elliptical orbit of the Earth and the attenuation by the Earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance (Eext), corrected for the elliptical orbit by using the day number of the year (dn), is:

E_{\rm ext}=E_{\rm sc}\left[1+0.034 \cdot \cos\left(2\pi\frac{{\rm dn}-3}{365}\right)\right],

where dn=1 on January 1; dn=2 on January 2; dn=32 on February 1, etc. In this formula dn-3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and therefore the maximum Eext, occurs around January 3 each year.

The solar illuminance constant (Esc), is equal to 128×103 lx. The direct normal illuminance (Edn), corrected for the attenuating effects of the atmosphere is given by:

E_{\rm dn}=E_{\rm ext}\,e^{-cm},

where c is the atmospheric extinction coefficient and m is the relative optical airmass.

Solar constant

A 1903 Langley bolograph with an erroneous solar constant of 2.54 calories/minute/square centimeter.
Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber.

The solar constant, a measure of flux, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to the Earth). When solar irradiance is measured on the outer surface of Earth's atmosphere,[2] the measurements can be adjusted using the inverse square law to infer the magnitude of solar irradiance at one AU and deduce the solar constant.[3]

The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1.366 kilowatts per square meter (kW/m²).[4][2][5][6] The actual direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year (from 1.412 kW/m² in early January to 1.321 kW/m² in early July) due to the Earth's varying distance from the Sun, and typically by much less than one part per thousand from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km²), the power is 1.740×1017 W, plus or minus 3.5%. The solar constant does not remain constant over long periods of time (see Solar variation), but over a year varies much less than the variation of direct solar irradiance at the top of the atmosphere arising from the ellipticity of the Earth's orbit. The approximate average value cited,[2] 1.366 kW/m², is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute.

The Earth receives a total amount of radiation determined by its cross section (π·RE²), but as it rotates this energy is distributed across the entire surface area (4·π·RE²). Hence the average incoming solar radiation, taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 342 W/m²). At any given moment, the amount of solar radiation received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude.

The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light (see Electromagnetic spectrum). It is linked to the apparent magnitude of the Sun, −26.8, in that the solar constant and the magnitude of the Sun are two methods of describing the apparent brightness of the Sun, though the magnitude is based on the Sun's visual output only.

In 1884, Samuel Pierpont Langley attempted to estimate the solar constant from Mount Whitney in California. By taking readings at different times of day, he attempted to remove effects due to atmospheric absorption. However, the value he obtained, 2.903 kW/m², was still too great. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1.322 and 1.465 kW/m². Abbott proved that one of Langley's corrections was erroneously applied. His results varied between 1.89 and 2.22 calories (1.318 to 1.548  kW/m²), a variation that appeared to be due to the Sun and not the Earth's atmosphere.[7]

The angular diameter of the Earth as seen from the Sun is approximately 1/11,000 radians, meaning the solid angle of the Earth as seen from the Sun is approximately 1/140,000,000 of a steradian. Thus the Sun emits about two billion times the amount of radiation that is caught by Earth, in other words about 3.86×1026 watts.[8]

Sunlight intensity in the Solar System

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun. A rough table comparing the amount of light received by each planet on the Solar System follows (from data in [1]):

Planet Perihelion - Aphelion
distance (AU)
Solar radiation
maximum and minimum
(W/m²)
Mercury 0.3075 – 0.4667 14,446 – 6,272
Venus 0.7184 – 0.7282 2,647 – 2,576
Earth 0.9833 – 1.017 1,413 – 1,321
Mars 1.382 – 1.666 715 – 492
Jupiter 4.950 – 5.458 55.8 – 45.9
Saturn 9.048 – 10.12 16.7 – 13.4
Uranus 18.38 – 20.08 4.04 – 3.39
Neptune 29.77 – 30.44 1.54 – 1.47

The actual brightness of sunlight that would be observed at the surface depends also on the presence and composition of an atmosphere. For example Venus' thick atmosphere reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".[9]

Sunlight on Mars would be more or less like daylight on Earth wearing sunglasses, and as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus it would give perceptions and "feel" very much like Earth daylight.

For comparison purposes, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise (see daylight for comparison table). Even on Pluto the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full moonlight on the Earth, a distance of about 500 AU (~69 light-hours) is needed; there is only a handful of objects in the solar system known to orbit farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.

Composition

Solar irradiance spectrum above atmosphere and at surface[what solar elevation?]

The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K. About half that lies in the visible short-wave part of the electromagnetic spectrum and the other half mostly in the near-infrared part. Some also lies in the ultraviolet part of the spectrum.[10] When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause damage to the skin known as sunburn or trigger an adaptive change in human skin pigmentation.

The spectrum of electromagnetic radiation striking the Earth's atmosphere is 100 to 106 nanometers (nm). This can be divided into five regions in increasing order of wavelengths:[11]

  • Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surface (Lithosphere). This spectrum of radiation has germicidal properties, and is used in germicidal lamps.
  • Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the photochemical reaction leading to the production of the Ozone layer.
  • Ultraviolet A or (UVA) spans 315 to 400 nm. It has been traditionally held as less damaging to the DNA, and hence used in tanning and PUVA therapy for psoriasis.
  • Visible range or light spans 400 to 700 nm. As the name suggests, it is this range that is visible to the naked eye.
  • Infrared range that spans 700 nm to 106 nm [1 (mm)]. It is responsible for an important part of the electromagnetic radiation that reaches the Earth. It is also divided into three types on the basis of wavelength:
    • Infrared-A: 700 nm to 1,400 nm
    • Infrared-B: 1,400 nm to 3,000 nm
    • Infrared-C: 3,000 nm to 1 mm.

Surface illumination

The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component from atmospheric scatter dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light photography where the principle source of illumination is sunlight as mediated by the atmosphere.

According to Craig Bohren, "preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".[12]

See diffuse sky radiation for more details.

Climate effects

On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body, the ground and other objects depends on the absorption (electromagnetic radiation) of the electromagnetic radiation in the form of heat.

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant due to Kepler's second law

\tfrac{2A}{r^2}dt = d\theta , where A is the "areal velocity" invariant.

I. e. the integration over the orbital period (also invariant) is a constant.

\int_{0}^{T} \tfrac{2A}{r^2}dt = \int_{0}^{2\pi} d\theta = const.

If we assume the solar radiation power P as a constant over time and the solar irradiation given by the inverse-square law, we obtain also the average insolation as a constant.

But the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies.[13] For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages (see: Milankovitch cycles).

Life on Earth

The existence of nearly all life on Earth is fueled by light from the sun. Most autotrophs, such as plants, use the energy of sunlight, combined with minerals and air, to produce simple sugars—a process known as photosynthesis. These sugars are then used as building blocks and in other synthetic pathways which allow the organism to grow.

Heterotrophs, such as animals, use light from the sun indirectly by consuming the products of autotrophs, either directly or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as respiration.

In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals which had previously only provided humans with meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans.

The more recent discoveries of coal, petroleum and natural gas are modern extensions of this trend. These fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within the earth for millions of years. Because the stored energy in these fossil fuels has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of primary energy. As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g. alternative fuels, Malthusian catastrophe, new urbanism, peak oil).

Cultural aspects

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sun is directly shining. Indeed, looking directly at the sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. Cars, many helmets and caps are equipped with visors to block the sun from direct vision when the sun is at a low angle.

In colder countries, many people prefer sunnier days and often avoid the shade. In hotter countries the converse is true; during the midday hours many people prefer to stay inside to remain cool. If they do go outside, they seek shade which may be provided by trees, parasols, and so on.

Sunshine is often blocked from entering buildings through the use of walls, window blinds, awnings, shutters or curtains.

Sunbathing

Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include beaches, open air swimming pools, parks, gardens, and sidewalk cafés. Sunbathers typically wear limited amounts of clothing or some simply go nude. An alternative some use to sunbathing is to use a sunbed that generates ultraviolet light and can be used indoors regardless of outdoor weather conditions and amount of sun light.

For many people with pale or brownish skin, one purpose for sunbathing is to darken one's skin color (get a sun tan) as this is considered in some cultures to be beautiful, associated with outdoor activity, vacations/holidays, and health. Some people prefer nude sunbathing so that an "all-over" or "even" tan can be obtained.

Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes and it is actually an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

Effects on human health

The body produces vitamin D from sunlight (specifically from the UVB band of ultraviolet light), and excessive seclusion from the sun can lead to deficiency unless adequate amounts are obtained through diet.

Excessive sunlight exposure has been linked to all types of skin cancer caused by the ultraviolet part of radiation from sunlight or sunlamps.[citation needed] Sunburn can have mild to severe inflammation effects on skin; this can be avoided by using a proper sunscreen cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called skin photodamage), which produces a difficult to treat cosmetic effect. Some people are concerned that ozone depletion is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer.[14]

A lack of sunlight, on the other hand, is considered one of the primary causes of seasonal affective disorder (SAD), a serious form of the "winter blues". SAD occurrence is more prevalent in locations further from the tropics, and most of the treatments (other than prescription drugs) involve light therapy, replicating sunlight via lamps tuned to specific (visible, not ultra-violet) wavelengths of light or full-spectrum bulbs.

A recent study indicates that more exposure to sunshine early in a person’s life relates to less risk from multiple sclerosis (MS) later in life.[15]

See also

References

  1. ^ "Chapter 8 – Measurement of sunshine duration" (PDF). CIMO Guide. World Meteorological Organization. http://www.wmo.int/pages/prog/www/IMOP/publications/CIMO-Guide/CIMO%20Guide%207th%20Edition,%202008/Part%20I/Chapter%208.pdf. Retrieved 2008-12-01. 
  2. ^ a b c Satellite observations of total solar irradiance
  3. ^ http://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarirradiance.html
  4. ^ Willson, R. C., and A. V. Mordvinov (2003), Secular total solar irradiance trend during solar cycles 21–23, Geophys. Res. Lett., 30(5), 1199, doi:10.1029/2002GL016038 http://www.acrim.com/Reference%20Files/Secular%20total%20solar%20irradiance%20trend%20during%20solar%20cycles%2021%E2%80%9323.pdf
  5. ^ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant. Retrieved 2005-10-05. 
  6. ^ Total Solar Irradiance 1976-2008
  7. ^ This article incorporates text from the article "Sun" in the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.
  8. ^ The Sun at nine planets.org
  9. ^ "The Unveiling of Venus: Hot and Stifling". Science News 109 (25): 388. 1976-06-19. http://www.jstor.org/pss/3960800. Retrieved 2010-02-03. "100 watts per square meter ... 14,000 lux ... corresponds to ... daytime with overcast clouds". 
  10. ^ Climate Change 2001: The Scientific Basis
  11. ^ Naylor, Mark; Kevin C. Farmer (1995). "Sun damage and prevention". Electronic Textbook of Dermatology. The Internet Dermatology Society. http://www.telemedicine.org/sundam/sundam2.4.1.html. Retrieved 2008-06-02. 
  12. ^ Craig F. Bohren. "Atmospheric Optics". http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf. 
  13. ^ Graph of variation of seasonal and latitudinal distribution of solar radiation
  14. ^ Ozone Hole Consequences retrieved 30 October 2008
  15. ^ NEUROLOGY 2007;69:381-388

Further reading

External links


Quotes

Up to date as of January 14, 2010
(Redirected to Sunshine (2007 film) article)

From Wikiquote

Sunshine is a 2007 film about a group of scientists trying to re-ignite the dying sun.

Directed by Danny Boyle and written by Alex Garland.

Contents

Robert Capa

  • Our sun is dying. Mankind faces extinction. Seven years ago, the Icarus project sent a mission to restart the sun, but that mission was lost before it reached the star. Sixteen months ago, I, Robert Capa, and a crew of seven, left Earth frozen in a solar winter. Our payload: a stellar bomb with a mass equivalent to Manhattan Island. Our purpose: to create a star within a star. Eight astronauts strapped to the back of a bomb. My bomb. Welcome to the Icarus Two.
  • So if you wake up one morning and it's a particularly beautiful day, you'll know we made it.

Cassie

  • Only dream I ever have... is it the surface of the sun? Everytime I shut my eyes... it's always the same.
  • We have an excess of manliness in the comm center right now.

Searle

  • Hey Capa, we're only stardust.
  • Prescription, two hours in the Earth Room. And get a haircut, Mace.
  • Everything about the delivery and effectiveness of that payload is entirely theoretical.
  • Kaneda! What do you see? Kaneda! What do you see? Kaneda! Kaneda!

Pinbacker

  • Are you an angel? Has the time come? I've been waiting so long.
  • For seven years I spoke with God. He told me to take us all to Heaven.
  • At the end of time, a moment will come when just one man remains. Then the moment will pass. Man will be gone. There will be nothing to show that we were ever here... but stardust.
  • [From trailer] Nothing will survive... not your parents... not your children... not even...stars!

Dialogue

Searle: It's invigorating. It's like... taking a shower in light. You lose yourself in it.
Corazon: Like a floatation tank?
Searle: Actually, no. More like... in psych tests on deep space, I ran a number of sensory deprivation trials, tested in total darkness, on floatation tanks - and the point about darkness is, you float in it. You and the darkness are distinct from each other because darkness is an absence of something, it's a vacuum. But total light envelops you. It becomes you. It's very strange. I recommend it.
Mace: What's strange, Searle, is that you're the psych officer on this ship and I'm clearly a lot saner than you are.

Cassie: Are you scared?
Capa: When a Stellar Bomb is triggered, very little will happen at first. And then a spark, will pop into existance, and it will hang for an instant, hovering in space and then, it will split into two, and those will split again, and again, and again. Detonation beyond all imaging - the big bang on a small scale. A new star born out of a dying one. I think it will be beautiful. No, I'm not scared
Cassie: I am.

Icarus: Capa - warning. You are dying. All crew are dying.
Capa: We know we're dying. Were OK with it, just as long as we have enough oxygen to reach the payload delivery point.
Icarus: Capa - warning, you do not have enough oxygen to survive until the payload delivery point.
Capa: Please clarify.
Icarus: Twelve hours before crew will be unable perform complex tasks. Fourteen hours before crew will be unable to perform basic tasks. Sixteen hours until death. Time to payload delivery point, 19 hours.
Capa: Negative, Icarus. We have enough oxygen for four crew members to survive.
Icarus: Affirmative. Four crew members could potentially survive.
Capa: Trey is dead. There are only four crew members; Cassie, Mace, Corazon and me.
Icarus: Negative. Five crew members.
Capa: Icarus?
Icarus: Yes?
Capa: Who is the fifth crew member?
Icarus: Unknown.
Capa: Where is the fifth crew member?
Icarus: In the observation room.

Corazon: [Last Lines] Icarus patch me throught to Mace I have something wonderful to show him or Cassie or Capa...Icarus?
Pinbacker: [As he stabs her] Don't fight...don't fight

Cast

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Sunshine
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This is a disambiguation page, which lists works which share the same title. If an article link referred you here, please consider editing it to point directly to the intended page.


Sunshine may refer to:


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

SUNSHINE. As a meteorological element sunshine requires some conventional definition. There is uninterrupted continuance of gradation from the burning sunshine of a tropical noon to the pale luminosity that throws no shadow, but just identifies the position and shape of the sun through the thin cloud of northern skies.

Table of contents

The Campbell-Stokes Sunshine Recorder

In the British Isles the sun is allowed to be its own timekeeper and the scorch of a specially prepared card used as the criterion for bright sunshine. The practice arose out of the use of the sunshine recorder which depends upon the scorching effect of a glass sphere in the sun's rays. The original form of the instrument was suggested by J. F. Campbell of Islay in 1857. He used a glass sphere within a hemispherical bowl of wood. The scorching of the wood along successive lines of the bowl as the sun alters its declination from solstice to solstice leaves a rugged monument of the duration and intensity of the sunshine during the half-year, but does not lend itself to numerical measurement. The design of a metal frame to carry movable cards and thus give a decipherable record of each day's sunshine is due to Sir G. G. Stokes. The excursions of the sun to the north and south of the equator are limited by the tropical circles, and the solar record on the hemispherical bowl will be confined within a belt 23 0 27' north and south of the plane through the centre parallel to the equator or perpendicular to the polar axis. Thus a belt 46° 54' in angular width will be suitable for a sunshine recorder for any part of the world. Whatever place be chosen for the observation the same belt will do if it is set up perpendicular to the earth's polar axis. But there can be no record if the sun is below the horizon; hence any part of the belt projecting above the horizon is not only useless for recording but is liable to shadow a part of the belt where there might be a record. Hence to meet the requirements of a particular locality the belt as set up round the polar axis should be cut in two by a horizontal plane through the centre and the half projecting above the horizontal removed. Reversed it makes a half belt, exactly similar to what is left, and thus each complete belt is cut by a horizontal plane through the centre into two frames suitable for sunshine recorders for the particular locality.

The cutting of the belt may, of course, vary between the direct transverse cut along the polar axis which gives a half-ring belt to be set vertical in order to receive the record for a point on the equator, and the cut perpendicular to the polar axis which 1 These claims to early origin are mere fables, like the claim of the Oweisi order to spring from Oweis, one of the oldest traditionalists, and so forth.

2 For the dervish orders see Dervish.

divides the belt into two similar rings suitable for recording the sunshine at the poles. Clearly, when the belt is so cut that two complete rings are formed, a continuous record of sunshine throughout the twenty-four hours may be expected, so that for the polar circles the cut will run diagonally between opposite points of the extreme circles of the sun's records. As examples of the cutting of the belt for different latitudes we may put side by side the recorder as used in temperate latitudes (fig. 1) and FIG. I. - Campbell-Stokes Sunshine Recorder.

the special form designed in the Meteorological Office, London, for use on the National Antarctic Expedition,1901-1904(fig. 2). A belt cut for a particular latitude is serviceable for some Io Antarctic Sunshine Recorder, to carry 24-hour record.

FIG. 2. - Antarctic Sunshine Recorder, to carry 12-hour record.

on either side of that latitude if the cards are not trimmed too closely to the cutting of the belt. The belt must always be adjusted round the parallel to the polar axis. If the cut of the belt is too oblique for the latitude of the place where it is exposed, and the cards are cut strictly to the belt, the northern side of the cut will be below the horizon and the southern side above it, regard to the equinoctial card. The section of the supporting surface by a plane through the polar axis is to be as in fig. 3.

The Sphere

The material for the sphere must be " crown glass, colourless, or of a very pale yellow tint. The diameter 4 in. The weight between 2.92 and 3.02 lb. The focal length from the centre of the sphere to the geometrical focus for parallel rays should be between 2.96 in. and 2.99 in.

Measurement of the Sunshine Record

It was mentioned that the Campbell-Stokes recorder involves a conventional definition of sunshine. The recorded day of sunshine is less than the actual time during which the sun is above the horizon by about twenty minutes at sunrise and sunset on account of the want of burning power of a very low sun. Some further convention is necessary in order to obtain a tabulation of the records which will serve as the basis of a comparison of results for climatological purposes. The spot which is scorched on the card by the sun is not quite limited to the image of the sun, and a few seconds of really strong sunshine will produce a circular burn which is hardly distinguishable in size from that of a minute's record. (See fig. 4.) Consequently with intermittent sunshine exaggeration of the actual duration of burning is very probable. Strictly speaking measurements ought to be between the diameters of the circular ends of the burns, but the practice of measuring all the trace that can be distinctly recognized as scorched has become almost universal in Great Britain, and appears to give a working basis of comparisons.

Missing image
Sunshine-1.jpg

some sunshine may be lost near sunrise or sunset in the winter because there is no card to receive it. The part projecting above the horizon in summer will partly shadow the globe, and faint sunshine may be lost, for at most only half the globe can be solarized at sunset. But the loss due to this cause is unimportant. Stokes designed the complete belt to use successively three cards (From the Observer's Handbook, by permission of the Controller of H.M. Stationery Office.) FIG. 3.

of different shape for different times of the year. The equinoctial card forms a portion of a cylinder round the polar axis for spring and autumn, the summer card and the winter card each forms a part of a cone making a vertical angle of 16° with the polar axis as indicated in fig. 3. Adjustments. - The adjustments of the instrument are to set the belt so that its axis is parallel to the polar axis and symmetrically adjusted with reference to the meridian of the place, and to set the sphere so that its centre coin cides precisely with the centre of the belt. No one of the three adjustments is easy to make or totest because neither the centre of the sphere nor 3' the centre (nor indeed the g 4? axis) of the belt can be easily identified. For an instrument for testing these adjustments 02t see Quart. Journ. Roy. Met. fi'R?P Soc. xxxii. 2 49.0'S Instruments differ according to the means provided for mounting or adjusting the positions of the belt or sphere, from in that known as the Whipple Casella instrument the fixed belt is replaced by a movable card holder. The chief advantage of Stokes's specification is the simplicity of the use of the instrument when once it has been properly adjusted and fixed.

It is essential that the glass sphere s and refractive index to give an image of the sun on the prepared card or within the 20th of an inch of it nearer the centre. It is also essential that the cards used should not only be of suitable material but also of the right dimensions for the bowl. The colour and material of the cards were selected by Stokes in consultation with Warren De la Rue, who was at that time his colleague on the Meteorological Council, and the cards used by the meteorological office are still supplied by Messrs De la Rue & Co. Accuracy in the comparative measurements of sunshine by this method depends upon the proper adjustment of the dimensions of the different constituent parts of the recorder and accordingly the following specification of standard dimensions has been adopted by the meteorological office.

The Time Scale

On the time scale of the equinoctial card twelve hours are represented by 9.00 in.

The Bowl. - The diameter of the bowl, measured between the centres of the 6 o'clock marks on a metal equi noctial card of thickness 0.02 in. when FIG. in its place, is to be 5.73 in. (t o. 01 in.). The distance between the exposure edges of the upper winter flange and the lower summer flange must not be less than 2.45 in., nor exceed 2.50 in. The distances from the middle line on the equinoctial card to the middle lines on the summer and winter cards are to be 0 70 in. (* 0.02 in.). The inclination of the summer card, in place, to the winter card, in place, is to be 32° 1°, s y mmetrically arranged with 5. - Sunshine Record (June 19 and 20, 1908).

cylinder about a line parallel to the polar axis. The effect thereby recorded is a photochemical one, and the composite character of the sun's radiation, modified by the elective absorption of the atmosphere makes the relation of the record to that of the sun's scorching power dependent upon atmospheric conditions and therefore on different occasions, so that the two records give different aspects of the solar influence. Other recorders use the thermal or photographic effect FIG. 4. - Records obtained by intervals varying rom one second burns increases from right to left o exposing a Campbell-Stokes Sunshine Recorder for measured to thirty minutes. The duration of the exposure of the separate f the diagram.

Loulu ye of the proper size traverse of a spot of itgnt over the sensitive paper, arrangea as a

10 II Noon

I 2 3 4 5 6789 10 II Mid. I 2 3 4 5 6 7 8 9

Other Types of Sunshine Recorder

There are, however, various other conventions as to sunshine which are used as the basis of recorders of quite different types. The Jordan recorder uses ferro cyanide paper and the sun keeps the time of its own record by the of the sun's rays and record duration by a clock instead of allowingexclusively local, and indeed the possible duration of sunshine at the sun to keep its own time. In the Marvin sunshine recorders of I any station is a local characteristic which it is desirable to know.

Forenoon, (a.m.), (p. m.) Consequently as evidence of the peculiarity of Noon Afternoon 6 8 10 12246 g the site the recorded sunshine might be referred to the total possible with a free horizon. On the other hand, taking the record of sunshine as an indication of the clearness of the sky for the purposes of general meteorology, the screening of the sun by hills must be regarded simply as limiting the time during which observation is possible and the duration of the sunshine recorded should be referred to the possible duration at the particular site. It would, therefore, be desirable in publishing records of the duration of sunshine recorded to note also the possible amount for the instrument as exposed (see Hourly Means at Five Observatories under the Meteorological Council, 1891, No. 113, p. io). The table shows the number of hours the sun is above the horizon during each month in the latitude of the British Isles.

By way of exhibiting the results obtained from sunshine records we reproduce (fig. 7) the sunshine map of the British Isles taken from the annual summary of the Monthly " Weather Report," 1908 (British Meteorological Year-Book, pt. ii.). Corresponding maps embodying data from over 130 stations are prepared each month; fig. 8 shows the variation in the distribution of ° sunshine that may take place in different months.

Further, fig. 9 represents the average weekly distribution of sunshine in different sections of the British Isles according to the average of the United States weather bureau an electrical contact is made by twenty-five years.

4 January February March April May June July August Septembe October Nouember December 25 24 9 10 FIG. 6. - Monthly Average Duration of bright Sunshine for each at Valencia (Ireland).

the thermal effect of the sun and the duration of the contact is recorded. An instrument which gives a corresponding result is described by W. H. Dines (Quart. Journ. Roy. Met. Soc. xxvi. 243). These define sunshine by the effect necessary to produce or maintain a certain thermal effect, but the definition once accepted there is no uncertainty as to the record. The Callendar sunshine recorder gives a record of the difference of temperature of two wires, one solarized and the other not, and it is therefore a continuous record of the thermal effect of solar and terrestrial radiation. It is vastly more detailed than that of other instruments (see fig. 5), but the interpretation of the record in terms suitable for meteorological or climatological purposes is a special study, which has not yet been attempted. In a somewhat similar way information about the duration and intensity of sunshine with an abundance of detail can be obtained from the record upon photographic paper passing under an aperture in a drum which revolves with the sun, as in the Lander recorder, but the study of such details has not been begun.

Sunshine Records for the British Isles

The interest in the use of sunshine recorders is more widely extended in the British Isles than elsewhere, and it is, so far as the public are concerned, the most important meteorological element, but it is singular that up to the present a knowledge of the total amount of sunshine recorded during the day, the week, the month or the year is all that is apparently required. Except for the observatories in connexion with the meteorological office and a few others the distribution of sunshine during the day is not taken out, so that we are still some distance from attacking the problems presented by the finer details of solar records. Fig. 6 shows the average duration of bright sunshine for each hour of the day for each month at Valencia. The expectation of sunshine is greatest at 1 p.m. and 2 p.m. in May, while there is a well-marked secondary maximum in September.

Exposure

We now consider what the daily sunshine record for a particular station means. An ideal exposure has an uninterrupted view of those parts of the horizon in which the sun rises or sets; and elsewhere the view of the sun must not be obstructed by the ground, buildings, trees or any other obstacle; but ideal exposures are not always to be obtained. In mountainous districts particularly it may be impossible to find a site in which the sun is not obstructed for an appreciable part of the day. In these circumstances it becomes a question whether the amount of sunshine recorded should be referred to the maximum possible for an uninterrupted horizon or the maximum possible for the particular exposure. The answer to the question really depends upon the purpose for which the information is wanted. As a climatological factor of the locality the shadow cast by the surrounding hills is of importance, it is part of the difference between the fertility of the southern and northern slopes of hill country. This importance is, of course, in many respects 1 Brit. Assoc. Report (1900), p. 44.

hour of the day FIG. 7. - Sunshine in the British Isles in 1908.

Missing image
Sunshine-2.jpg

e

1 4

/I®

tude.

Jan.

Feb.

y ea

March

April

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

50°

262

278

288

365

410

473

4 82

4 8 5

44 2

373

327

.266

246

51°

257

276

286

365

411

477

4 8 7

4 8 9

444

373

325

262

241

52°

251

273

284

365

412

481

49 1

494

44 6

374

324

258

236

53°

2 47

271

281

364

4 1 4

4 86

49 8

499

45 0

375

3 2 3

2 54

231

54°

2 43

268

2 79

3 6 3

4 1 7

49 0

5 0 3

5 0 5

453

375

322

251

225

55°

2 37

26 5

2 7 6

3 6 3

4 18

494

510

511

456

376

3 1 9

2 45

218

56°

232

263

2 73

362

420

499

516

516

459

37 6

316

239

211

57°

226

260

270

362

423

5 0 4

5 2 4

5 2 3

4 6 3

377

314

236

205

58°

219

257

267

361

426

510

53 2

53 0

4 6 7

37 8

312

232

197

59°

211

2J3

263

361

429

5 1 7

54 1

53 8

47 1

379

309

225

187

Sunshine in the Antarctic Regaons

It is clear that so far as concerns the zone from 50° to 60° N. in this particular region, the annual amount of sunshine diminishes as one goes northward. It would, however, not be safe to conclude that this diminution in the aggregate duration of sunshine during the year goes on without interruption as one proceeds northward. At least the corresponding statement would not be true of the southern hemisphere. No doubt lsohels are shown for 1000,1300.1600 and 1900 hrs. The unit-for the values at stations is one thousand hours 1000 000 1300 1600 Possible Duration of Bright Sunshine in the Latitude of the British Isles. the frequency of cloud and the consequent loss of duration of sunshine would increase for corresponding latitudes from the tropical anticyclone southward, but beyond the region of minimum pressure at the winter quarters of the " Discovery " in latitude 77° 51' S., longitude 166° 45' E., the amount of bright sunshine recorded during the two years 1902 and 1903 was remarkably large. The total for 1903 equalled that for Scilly, and in December of that year an average of 16 hours per day was registered.

May 1909.

June 1909. FIG. 8. - Sunshine in the British Isles in May and June 1909.

Sunshine Results for Other Parts of the World

Maps showing the average annual distribution of sunshine over Europe and North America are given in Bartholomew's Physical Atlas, vol. iii. Atlas of Meteorology. Over Europe the largest totals, over 2750 hours per annum, are shown over central Spain. In North America, values exceed 3250 hours per annum in the New Mexico region. For other parts of the world the information available is not sufficiently extensive for the construction of charts.

Effect upon Sunshine Records of the Smoke of Great Cities

Much discussion has taken place from time to time as to whether the climate of a locality can be altered by artificial means. Questions have been raised as to the effect of forests upon rainfall, as to the indirect effect of irrigation or the converse process, the obliteration of natural irrigation by blown sand, and as to the possibility of producing, arresting or modifying rainfall by the discharge of explosives.

11 20

25 30 35 40 45

50 3 8

Spring

Summer

Autumn

Winter

Extreme

North

so

50

40

30

20

10

Extreme

South

40

30

20

10

-

Western

Section

- Nr.

40

Eastern

Section

The one question of the kind to which the sunshine recorder gives an absolutely incontrovertible answer is as to the effect of the smoke of great cities in diminishing the sunshine in the immediate FIG. 9. - Average Duration of bright Sunshine in the British Isles for each week.

neighbourhood. This may be illustrated by the figures for sunshine during the winter months off Bunhill Row, E.C., in the middle of London, Westminster, Kew and Cambridge.

Station.

November.

December.

January.

February.

Bunhill Row. .

22.8

7.5

14.1

30.6

Westminster. .

27.7

13 I

18.4

32.8

Kew.. .

50.8

38.1

40.3

54.6

Cambridge. .

61.0

40.6

48.9

73.8

Missing image
Sunshine-3.jpg

Monthly Average Duration of Bright Sunshine derived from Observations extending over Twenty Years. This is not a quest i on which comes out merely by taking averages. The answer can be seen directly by comparing the daily cards (see fig. 10, Sunshine Cards for Cambridge, Westminster and Bunhill Row for December 1904). Thus it appears that the direct effect of the local contamination of the London atmosphere results in the !sohels are shown for 150,200,250,300 and 350 hrs. /sohe/s are shown for 100,140,180 and 220 hrs. diminution of the recorded sunshine for the whole year by 37%, and it is clear that the contamination extends in some degree as far as Kew, where the loss amounts to about to %. There is evidence of various kinds to show that the effect of the smoke cloud of cities Cambridge. Westminster. Bunhill Row.

?I!?? At ' '?ft?r'a41n ?.°? A?llni t?e:?,  ?

!n e .

Ai?Riiin A ???? "' ? ?`?i!

A ????? ? ? 11 FIG. IO.

can be traced sometimes for great distances, and in special conditions of weather with easterly winds the effect is sometimes remarkably persistent. (W. N. S.)


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