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Irradiance vs. Insolation
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Irradiance and Insolation
Irradiance is to power as insolation is to energy. Or in other words:
Irradiance is an instantaneous measurement of solar power over some area. The units of
irradiance are watts per square meter. For practical purposes of measurement and
interpretation, irradiance is expressed and separated into different components
Insolation is a measurement of the cumulative energy measured over some area for a
defined period of time (e.g., annual, monthly, daily, etc.). The common unit of Insolation
is kilowatt hours per square meter. But for these units to make sense the time interval
must be clearly stated (e.g., kW-hr per square meter annual insolation).
Irradiance vs. Insolation
There are multiple different ways to measure light intensity/radiation, however in terms
of Solar and PV-technology specifically, there are two main practical and applicable
means of measuring light: irradiance and insolation. A third useful means of
measurement is the color, or frequency of the light being measured, however this is used
in the industry far less often. Frequency measurements focus on the energetic difference
between red, green, blue, or other forms of light, like UV or inferred. Higher frequency
photons contain more energy than lower energy photons. Consequently, blue or violet
light has more energy per photon than red or inferred light. For the most part, many of
the PV-modules on the market in the past few years can only absorb mostly red and
green light, reflecting some of the blue light. This is why most PV modules appear to be
somewhat dark blue. The darker the PV module appears, generally the more efficient it is
as absorbing light.
Since there is no practical way to measure the energy in each photon directly, a more
effective means of measurement is needed. The easiest form is to start off with the area
of material that is intended to absorb the light to be measured, like a square meter, and
then determine the amount of energy that strikes that surface. However one important
point to note is that light can be different brightness at different times. Should the
measurement focus on the total energy being absorbed by the surface area, or just
what’s striking the surface at any given moment? This question gives rise to the two
different forms of measurement of light: irradiance and insolation.
Irradiance is the rate of energy that is being delivered to a surface area at any given time.
Its units are Watts per square meter. That means that for any given surface area A, there
exists a specific amount of power P that is being delivered to that area in photonic form.
The equation is shown below:
Irradiance = Power / Area
Insolation is the total amount of energy that has been collected on a surface area within a
given time. While the irradiance denotes the instantaneous rate in which power is
delivered to a surface, the insolation denotes the cumulative sum of all the energy
striking the surface for a specified time interval. This interval must be specified in order to
make sense, and the typical unit of time measurement is the hour. Since energy is equal
to the rate of power P being delivered for a specified time T, the resultant insolation
equation is as follows:
Insolation = Power * Time / Area or
= Power / Area (=irradiance) * Time
An important point to note about insolation is that while delivering the light data in a
more simplistic form, when compared to an irradiance measurement, data is lost. The
irradiance measurement ideally is an instantaneous measurement of light intensity. So
for example if the light being measured changed or varied its intensity over time, an
irradiance measurement would be able to detect and display such an event, while an
insolation measurement would not. In the example shown below, two days of irradiance
and insolation measurements are shown. The first day shows a cloudy day, with the sun
mostly obscured by clouds and intermittent measurements. The second day recorded a
hardware malfunction occurring, and the measurements suddenly stopping.
With the irradiance measurement, it is very easy to see the difference between these two
days of data. The first day is a non-issue from the perspective of maintenance and repair;
but the second day denotes a very clear and lasting problem that needs an immediate
response. However, since the measurements of insolation sum up to the same value for
both days, any repair personnel relying on insolation data to look for problems wouldn’t
even notice that there is an issue.
Not to be confused with Insulation (disambiguation).
Annual mean insolation at the top of Earth's atmosphere (top) and at the planet's surface
US annual average solar energy received by a latitude tilt photovoltaic cell (modeled)
Average insolation in Europe; also see Insolation map of Europe and Africa for freerlicence map.
Insolation is a measure of solar radiation energy received on a given surface area and
recorded during a given time. It is also called solar irradiation and expressed as "hourly
irradiation" if recorded during an hour or "daily irradiation" if recorded during a day. The
unit recommended by the World Meteorological Organization is megajoules per square
metre (MJ/m2) or joules per square millimetre (J/mm2) .[1] Practitioners in the business of
solar energy may use the unit watt-hours per square metre (Wh/m2). If this energy is
divided by the recording time in hours, it is then a density of power called irradiance,
expressed in watts per square metre (W/m2).
Contents
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1 Absorption and reflection
2 Projection effect
3 Earth's insolation
4 Distribution of insolation at the top of the atmosphere
o 4.1 Application to Milankovitch cycles
5 Applications
6 See also
7 References
8 External links
[edit] Absorption and reflection
The object or surface that solar radiation strikes may be a planet, a terrestrial object
inside the atmosphere of a planet, or an object exposed to solar rays outside of an
atmosphere, such as spacecraft. Some of the radiation will be absorbed and the
remainder will be reflected. Usually the absorbed solar radiation is converted to thermal
energy, causing an increase in the object's temperature. Manmade or natural systems,
however, may convert a portion of the absorbed radiation into another form, as in the
case of photovoltaic cells or plants. The proportion of radiation reflected or absorbed
depends on the object's reflectivity or albedo.
[edit] Projection effect
The insolation into a surface is largest when the surface directly faces the Sun. As the
angle increases between the direction at a right angle to the surface and the direction of
the rays of sunlight, the insolation is reduced in proportion to the cosine of the angle; see
effect of sun angle on climate.
Figure 2
One sunbeam one mile wide shines on the ground at a 90° angle, and another at a 30°
angle. The oblique sunbeam distributes its light energy over twice as much area.
In this illustration, the angle shown is between the ground and the sunbeam rather than
between the vertical direction and the sunbeam; hence the sine rather than the cosine is
appropriate. A sunbeam one mile (1.6 km) wide falls on the ground from directly
overhead, and another hits the ground at a 30° angle to the horizontal. Trigonometry tells
us that the sine of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the
sunbeam hitting the ground at a 30° angle spreads the same amount of light over twice as
much area (if we imagine the sun shining from the south at noon, the north-south width
doubles; the east-west width does not). Consequently, the amount of light falling on each
square mile is only half as much.
This 'projection effect' is the main reason why the polar regions are much colder than
equatorial regions on Earth. On an annual average the poles receive less insolation than
does the equator, because at the poles the Earth's surface are angled away from the Sun.
[edit] Earth's insolation
Solar radiation map of Africa and Middle East
A pyranometer, a component of a temporary remote meteorological station, measures
insolation on Skagit Bay, Washington.
Direct insolation is the solar irradiance measured at a given location on Earth with a
surface element perpendicular to the Sun's rays, excluding/less diffuse insolation (the
solar radiation that is scattered or reflected by atmospheric components in the sky).
Direct insolation is equal to the solar constant minus the atmospheric losses due to
absorption and scattering. While the solar constant varies with the Earth-Sun distance
and solar cycles, the losses depend on the time of day (length of light's path through the
atmosphere depending on the Solar elevation angle), cloud cover, moisture content, and
other impurities. Insolation is a fundamental abiotic factor[2] affecting the metabolism of
plants and the behavior of animals.
Over the course of a year the average solar radiation arriving at the top of the Earth's
atmosphere at any point in time is roughly 1366 watts per square metre[3][4] (see solar
constant). The radiant power is distributed across the entire electromagnetic spectrum,
although most of the power is in the visible light portion of the spectrum. The Sun's rays
are attenuated as they pass through the atmosphere, thus reducing the irradiance at the
Earth's surface to approximately 1000 W /m2 for a surface perpendicular to the Sun's rays
at sea level on a clear day.
The actual figure varies with the Sun angle at different times of year, according to the
distance the sunlight travels through the air, and depending on the extent of atmospheric
haze and cloud cover. Ignoring clouds, the daily average irradiance for the Earth is
approximately 250 W/m2 (i.e., a daily irradiation of 6 kWh/m2), taking into account the
lower radiation intensity in early morning and evening, and its near-absence at night.
The insolation of the sun can also be expressed in Suns, where one Sun equals 1000 W/m2
at the point of arrival, with kWh/m2/day expressed as hours/day.[5] When calculating the
output of, for example, a photovoltaic panel, the angle of the sun relative to the panel
needs to be taken into account as well as the insolation. (The insolation, taking into
account the attenuation of the atmosphere, should be multiplied by the cosine of the
angle between the normal to the panel and the direction of the sun from it). One Sun is a
unit of power flux, not a standard value for actual insolation. Sometimes this unit is
referred to as a Sol, not to be confused with a sol, meaning one solar day on a different
planet, such as Mars.[6]
[edit] Distribution of insolation at the top of the atmosphere
Spherical triangle for application of the spherical law of cosines for the calculation the
solar zenith angle Θ for observer at latitude φ and longitude λ from knowledge of the
hour angle h and solar declination δ. (δ is latitude of subsolar point, and h is relative
longitude of subsolar point).
, the theoretical daily-average insolation at the top of the atmosphere, where θ is
the polar angle of the Earth's orbit, and θ = 0 at the vernal equinox, and θ = 90° at the
summer solstice; φ is the latitude of the Earth. The calculation assumed conditions
appropriate for 2000 A.D.: a solar constant of S0 = 1367 W m−2, obliquity of ε = 23.4398°,
longitude of perihelion of ϖ = 282.895°, eccentricity e = 0.016704. Contour labels (green)
are in units of W m−2.
The theory for the distribution of solar radiation at the top of the atmosphere concerns
how the solar irradiance (the power of solar radiation per unit area) at the top of the
atmosphere is determined by the sphericity and orbital parameters of Earth. The theory
could be applied to any monodirectional beam of radiation incident onto a rotating
sphere, but is most usually applied to sunlight, and in particular for application in
numerical weather prediction, and theory for the seasons and the ice ages. The last
application is known as Milankovitch cycles.
The derivation of distribution is based on a fundamental identity from spherical
trigonometry, the spherical law of cosines:
where a, b and c are arc lengths, in radians, of the sides of a spherical triangle. C is the
angle in the vertex opposite the side which has arc length c. Applied to the calculation of
solar zenith angle Θ, we equate the following for use in the spherical law of cosines:
The distance of Earth from the sun can be denoted RE, and the mean distance can be
denoted R0, which is very close to 1 AU. The insolation onto a plane normal to the solar
radiation, at a distance 1 AU from the sun, is the solar constant, denoted S0. The solar flux
density (insolation) onto a plane tangent to the sphere of the Earth, but above the bulk of
the atmosphere (elevation 100 km or greater) is:
and
The average of Q over a day is the average of Q over one rotation, or the hour angle
progressing from h = π to h = −π:
Let h0 be the hour angle when Q becomes positive. This could occur at sunrise when
, or for h0 as a solution of
or
If tan(φ)tan(δ) > 1, then the sun does not set and the sun is already risen at h = π, so ho =
π. If tan(φ)tan(δ) < −1, the sun does not rise and
.
is nearly constant over the course of a day, and can be taken outside the integral
Let θ be the conventional polar angle describing a planetary orbit. For convenience, let θ
= 0 at the vernal equinox. The declination δ as a function of orbital position is
where ε is the obliquity. The conventional longitude of perihelion ϖ is defined relative to
the vernal equinox, so for the elliptical orbit:
or
With knowledge of ϖ, ε and e from astrodynamical calculations [7] and So from a
consensus of observations or theory,
can be calculated for any latitude φ and θ.
Note that because of the elliptical orbit, and as a simple consequence of Kepler's second
law, θ does not progress exactly uniformly with time. Nevertheless, θ = 0° is exactly the
time of the vernal equinox, θ = 90° is exactly the time of the summer solstice, θ = 180° is
exactly the time of the autumnal equinox and θ = 270° is exactly the time of the winter
solstice.
[edit] Application to Milankovitch cycles
Obtaining a time series for a
for a particular time of year, and particular latitude, is a
useful application in the theory of Milankovitch cycles. For example, at the summer
solstice, the declination δ is simply equal to the obliquity ε. The distance from the sun is
Past and future of daily average insolation at top of the atmosphere on the day of the
summer solstice, at 65 N latitude. The green curve is with eccentricity e hypothetically set
to 0. The red curve uses the actual (predicted) value of e. Blue dot is current conditions, at
2 ky A.D.
For this summer solstice calculation, the role of the elliptical orbit is entirely contained
within the important product
, which is known as the precession index, the
variation of which dominates the variations in insolation at 65 N when eccentricity is
large. For the next 100,000 years, with variations in eccentricity being relatively small,
variations in obliquity will be dominant.
[edit] Applications
In spacecraft design and planetology, it is the primary variable affecting equilibrium
temperature.
In construction, insolation is an important consideration when designing a building for a
particular climate. It is one of the most important climate variables for human comfort
and building energy efficiency.[8]
Insolation variation by month; 1984-1993 averages for January (top) and April (bottom)
The projection effect can be used in architecture to design buildings that are cool in
summer and warm in winter, by providing large vertical windows on the equator-facing
side of the building (the south face in the northern hemisphere, or the north face in the
southern hemisphere): this maximizes insolation in the winter months when the Sun is
low in the sky, and minimizes it in the summer when the noonday Sun is high in the sky.
(The Sun's north/south path through the sky spans 47 degrees through the year).
Insolation figures are used as an input to worksheets to size solar power systems for the
location where they will be installed.[9] This can be misleading since insolation figures
assume the panels are parallel with the ground, when in fact they are almost always
mounted at an angle[10] to face towards the sun. This gives inaccurately low estimates for
winter.[11] The figures can be obtained from an insolation map or by city or region from
insolation tables that were generated with historical data over the last 30–50 years.
Photovoltaic panels are rated under standard conditions to determine the Wp rating
(watts peak),[12] which can then be used with the insolation of a region to determine the
expected output, along with other factors such as tilt, tracking and shading (which can be
included to create the installed Wp rating).[13] Insolation values range from 800 to 950
kWh/(kWp·y) in Norway to up to 2,900 in Australia.
In the fields of civil engineering and hydrology, numerical models of snowmelt runoff use
observations of insolation. This permits estimation of the rate at which water is released
from a melting snowpack. Field measurement is accomplished using a pyranometer.
Conversion factor (multiply top row by factor to obtain side column)
W/m2 kW·h/(m2·day) sun hours/day kWh/(m2·y) kWh/(kWp·y)
W/m2
1
41.66666
41.66666
0.1140796 0.1521061
kW·h/(m2·day) 0.024
1
1
0.0027379 0.0036505
sun hours/day 0.024
1
1
0.0027379 0.0036505
kWh/(m2·y) 8.765813 365.2422
365.2422
1
1.333333
kWh/(kWp·y) 6.574360 273.9316
273.9316
0.75
1
Fundamental Insolation Terms and Concepts
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This Section:
Insolation
Definitions
Types of Radiation
Tilt Angle
Sun Angle and Insolation
Air Mass
Day Length
Clouds and Pollution
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This page is one of three introducing insolation-concepts. An overview is at insolation.
The other two pages are: insolation definitions and types of radiation.
.
"Talking about how sunny it is is
rather too jejeune for me. Let's
discuss insolation instead."
.
.
This page defines some terms that are often used when discussing solar energy. After
defining "solar radiation" and "insolation", we differentiate between "irradiance" and
"irradiation."
We then explain how we'll be using "solar intensity" on all of our insolation pages.
Finally, we explain the difference between "solar elevation angle", "zenith angle" and
"solar azimuth angle".
Solar radiation and Insolation
Solar radiation is electromagnetic radiation made by the sun.
What exactly that is is complicated. For our purposes, we just need to remember that
electromagnetic radiation is a type of energy that can move through empty space.
"Solar radiation" is sometimes thought of in terms of "energy" and sometimes in terms of
"power". "Power" is "the rate of energy transfer" - how quickly energy is transferring
from one thing to another.
Insolation measures solar radiation per unit area (the density of the solar radiation - how
much of it is falling on or has fallen on a square meter, square foot or etc). As we discuss
below, this measurement of solar radiation is sometimes given in terms of "energy" and
sometimes in terms of "power".
"No! I'm not saying that the sun is particularily
strong right now! I'm saying that over the course
of the day we've gotten a lot of sun!"
Sometimes people use "insolation" to describe the sunlight's power per unit area at a
given place in a given instant. When this is the case, "insolation" is measured in watts per
square meter (W/m2) and can track moment-to-moment changes in the strength of the
sunlight.
Other times when people say "insolation" they are talking instead about how much solar
energy per unit area a place receives over some period of time.
In this second case, "insolation" is measured in kilowatt-hours per square meter
(kWh/m2) and is used to discuss how much solar energy a place receives in a day, a
month, a year, or etc.
Irradiance and Irradiation
Since "insolation" is often used to describe either power or energy, people sometimes
use more exacting terms when they want to be extra careful.
If you want to make sure that people know you are describing the strength (power) of the
sunlight on your nose right this moment, say "irradiance". If you want to be certain
everyone knows that you are talking about how much total solar energy your nose
absorbed yesterday, say "irradiation".1
"Now, look, as I've said, you've nothing to worry about.
I will constantly apply a very small amount of force to
your head for five or six hours. I grant you that that's a
long time, but since the force will always be very small,
you'll barely notice it. There won't be a single intense
moment.
Policy regarding 'Intensity'
People often use solar intensity or just "the intensity of the sunlight" or even "the
strength of the sunlight" as synonyms for irradiance. We will be adopting that strategy for
this website. "Intensity" and "strength" are more common words and they capture the
sense of "irradiance" well.
If, in a given moment, a very high number of watts of solar radiation are striking a square
meter, the sun will feel very intense/strong to someone standing in that square meter.
Since irradiation is a measurement of energy accumulated over some period of time, it
isn't an instantaneous measurement and so it doesn't have quite the same connotation as
"intensity" or "strength" - at least not to me.
Solar Angle
On this website, we use "solar angle" or "sun angle" when we just want to talk in general
about the vertical angle of the sun in the sky (how high it is in the sky).
If we want to say exactly how high the sun is in the sky, we will use either "solar elevation
angle" (SEA°) or "zenith angle" (ZA°). If we are talking about the horizontal angle of the
sun (where along the horizon it is), we use "azimuth angle". The diagrams below should
help to illustrate these concepts.
Before we introduce the angles, we quickly introduce the concept of "zenith" as not
everyone is familiar with it.
"Zenith" means "Up"
Besides keeping track of "North, South, East and West", we sometimes need to pay
attention to the zenith direction - the direction directly overhead of you. To see it you
have to crane your head back and look straight up. The zenith is always 90° above the
horizon.
Precise Sun Angles
To communicate how high the sun is in the sky, people often tell you the "solar elevation
angle". This is how many degrees above the horizon the center of the sun is.
They can communicate the same information with reference to the zenith. The "zenith
angle" is how many degrees below the zenith the center of the sun is (how far below
"directly overhead" the sun is). The diagram should help make these definitions clear.
Note that because the solar elevation angle (SEA) takes the horizon as its reference and
the zenith angle (ZA) takes the zenith as its reference point and because the zenith is
always 90 degrees above the horizon, SEA° + ZA° = 90.°
That means if we know the zenith angle and want to know the solar elevation angle, we
can just use this formula:
90° - ZA° = SEA°.
The "solar azimuth angle" (Azimuth) measures the horizontal angle of the sun along the
horizon (as opposed to the vertical angle of the sun above the horizon which is what
"solar elevation angle" measures).
The convention is to measure the solar azimuth clockwise starting from due North. So,
due North is "0°" or "360°", due East is "90°", due South is "180°" and due West is "270°."
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This Section:
Insolation
Definitions
Types of Radiation
Tilt Angle
Sun Angle and Insolation
Air Mass
Day Length
Clouds and Pollution
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Footnotes
1. The concepts behind the definitions for "irradiance" and "irradiation" come from the
website of Volker Quasching, an author of books and articles in the renewable energy
field. The page is: fundamentals.
©Copyright 2011. AM Watson & David E. Watson. All rights reserved. Just about
everything in the FT Exploring web site is copyrighted. For information concerning use of
this material, click on the word Copyright.
Applications author's contact page: Applications Contact.
Disclaimer: All information given on these pages is given in good faith. While we have
tried to be accurate, we do not guarantee the accuracy of the information nor are we
liable for any use of the information. If you are undertaking a real-world project, you are
responsible for either knowing what you are doing or hiring someone who does.
Insolation and Total Solar Irradiance
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Total solar irradiance is defined as the amount of radiant energy emitted by the Sun over
all wavelengths that fall each second on 11 ft2 (1 m2) outside Earth's atmosphere.
Insolation is the amount of solar energy that strikes a given area over a specific time, and
varies with latitude or the seasons.
By way of further definition, irradiance is defined as the amount of electromagnetic
energy incident on a surface per unit time per unit area. Solar refers to electromagnetic
radiation in the spectral range of approximately 1 ft (0.3 m), where the shortest
wavelengths are in the ultraviolet region of the spectrum, the intermediate wavelengths
in the visible region, and the longer wavelengths are in the near infrared. Total solar
irradiance means that the solar flux has been integrated over all wavelengths to include
the contributions from ultraviolet, visible, and infrared radiation.
By convention, the surface features of the Sun are classified into three regions: the
photosphere, the chromosphere, and the corona. The photosphere corresponds to the
bright region normally visible to the naked eye. About 3,100 mi (5,000 km) above the
photosphere lies the chromosphere, from which short-lived, needle-like projections may
extend upward for several thousands of kilometers. The corona is the outermost layer of
the Sun; this region extends into the region of the planets. Most of the surface features of
the Sun lie within the photosphere, though a few extend into the chromosphere or even
the corona.
The average amount of energy from the Sun per unit area that reaches the upper regions
of Earth's atmosphere is known as the solar constant; its value is approximately 1,367
watts per square meter. As Earth-based measurements of this quantity are of doubtful
accuracy due to variations in Earth's atmosphere, scientists have come to rely on
satellites to make these measurements.
Although referred to as the solar constant, this quantity actually has been found to vary
since careful measurements started being made in 1978. In 1980, a satellite-based
measurement yielded the value of 1,368.2 watts per square meter. Over the next few
years, the value was found to decrease by about 0.04% per year. Such variations have
now been linked to several physical processes known to occur in the Sun's interior, as will
be described below.
From Earth, it is only possible to observe the radiant energy emitted by the Sun in the
direction of our planet; this quantity is referred to as the solar irradiance. This radiant
solar energy is known to influence Earth's weather and climate, although the exact
relationships between solar irradiance and long-term climatological changes, such as
global warming, are not well understood.
The total radiant energy emitted from the Sun in all directions is a quantity known as
solar luminosity. The luminosity of the Sun has been estimated to be 3.8478 1026 watts.
Some scientists believe that long-term variations in the solar luminosity may be a better
correlate to environmental conditions on Earth than solar irradiance, including global
warming. Variations in solar luminosity are also of interest to scientists who wish to gain
a better understanding of stellar rotation, convection, and magnetism.
Because short-term variations of certain regions of the solar spectrum may not accurately
reflect changes in the true luminosity of the Sun, measurements of total solar irradiance,
which by definition take into account the solar flux contributions over all wavelengths,
provide a better representation of the total luminosity of the Sun.
Short-term variations in solar irradiation vary significantly with the position of the
observer, so such variations may not provide a very accurate picture of changes in the
solar luminosity. But the total solar irradiance at any given position gives a better
representation because it includes contributions over the spectrum of wavelengths
represented in the solar radiation.
Variations in the solar irradiance are at a level that can be detected by ground-based
astronomical measurements of light. Such variations have been found to be about 0.1%
of the average solar irradiance. Starting in 1978, space-based instruments aboard the
Nimbus 7 Solar Maximum Mission, and other satellites began making the sort of
measurements (reproducible to within a few parts per million each year) that allowed
scientists to acquire a better understanding of variations in the total solar irradiance.
Variations in solar irradiance have been attributed to the following solar phenomena:
Oscillations, granulation, sunspots, faculae, and solar cycle.
Oscillations, which cause variations in the solar irradiance lasting about five minutes,
arise from the action of resonant waves trapped in the Sun's interior. At any given time,
there are tens of millions of frequencies represented by the resonant waves, but only
certain oscillations contribute to variations in the solar constant.
Granulation, which produces solar irradiance variations lasting about 10 minutes, is
closely related to the convective energy flow in the outer part of the Sun's interior. To the
observer on Earth, the surface of the Sun appears to be made up of finely divided regions
known as granules, each from 311,864 mi (500,000 km) across, separated by dark regions.
Each of these granules makes its appearance for about 10 minutes and then disappears.
Granulation apparently results from convection effects that appear to cease several
hundred kilometers below the visible surface, but in fact extend out into the
photosphere, i.e., the region of the Sun visible to the naked eye. These granules are
believed to be the centers of rising convection cells.
Sunspots give rise to variations that may last for several days, and sometimes as long as
200 days. They actually correspond to regions of intense magnetic activity where the
solar atmosphere is slightly cooler than the surroundings. Sunspots
Visible light image of the Sun, showing sunspots. U.S. National Aeronautics and Space
Administration (NASA).
appear as dark regions on the Sun's surface to observers on Earth. They are formed when
the magnetic field lines just below the Sun's surface become twisted, and then poke
though the solar photosphere. Solar irradiance measurements have also shown that the
presence of large groups of sunspots on the Sun's surface produce dips ranging in
amplitude from 0.1 to 0.25% of the solar constant. This reduction in the total solar
irradiance has been attributed both to the presence of these sunspots and to the
temporary storage of solar energy over times longer than the sunspot's lifetime. Another
key observation has been that the largest decreases in total solar irradiance frequently
coincide with the formation of newly formed active regions associated with large
sunspots, or with rapidly evolving, complex sunspots. Sunspots are especially noteworthy
for their 11-year activity cycle.
Faculae, producing variations that may last for tens of days, are bright regions in the
photosphere where high-temperature interior regions of the Sun radiate energy. They
tend to congregate in bright regions near sunspots, forming solar active regions. Faculae,
which have sizes on the order of 620 mi (1,000 km) or less, appear to be tube-like regions
defined by magnetic field lines. These regions are less dense than surrounding areas.
Because radiation from hotter layers below the photosphere can leak through the walls
of the faculae, an atmosphere is produced that appears hotter, and brighter, than others.
The solar cycle is responsible for variations in the solar irradiance that have a period of
about 11 years. This 11-year activity cycle of sunspot frequency is actually half of a 22year magnetic cycle, which arises from the reversal of the poles of the Sun's magnetic
field. From one activity cycle to the next, the north magnetic pole becomes the south
magnetic pole, and vice versa. Solar luminosity has been found to achieve a maximum
value at the very time that sunspot activity is highest during the 11-year sunspot cycle.
Scientists have confirmed the length of the solar cycle by examining tree rings for
variations in deuterium-to-hydrogen ratios. This ratio is temperature-dependent because
deuterium molecules, which are a heavy form of the hydrogen molecule, are less mobile
than the lighter hydrogen molecules, and therefore less responsive to thermal motion
induced by increases in the solar irradiance.
Surprisingly, the Sun's rotation, with a rotational period of about 27 days, does not give
rise to significant variations in the total solar irradiance. This is because its effects are
over-ridden by the contributions of sunspots and faculae.
Scientists have speculated that long-term solar irradiance variations might contribute to
global warming over decades or hundreds of years. More recently, there has been
speculation that changes in total solar irradiation have amplified the greenhouse effect,
i.e., the retention of solar radiation and gradual warming of Earth's atmosphere. Some of
these changes, particularly small shifts in the length of the activity cycle, seem to
correlate rather closely with climatic conditions in pre- and post industrial times.
Whether variations in solar irradiance can account for a substantial fraction of global
warming over the past 150 years, however, remains a highly controversial point of
discussion.
See also Electromagnetic spectrum; Greenhouse gases and greenhouse effect; Solar
energy; Solar illumination: Seasonal and diurnal patterns
Source: World of Earth Science, ©2003 Gale Cengage. All Rights Reserved. Full copyright