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Solar Radiation Climate

Paper Type: Free Essay Subject: Environmental Studies
Wordcount: 3787 words Published: 1st Jan 2015

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Incoming solar radiation is a key component of the Earths Climatology. From maintaining the Earth’s climate, living forms are able to survive as Hulstrom (1989, p.1) points out solar radiation is a key principle for sustaining life and as a renewable source of energy it can prevent exploitation of the non-renewable sources e.g. oil. Depending on the landscape, solar radiation can create varies microclimates as explained by Chen, Saunders, Crow, Naiman, Brosofske, Mroz, Brookshire, and Franklin (1999, p.288) where a canopy of vegetation will absorb the short-wave radiation, increasing the sensitivity of the ground surface temperatures below. Chen, Hall and Liou (2006, p.1) state it is the spatial and temporal elements of incoming surface solar radiation that can determine many landscape scale processes.

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An area of interest where incoming radiation can create or trigger several processes and climates is mountainous terrain. Even though the total surface area of mountain systems around the world is a very small percentage of the total Earth surface, they can still create an input to the climate system experienced globally. The intensity of solar radiation reached at the surface plays a vital role in mountain climates. It is variations in elevation, slope, aspect, and shadowing that can affect the amount of radiation received at the surface (Dubayah, 1994, p.627, White, Mottershead & Harrison, 1994, p. 207, and Chen et al., 2006, p. 1). This study will focus on incoming radiation and analyse the extent to which each of the factors above affect incoming radiation received upon uneven terrain. The focus will be on incoming radiation because, as Duguay points out (1993, p.339) any progress in the determination of surface radiation in mountainous terrain has to begin with incoming shortwave radiation. Another aspect that this study will approach is the extent to which vegetation canopy cover can intercept radiation before reaching the Earth’s surface. A study created by Mariscal, Orgaz, and Villaobos (2000, p.183) states the importance of measuring radiation received at the surface for purposes of photosynthesis and proposed 70% (p.184) of solar radiation can be intercepted by canopy cover. This study will be analysing the amount of radiation received beneath a forest canopy to aid the understanding between incoming radiation and vegetation cover.

This study is designed to examine the intensity of incoming solar radiation received within Cwm Llysiog valley (51˚49N 3˚25W), located in the Brecon Beacons in South- East Wales. A Coniferous tree forest, located at the Southern end of the valley provides a canopy of vegetation to record radiation measurements beneath. The northern part of the valley is mostly grasslands and shrubs, providing a transect to measure radiation without vegetation inception. Across Britain in the 1970s Harding (1979,p.161) discovered there was very few actual observations of radiation reached on the surface across the uplands due to there be a lack of “robust automatic instruments, capable of withstanding the extremes of an upland environment”. A problem that was crossed in this study was the availability of automatic instruments for recording solar radiation, disallowing me to achieve the quantity of radiation data required.

Literature Review


Radiation is the main input to the black-box closed- system, planet Earth, received from the Sun, in the form of electromagnetic radiation waves ranging from 0.25-3.5 micrometres (Oke, 1987, p.8-9). These waves travel towards the Earth away from the source, at a speed of 299,800kilometers/second (Strobel, 2001). There is a large distance between the Sun and the Earth, resulting in only 0.002% of the total radiation secreted from the Sun is an input to the Earth’s system. The ozone is an important component for protecting the Earth’s atmosphere from captivating harmful amounts of solar radiation, by absorbing the majority of the radiation around wavelengths of 300mm. Each different wavelength is absorbed at different points of the Earths atmosphere. Shorter wavelength UV radiation and solar energetic particles are deposited mainly above the troposphere, where gases such as O2 (Oxygen) act as an absorber of the UV radiation (Lean & Rind, 1998, p. 3072). Visible light is what can be seen by the eye and is centred on wavelengths of about 0.5µm (McIlveen, 1998, p.244). Acra et al. (1990), researched into how atmospheric interventions can cause this change in wavelength and how different colours can relate to the wavelength Blue skies are present when the degree of scattering is sufficiently high within the blue region of the spectrum (McVeigh, 1977, cited by Acra et al., 1990).

The intensity of radiation reaching the Earth surface as a single value is 1353W/m5 and continues to be relatively constant (Rich, Hetrick & Saving, 1995, p.3). Nunez (1980, p. 173) expresses the need for reliable knowledge of solar and terrestrial radiation at the Earth’s surface and looks into approaches that concentrate on the radiation fluxes over a unit of horizontal area, and some index of atmospheric turbidity to derive a climatic radiation model. It is analysed that in most of these cases the radiation fluxes at ground level are assumed to be non-related to the properties of the receiving surface. It would only be the reflected and outgoing terrestrial radiation that the surface would initiate changes (Nunez, 1980, p.173.). The surface properties aspect and gradient will be measured to analyse whether Nunez (1980) has the right idea.

The receiving of energy emitted by the Sun, at the Earth surface is controlled by three sets of factors. Spatial and temporal variation in insolation at specific sites is predictable from basic geometric principles, and can cause variation in climatic conditions across local topography. Insolation is commonly expressed as the average irradiance and is a function of latitude, day of year, time of day, slope and aspect of the receiving surface, and horizon obstruction (Rich, Hetrick & Saving, 1995, p.1). At different times throughout the day the Sun’s height appears to change, and is at it’s highest in around noon. At this point the sun rays have the least distance to travel through the atmosphere and UVB are at their highest. In the early morning and late afternoon the Sun’s rays pass through the atmosphere at an angle resulting in a reduction in intensity. The second is the scattering and absorption of incoming radiation within the atmosphere, through gases, aerosols and cloud particles. This results in three forms of incoming radiation received on inclined surfaces, including: direct (beam) radiation, which is the part of solar radiation that is not absorbed or scattered by the atmosphere and has a direct path from the sun to the surface (Allen, Trezza & Tasumi, 2006, p.55). This study will be focusing on the factors influencing radiation once its nearer to the surface.

Mathematical models have been used to estimate solar radiation. Alam, Saha, Chowdhury, Saifuzzaman and Rahman (2005) present a mathematical model to simulate the availability of solar radiation in Bangladesh using system dynamics methodology.

describes the formulation of the mathematical model used for the study. It takes into account slope angles, atmospheric absorption and scattering by diffused radiation, and the amount of extraterrestrial radiation that would be received. One problem with these models is that the outcomes are only predictions of radiation intensity. Surface based measurements avoid estimations from modelling on radiation, but are more labour intensive.

Holst, Rost and Mayer (2005) used both surface based measurements and empirical modelling, because it was recognised that modelling did not reach a standard of accuracy on its own. For this study field based measurements were carried out over the period of two days to measure the intensity of radiation received at the surface.

Mountain Climates

Observations made in mountains are very important for the understanding of solar radiation and solar constant. Data collection on mountains and their climates over many years has been seen to be quite problematic. The areas tend to be remote from major centres of human activity, have limited physical access, difficult to install and maintain weather stations, and can experience extreme climates. Recent studies have used satellite remote sensing and digital terrain data for analysing mountain climates (Duguay, 1994, Haefner, Seidel, & Ehrler, 1997, & Dubayah, 1994). Digital and satellite imagery has confirmed many climatic conditions that have emerged over thousands of years from the analysis of synoptic data, and has increased the understanding of cloud cover influencing radiation at the surface.

Geographical controls that vary the intensity of solar radiation reaching the surface are Latitude and Altitude. Barry (1992, p.18) explains that latitude has a great influence on mountain climates with solar radiation and temperature decreasing with increasing latitudes. The Ozone becomes increasingly rich with altitude resulting in the mid and higher altitude regions reaching less radiation because the sun is lower in the sky and therefore the rays must travel a greater distance through the Ozone. This gives reason to why Holland and Steyn (1975, p.181) discovered aspect as being an important parameter in the mid- latitudes. Barry (1992, p. 77) also pronounces slope effects changes with latitude. Around latitudes of 40ºN in the northern hemisphere, north facing slopes receive a greater duration of direct radiation throughout the day compared to south facing slopes (Barry, 1992, p.77). The Brecon Beacons is 51˚N so the duration of direct radiation will be shorter on the north facing slope, but the differences between intensities on each slope will be compared for the duration of the day.

Cloud cover is recognised as being a limiting parameter of incoming radiation (Arking & Childs, 1984, & Rieland & Stuhlmann, 1992) and a main contributor to diffuse radiation. This research believes cloud cover plays a vital role in scattering and preventing direct solar radiation reaching the Earth’s surface. Rumney (1968, p. 89) exemplifies the fact that the amount of radiation and sunshine from one year to another would be the same were it not for variable amounts of cloud cover. Cloud cover is thought to cause “back scattering, and can reduce the solar power reaching the underlying surface by as much as 90%,” (McIlveen, 1998, p.244). Fritz (1951, cited by Garnier & Ohmura, 1968, p.798) noted that cloudless skies are appropriate in climate studies to limit the atmospheric tranmissivity influence on incoming radiation.

Spatial characteristics of mountainous terrain such as orientation, angle, vegetation cover and shadowing from neighbouring slopes have been the subject of many observational and analytical studies, Duguay (1993) by modelling downward fluxes (pp.341- 347), Churchill (1982) with aspect influence on hill slope process, Holland and Steyn (1975), vegetation response to angle and aspect, and Wendler and Ishikawa (1974) with the effects on slope and exposure on solar radiation. Figure 3 illustrates the three sources of illumination that can occur on slopes. Variability in slope angle can lead to strong local gradients in solar radiation (Ralph, 1994, p.627 & Kumar, Skidmore, & Knowles, 1997, 467). Holland and Steyn (1975, p.181) found that the differences in incident solar radiation in mountainous areas of different slopes and aspects were maximum in the mid-latitudes and the least in equatorial and polar regions. The mid-latitudes in the northern hemisphere are closer to the Sun in July (summer solstice) resulting in greater amount of radiation received on slopes north facing slopes receive more radiation in early hours (0600hrs) of the day and later hours in the evening (1800hrs) compared to the south facing slopes. The southern facing slopes, of an angle greater than 55º receives a greater intensity of radiation at midday, where the north facing slopes are not illuminated, as displayed in figure 4. Barry (1992, p. 76) acknowledges the fact that “South- facing slopes at the equinoxes show a symmetrical diurnal pattern,” from the time the sun rises in the east, limiting the intensity shining on south facing slopes with increasing steepness in the early hours of the day. By comparing the north west and south east facing slopes through the duration of a day, I will be able to analyse whether the patterns found within research have correlated with my own findings.

It shows the steep south facing slopes reach a greater amount of radiation compared to average south facing slopes, but it is clear the greatest difference between aspects is when the sun is either highest in June, or lowest in December in the sky (Ralph, 1994, p.633). Surface temperature is a useful parameter to estimate the amount of radiation received on varying slope aspects. Safanda (1999, p.367) expresses that the north facing slopes in the middle latitudes in the northern hemisphere are a few ºC colder at similar elevations as on South-facing slope surfaces. Reason for this is that less solar radiation falls on a unit area of the slope surface (Safanda, 1999, p.367). By recording near surface temperatures for the two valley transects, it will allow me to correlate the differences between two aspects by comparing temperatures at the same elevation.

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Shadowing from neighbouring slopes or valleys is thought to be a “spatio-temporal function” because it depends on both topography and solar geometry (Ranzi & Rosso, 1995, p.464). Shadowing, introduced by Ranzi and Rosso (1995, p. 468) for a catchment basin that has shadowing occurring across the surface from projected horizons within the catchment area, is ‘Self Shadowing’. This should only occur in a valley with east and west facing slopes as the sun will rise in the east projecting a shadow onto the east facing slope if elevation is great enough. By knowing the different slope angles and orientations of the Cwm Llysiog valley, the effect of exposure and shadowing can be assessed. The McCall Glacier (Alaska) was studied (Wendler & Ishikawa, 1974) for the effect of slope, exposure and mountain screening on solar radiation and discovered that the screening effect of mountains was much more important than the northerly exposure reducing radiation reaching the glacier. It is not only slope shadowing that could limit the intensity of radiation received at the surface in the Cwm Llysiog valley, vegetation cover will also reflect radiation. Cannell, Milne, Sheppard, and Unsworth (1987), and Bartelink (1998) explain with increasing canopy cover, radiation interception is increased resulting in a decrease of radiation at the surface (Jordan, 1969, p.663). Vegetation cover is thought to be the greatest limiting factor in the Vegetated valley and will be compared to the non- vegetated valley radiation readings to verify this prediction.

2. Aims and Objectives

The aim of this study is to investigate how the variability in slope, aspect and shadowing comprise to create a changing affect on the gradients of incoming radiation in forested and non- forested valleys. This will be assessed by comparing north and south facing slopes within a South Wales valley with forested and non-forested slopes in the summer with cloudless skies. Below is each Hypothesis set before measurements were taken and research that backups the reasoning for the hypothesis.

Hypothesis A: – The vegetated slopes will decrease the intensity of solar radiation received at the surface compared to the non- vegetated slopes. This will reflect in the surface temperature, with an increase in solar radiation resulting in an increase in temperature.

  • Safanda (1999, p.367) concluded that north facing slopes achieve a low temperature then south facing slopes.
  • Bartelink (1998) is one of many that has proven vegetation cover will decrease the intensity of radiation received at the surface.

Hypothesis B: – The intensity of solar radiation will be greater on the south east facing slopes compared to the total solar radiation received on the north west facing slopes. The orientation of slope faces will be the most influential factor on incoming solar radiation.

  • White et al. (1994, p.207) describes the azimuth (orientation of the surface) as being the most influential component in the intensity of solar radiation received at the surface. It is stated that a southerly facing aspect will receive a greater intensity of radiation at the surface compared to a northerly aspect, which might not receive any at all.
  • On the other hand Whiteman, Allwine, Fritschen, Orgill, and Simpson (1988) compared radiation components from five stations situated in a single valley during September of 1984 and concluded that slope faces have distinctly different diurnal courses of radiation. Slopes facing north east, experience downward solar fluxes directly after the slope is illuminated during sunrise but the fluxes become weaker during the afternoon as a result sunset. In contrast the south west facing slopes, has weaker direct radiation in the morning but attains a strong peak in the early afternoon. This view is slightly different to White’s et al. theory on aspect.

Hypothesis C: – Slope angle will have a less influential impact on radiation intensity compared to slope aspect. It is thought with an increase in gradients the intensity of solar radiation will decrease and become less direct.

  • White et al. (1994, p.208) explains that these two factors (aspect and gradient) combined have a greater effect on the amount of direct radiation on north facing slopes in the northern hemisphere. It is made clear, with increasing slope angles, there is a decrease in solar intensity directed at the north facing aspect.
  • Dubayah (1994, p. 634) displays a time series of monthly incoming solar radiation for different slope terrains within the Rio Grande River Basin. The study shows steep south facing slopes receive around 140W/m2 more radiation than a steep north-facing slope in July. The differences displayed in these findings are thought to be due to slope gradient.

Hypothesis D: – Within the forested valley, the vegetation cover will cause a great deal of shadowing on the surface decreasing solar radiation received at the surface. The greatest shadowing in the non- vegetated valley will occur in the lower sites, near the valley floor where the surrounding horizons are at a higher elevation, decreasing the sky view factor.

  • Ranzi and Rosso’s (1995, p.464) study in a drainage basin realised that shadowing occurs at low altitudes, as the “direct radiation is less important in relation to the other radiative fluxes, i.e. diffuse irradiance from the sky and direct and diffused irradiance reflected from nearby terrain”. This means any horizon at a higher elevation then the site being studied will reduce the intensity of solar radiation received at the particular site.
  • White et al. (1989, p.419) agrees with Ranzi and Rosso views where changes in orientation or positioning on a slope, affects the view of surrounding topography, thus affecting receipt of reflected radiation.
  • Jordan (1969, p. 663) explains The greater the vegetation cover the greater the greater the difference in radiation above and below the canopy.

Null Hypothesis: – There will be no correlation between radiation received at the surface on the vegetated and non- vegetated slopes. Factor such as slope orientation, slope angle and shadowing will not influence the amount of radiation reached at the surface.


The relationship between direct radiation and surface elevation is complex and depends on the atmospheric conditions such as cloud cover. With an increase in surface elevation an increase in direct solar radiation will occur, because the solar path through the atmosphere is shortened. This only tends to occur under cloudless skies. Batlles, Bosch, Tovar-Pescador, Martinez-Durban, Ortega and Miralles (2008, p.341) studied atmospheric parameters to estimate radiation in areas of complex topography and came to the conclusion that only the global radiation changes with increase of 1000m in elevation. It was thought that in the current microclimate being studied, elevation variations are less significant than other topographic variables, such as shadowing affects. Measurements recorded in the field for this study only reached 40m up the slope, meaning the effect of elevation on radiation would be very little. Due to these findings elevation will not be included in this study as a control.


The main objective is to examine any correlation between slope aspect, slope angle, shadowing and vegetation cover and to analyse the influence they might have on the intensity of incoming solar radiation reaching the surface. To assess these factors affecting incoming radiation on sloping terrain, this study will test the hypotheses determined by studying the previous research.

The hypotheses will be tested by recording a set of incoming solar radiation readings along a transect across a valley over the period of a day. It was difficult to locate a valley with north and south facing slopes in Wales. The Cwm Llysiog presents north west and south east aspects. The increase in elevation will be measured to display differences in radiation at the base of the valley and the valley slopes. The gradient is also important to analyse the correlation between slopes and radiation. A set of temperature results at the nine sites along the transect will determine if there is a link between solar radiation intensity and near surface temperatures.

Another main objective is to provide readings for all the above, on a slope covered by a forest canopy, creating a shadowing affect. Exposure readings for all sites will be recorded to assess the extent of shadowing from near surfaces and objects.


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