Climate information for solar energy

The Bureau of Meteorology undertakes solar measurements using a combination of high quality, ground-based instruments and a computer model which processes satellite images. Apart from the one minute solar data product, all solar data on this website have been derived from satellite observations. While the ground stations provide observations with lower uncertainty than the satellite-derived data, they are relatively sparcely distributed. In a typical agricultural area such as that around Wagga Wagga, the satellite method becomes more accurate than using the surface station values at a distance typically 40 km from the pyranometer.

Solar energy

Solar energy is the primary energy source for the Earth's environment and drives weather and climate. The term is commonly used in the community to refer to both the energy being radiated by the sun and received on Earth, and the energy produced by solar power technologies such as electricity from domestic roof-mounted photovoltaic panels.

The first measurement of sunlight performed by the Bureau commenced in Perth back in 1898. An instrument known as a Campbell-Stokes recorder measured the length of time during the day when the sun was bright enough to burn a trace on a paper chart. This instrument is a binary recorder; at any given time the sun is either bright enough to burn the paper or it isn't.

The next level of sophistication occurred in the 1960s when the Bureau installed a network of 28 pyranometers at its weather stations around the country. Not only could these instruments measure both direct (shadow-causing) and diffuse (indirect light scattered primarily by clouds), but they provided information about the amount of energy received every 30 minutes.

At the beginning of 1990 the Bureau commenced a service to provide solar energy data derived from satellite images, which was supported by a new network ground stations with significantly higher quality data than that previously obtained. While the uncertainty around the data obtained from the satellite was larger than that from the new ground network the service offered the significant benefit of covering the entire country.

The majority of the solar information on this website was obtained from satellite data and is a measure of the amount of energy over a period of time (usually a day) which has been received on a flat horizontal surface (eg flat ground). There are many other factors which must be considered when estimating the energy generated from a device such as a solar panel. These include the angle of the panel, whether part of the sky is blocked or shading occurs from nearby trees or buildings, and how effiicent the device is in converting sunlight to electricity.

Climate information for solar energy is arranged as a number of tabbed sections, each of which provides a general overview of the topic, together with a number of sections with more specific, and sometimes quite technical information. For further information please use our Enquiry form.

General characteristics of solar data

The Bureau of Meteorology maintains a network of high quality ground stations which provide one minute statistics on a range of solar parameters. This network is complemented by a system which provides global solar irradiance and derived products from satellite imagery. Many of these data can be obtained online via the links on the introductory page of this solar information portal.

Ground observations include direct, diffuse and global solar irradiance and terrestrial (longwave) irradiance. All measurements provided from this network are traceable to SI units. As a result, uncertainties are also provided for each set of minute statistics and enable specific uncertainties to be estimated for other derived quantities. Data quality assurance and quality control of the station measurements are such that the initial target of 95% uncertainties for any minute exposure quantity of 3% or 900 Jm-2 (which ever is the greater) have been achieved and improved upon.

As part of the tiered approach, the Bureau derives daily solar exposure over the Australian region from geostationary satellite data with an areal resolution of 25 km2. The algorithm is satellite-specific and as satellite characteristics change, the derived data are adjusted to the ground network data. Studies indicate that, for monthly satellite-derived global exposures, data have an uncertainty of approximately 7%. Since 2010 the frequency of satellite-derived global and direct solar exposure data has expanded from daily to hourly.

As one example of testing the satellite method an intercomparison was undertaken using pyranometer data from 9 network sites from July and August 1997. On average the model agreed with the measurements to within 0.17% (around 0.04 MJ/m2 on a typical clear day) and the majority of measurements agreed within 6% (around 1.5 MJ/m2 on a typical clear day). The satellite method tends to slightly over-estimate the radiant exposure in wet, cloudy conditions, and under-estimate it in dry conditions. On the basis of these and subsequent intercomparisons it is concluded that the satellite model provides useful daily global solar exposure estimates in all conditions, with an error of 7% or better in clear sky conditions and up to 20% in cloudy conditions.

To put these numbers into perspective, one can imagine using the measured radiant exposure at a pyranometer location to estimate the radiant exposure at a point some distance away. The accuracy of the estimation will decrease as we move away from the radiation station. The further we go, the less reliable the estimate will be. In a typical agricultural area such as that around Wagga Wagga, the satellite method becomes more accurate than using the surface station values at a distance typically 40 km from the pyranometer.

Both the satellite and ground-based data are provided with relevant metadata to assist in interpretation of the data and their fitness for purpose. Ground data observed since 1993 are also provided with observation-specific uncertainties. There are periods of missing data, which may be caused by a variety of factors including equipment failure, loss of the transmitted signal from satellite, and temporary site closures.

Detailed information

Overview of the Bureau's solar observation program

Solar irradiance and exposure monitoring by the Bureau of Meteorology has undergone several transformations since it begun in the mid-1960s. At that time a 28 station network was established using Eppley Black & White pyranometers, which provided measurements of 30 minute global and diffuse solar exposure. This network was supplemented by a small network taking data in the capital cities, but using modest quality black-only Middelton EP07 thermopile pyranometers.

The combined network relied on initial calibration of the pyranometers, and subsequent correction of data using modelled climatology based on measurements in South Africa. Instruments were changed when it was clear they had failed, or their sensitivities were undergoing significant shifts when compared to a theoretical climatology.

Trends suspected to be instrumental were modified to reflect the theoretical climatology rather than by recalibrating the instrument observing the trend. Unfortunately, some of the trends are now known to have been environmental, not instrumental. As a result, there is evidence to suggest that the aim of 7% uncertainty for global solar exposure measurements was not met.

As resources declined, by the mid-1980s the number of station closures increased. From 1987 data recorded by the remaining stations were no longer corrected to climatological estimates and investigations into inconsistency of quality control checks were not possible. By the early 1990s only 6 old network stations were operating, and only monitoring global solar exposure.

Plans were developed to arrest the decline and provide a network of high quality ground sites and a satellite system capable of providing complete areal coverage of the Australian region. The upgraded surface network monitoring direct, diffuse and global exposure and terrestrial irradiance was initiated in 1993 and continues to operate, although the number of open stations at any one time has fluctuated over the years. Most recently, eight stations were opened for a limited time as part of a collaborative project with Geoscience Australia.

Daily estimates of global solar exposure derived from satellite images exist from 1990. Satellite-derived estimates are based on images from the Geostationary Meteorological Satellites GMS-4 and GMS-5, Geostationary Operational Environmental Satellite (GOES-9), and the MTSAT-1R and MTSAT-2 satellites, which are provided with permission of the Japan Meteorological Agency (JMA) and the United States National Oceanic & Atmospheric Administration (NOAA).

Monthly climatologies of hourly global and direct beam solar exposure were first produced in 2009 for the Australian Renewable Energy Atlas (no longer operational). The underlying hourly spatial data became publically available in 2010. Work continues within the Bureau to enhance the computer model and associated data processing, as well as integration of Himawari-8 data which became operational in 2015.

The solar climate

While the sun's output is relatively stable, the amount of solar energy reaching the ground depends on a number of factors; two of the most important are the position of the sun in the sky and the extent of cloud cover. Cloud cover can be quite variable and is determined by the local weather conditions and seasonal patterns. However the position of the sun in the sky is predictable and changes over the course of a day, and over the year.

The solar constant

The radiative output from the Sun is essentially stable. It is defined by the solar constant (1367 Wm-2), which varies by only about 0.1% over the 11 year sunspot cycle. Small variations may also occur over longer period cycles, but our observational records are not sufficiently long to detect their magnitudes directly. Because the Earth's orbit is elliptical, the irradiance at the top of the Earth's atmosphere at any time of the year will vary, being below the solar constant when the Earth is further from the sun than the mean distance, and greater than the solar constant when it is closer. This value is known as the extra-terrestrial irradiance.

Influence of the atmosphere

Enegy budget
Figure One. A schematic diagram illustrating the main components of the Earth's energy budget, with the long-wave radiative components shown in red. Source: NASA.

The various paths sunlight may take after reaching the atmosphere are shown in Figure One. Of the incoming solar energy, 30% of the total energy received by the Earth is reflected back into space by clouds, other components of the atmosphere, and the Earth's surface. A further 19% is absorbed by the atmosphere (water vapour, dust and ozone) and clouds. The nett result of this is that, when averaged over the globe, only 168Wm-2 of the 342 Wm-2 incoming energy is absorbed by the Earth's surface.

Because the temperature of the Earth is relatively constant, the system must be close to equilibrium, and the amount of energy radiated back into space must be approximately equal to the energy incident at the top of the atmosphere.

The reduction in energy does not occur equally across the solar spectrum. of light. Water vapour, for instance, absorbs energy in bands at the longer, red end of the spectrum. Scattering of light by molecules in the air is greater at the short-wave end of visible light, which is why the sky appears blue.

Variability over time

Solar exposure varies over many time scales. On the short end of the time scale, irradiance can suddenly drop as a cloud passes in front of the sun, while the changing position of the sun over the course of a day will affect global solar exposure more so than it does direct normal exposure (where the plane of incidence is kept perpendicular to the sun). The tilt of the Earth's axis, which causes the seasons, results in the sun being lower in the sky during winter than it is in summer (except at the Equator), with a corresponding reduction in incident energy on the ground. Longer term variations are associated with the passage of weather systems (cold fronts, for example), out to climate variability occurring over multi-decadal periods.


Solar radiation spectrum graph

Applications for solar data

Solar data from both the Bureau's ground observing network and satellite imagery have a wide range of applications; from scientific climate research to modelling of plant growth, location of good sites for building solar power stations and design of energy efficient buildings.

Renewable energy

Renewable energy applications make widespread use of solar data. Online data portals, such as the Australian Solar Energy Information System mananged by Geoscience Australia assist in providing the information required to site large-scale solar power systems in the most economically viable areas of Australia. On the smaller scale, solar data have been used to provide estimates of electricity generation capacity to the more than one million Australian households with rooftop solar photovoltaic systems. The design of energy efficient buildings also relies in part on access to solar data which assist in modelling the thermal performance of a building (where energy from the sun is a major factor in heat gain during summer) and optimisation of window design to provide good ambient lighting while managing energy flow through the glass.

Evapotranspiration

Radiative processes in the atmosphere play a key role in the Earth's water balance, where moisture moves in a cycle from rain through the soil, plants and oceans back to clouds. Evapotranspiration (ET) is the term used to describe the transfer of water, as water vapour, from both vegetated and un-vegetated land surfaces to the atmosphere. It is different from, but includes the moisture transfer through evaporation from water bodies such as lakes, which the Bureau measures using an evaporation pan. Evapotranspiration is affected by climate, availability of water and type of vegetation, and is relatively difficult to measure directly. However, a number of methods exist to estimate it from other environmental parameters such as wind speed, temperature, vapour pressure and global solar exposure.

Climate and weather applications

Three of the Bureau's high quality ground stations are among about forty stations around the world which form the Baseline Surface Radiation Network, a component of the Global Climate Observing System established in 1992 to meet evolving national and international requirements for climate observations. Data from the Bureau's network help validate solar forecasting components of the numerical weather models used to produce Australia's weather forecasts, and are vital in reducing the uncertainties of the satellite-derived solar exposure estimates.

Where to seek information

Most Australian States and Territories have a Government department which has responsibility for solar energy related issues. Names of these Departments are not consistent across Australia, and may change over time, but often includes Sustainability, Environment or Energy in the Department title.

The majority of solar energy conferences have a section devoted to solar resource assessment. Papers generally include topics on regional data analyses, new tools and techniques. The International Solar Energy Society's Solar World Congress is one such event. Within Australia, several previous conferences organised by the Australian Solar Energy Society included discussion about the Australian solar resource.

The Bureau's Data Services web page provides information to assist with obtaining a wide range of climate data and past weather information. It covers station data, maps and gridded data, and metadata, as well as various analysed data, and includes content available on this website as well as those data available on request to the Bureau.

Updated: 16 Nov 2016