Solar Radiation of Hawaiʻi
This website provides a set of maps of the spatial patterns of solar radiation for the major Hawaiian Islands. We estimated solar radiation as part of a larger project on evapotranspiration. In addition to solar radiation, numerous other variables, such as air temperature and relative humidity, had to be estimated. Those, along with evapotranspiration, are included here, too. Most variables are mapped for each hour of the average 24-hour cycle of each month and for each hour of the average 24-hour cycle for the whole year. The average value for each month and the annual average are also mapped. Many of those maps are available via this website, in the form of downloadable files and, for a selection of variables, on the interactive mapping tool.
Be sure to check out the interactive map! It may need a few minutes to load on your first visit.
Importance of Solar Radiation
|Photovoltaic panels mounted on the roof of a home in
Hawai‘i. Image from Mauisolarproject.org.
Nearly all the energy available on Earth is derived from solar radiation. That includes the energy the drives natural process, fron wind systems and the hydrological cycle to leaf photosynthesis. Knowing the amount, spatial patterns, and temporal variations in solar radiation is necessary for many scientific fields of study, including meteorology, ecology, and materials science, and for many practical purposes. These days, solar energy has become one of the most important sources of alternative energy for generating electricity and heat water. Whether it’s for rooftop photovoltaic and hot water systems or centralize solar power generation plants, knowledge of the patterns and variability of solar radiation is essential.
Why Solar Radiation Varies
Solar radiation differs from place to place and from time to time for several reasons, including the angle at which the sun’s rays strike, absorption and reflection by the atmosphere as the radiation passes through it, absorption and reflections by clouds, and shading by the surrounding terrain. Sun angle is a key variable and it affects the intensity of solar radiation in a very predictable way. Incident radiation is most intense when it is perpendicular to the surface. Sun angle varies according to location, time of day, and time of year. Atmospheric influences on radiation are affected by changing concentrations of certain gases, especially water vapor, and the amounts and types of suspended particles (aerosols). In Hawai‘i, except areas near active volcanic eruptions, the air is relatively clear and atmospheric effects on radiation are fairly constant. The main thing affecting place to place and time to time variations in solar radiation, besides sun angle, is the amount of cloud cover.
|Visible GOES imagery of the State of Hawai‘i at 2100 UTC
(11:00 AM HST) on June 6, 2014. NOAA-NASA GOES Project.
Clouds patterns in Hawai‘i are very distinct, with persistent cloud cover in areas where winds forced to rise up mountain slopes, i.e., along windward (east and northeast-facing) slopes and along the west-facing slopes of the Kona region of Hawai‘i Island. Scattered clouds are typically seen over leeward and high elevation areas.
Estimating Solar Radiation
We mapped solar radiation by first estimating clear-sky solar radiation, the amount of sunlight received with no clouds at a given location, time of year, and time of day. Then the effects of clouds and shading by surrounding terrain were incorporated to produce maps of solar radiation. We analyzed patterns of cloud frequency based on imagery from the MODIS and GOES satellite platforms. More information can be found on our methods page.
The Solar Radiation of Hawai‘i website provides access to a set of maps of the spatial patterns of solar radiation, as well as evapotranspiration (ET), potential evapotranspiration (PET), and the climatic and land characteristic variables used to estimate them for the major Hawaiian Islands. In general, each variable is presented in the form of mean hourly maps for each hour of the diurnal cycle of each month and of the whole year, mean monthly maps for each month, and a mean annual map. The maps represent our best estimates of the mean values of each variable based on observations taken during the past decade or two.
This web site was developed to make the solar radiation, ET, PET, and climate maps, data, and related information easily accessible. The maps depict patterns by color. The interactive map allows users to see the spatial patterns of each variable, zoom in on areas of particular interest, navigate to specific locations with the help of a choice of different base maps, and click on any location to get the mean value of the selected variable, graphs of the mean annual cycle (mean monthly values) and mean diurnal cycle (mean hourly values) of the selected variable, and obtain tables of the mean hourly, monthly, and annual values of all variables for the selected location.
The maps can also be downloaded in various forms. Our analysis produced digital maps called rasters or grids. On these maps, the islands are divided into 8.1-arcsecond spatial units, or approximately 234 × 250 m (770 × 820 ft). Each variable is estimated for each spatial unit. GIS (Geographic Information Systems) users can obtain as raster files. Alternatively, image files showing spatial patterns by color can be downloaded.
This website is part of a family of websites providing data on the climate of Hawai‘i. The Rainfall Atlas of Hawai‘i covers only rainfall. The other three websites each provide data for all variables, but each is presented with a particular focus.