How to obtain solar radiation data

We offer solar radiation data in both our 15-day forecast and historical observations datasets. They can be included in Weather API requests, viewed directly in the browser or downloaded as a CSV or Microsoft Excel format file.

What is solar radiation?

Solar radiation is the general term for the energy emitted from the sun. It is a form of electromagnetic radiation covering many wavelengths of the electromagnetic spectrum ranging from radio waves to X-rays. To life on Earth, solar radiation is the energy that gives the world light and heat – 99% of the solar energy ranges from near ultraviolet, visible and near infrared.

Solar radiation is measured as the amount of solar radiation per unit area per second. This is sometimes named ‘solar irradiance’ and is typically measured in Watts per meter squared (W/m2).

Solar radiation arrives at the top of the atmosphere at approximately constant value of 1361 W/m2 . The radiation is then reduced by interaction of the radiation with the Earth’s atmosphere. The typical maximum that will arrive at the surface on a clear summer day is 1000 W/m2.

This maximum will be reduced based the amount of atmosphere that the sunlight passes through to reach the ground plus any atmospheric conditions that will reflect, scatter or absorb the radiation.

The amount of cloud and atmospheric pollution will significantly reduce the radiation by scattering the incoming sunlight.

The latitude of a location will affect the amount of atmosphere that the radiation will travel through. Locations near the equator will experience the maximum values of solar radiation as the sun passes through the least atmosphere. Locations near the poles, where the sun is at a lower angle in the sky, will measure less radiation as the sun must travel through more atmosphere. This is exacerbated by the seasons and time of day. Winter time will further the amount of atmosphere that the radiation must travel.

The time of day also affects the amount of solar energy as again the lower angle of the sun in the sky causes the more atmospheric absorption and scattering as the energy travels through more atmosphere than at midday.

Why is solar radiation important for weather observations?

It is the heat from the sun that drives the energy of life on earth by providing plants with energy via photosynthesis as well as being the primary energy source of the weather. As a large source of energy, it is also the energy behind many renewable sources of energy including solar power and, by driving the weather systems, wind power.

Understanding the amount of solar radiation that falls on a location is therefore of great important to agriculture and renewable energy such as solar power.

How we collect and process the solar radiation data

We collect solar radiation data from 1000s of weather stations around the world. Typically these observations include the instantaneous measurement of solar radiation at the time of the observation (for example every hour or every 15 minutes depending how often the station reports).

The total amount of solar radiation energy for a day is found by summing the individual solar radiation values for the day. As measurements are not record for every second, it is generally assumed that the same solar energy was recorded for the whole previous interval. For example, if the observations are hourly, the total solar energy can be found by:

Energy=power x time

If our observations are hourly, the time is 3600 seconds. Therefore if our observations are hourly, we estimate the total energy for a single hour by multiplying by 3600

Hourly Energy (Joules, J) = observed instantaneous power x 3600
For a hourly measurement of   150W/square meter:
Energy (J) =  150 x 3600 =  540000J 

Energy is typically expressed in megajoules per square meter (0.54 MJ/m2) or kilowatt hours (kWh/ m2 ) . To find the kilowatt hour value from megajoules, divide by 3.6.

If you retrieve data using the ‘base’ unit group, then you will retrieve the solar energy in MJ/ m2 otherwise you will retrieve the data in kWh/m2 as Kilowatt-hours are generally more useful for solar power calculations and comparisons.

The default daily solar radiation display is the mean value of the solar radiation for the day. For solar energy, the daily value is the sum of the hourly values.

Viewing and downloading solar radiation data

Solar radiation and energy data can be viewed and downloaded directly within the Weather Data Services pages. The data is available for both historical observation and weather forecast data. The solar radiation data is not available in historical summaries or statistical daily forecasts at this time.

To view and download solar radiation data:

1. Log in to Weather Data Services.

2. Create a timeline, forecast or historical data query

3. On the view data page, click ‘Show additional columns’ to view the solar radiation and solar energy columns.

4. (Optional) You may download the data as a CSV or Microsoft Excel workbook by clicking on the download buttons below the grid.

Retrieving solar radiation data using the Weather API

You can also retrieve the solar radiation and solar energy data in the Timeline API and the forecast and history APIs.

If you make a request for forecast or historical data with a JSON return type, the solar radiation and energy data will be included. If you request the data in Comma Separated Values (CSV) format, then you must change the ‘elements’ parameter from ‘default’ to ‘all’.