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By far the best way to decide whether the sun is hot enough to make your solar tetroon fly is to use a solar power meter. You cannot rely on solar power data or even local weather forecasts. It takes just one thick cloud between you and the sun for your tetroon to stay firmly on the ground. Either make the simple power meter from Chapter 2 or use a silicon photocell type meter and find out the likely temperature rise inside your tetroon. Then use the lift calculator spreadsheet to see if the temperature rise is enough to get your tetroon off the ground.

It can be hard to find more general information about how much solar power your area receives. You can find solar power data from several Government organisations, but the data can be buried in clunky and difficult to use websites. Luckily the two websites listed below are quite good.

Solar radiation data comes from two sources: satellites and ground-stations.

Scientists have to do some clever modelling to convert the satellite measurements of solar radiation, cloud cover, haze and solar angle made from space into a figure for the likely solar radiation reaching the earth's surface. But because satellites can easily cover large areas there are satellite solar power measurements for the whole of the earth's surface. You can even get data averaged over the 22 year period between  1983 and 2005, giving a very reliable estimate of the likely radiation at any location in the world.

Ground-stations give a direct measurement of the actual solar radiation at one place. Although you only get data for one place there are a lot of ground stations across the USA and Europe, and all government funded ground-stations publish the data on the internet.
Satellite measurements of solar radiation, air temperature and humidity. Measurements are also listed from ground stations all around the world. You will need to enter the latitude and longitude of your location. If you don't know your latitude and longitude you can look up your local town on Wikipedia - it usually lists the latitude and longitude as part of the general information.
National (USA) solar radiation database for the years 1961 to 1990. A fantastic resource that lists a huge number of measurements from ground stations all across the USA. You will need to read the description of the file format to understand what information is in which column in the data file. There is an update to the database on the home page that gives data for the years 1991 to 2005. The solar radiation data is free to download.