These days near the summer solstice and when there are big puffy clouds in the sky are typically when we get the highest values of incoming solar radiation all year.
In fact the readings are sometimes higher than the maximum incoming solar radiation which confuses some people. One has to remember that the maximum values are the maximum "direct" incoming solar radiation while the instruments we have at the UW weather station measure both "direct" and "reflected" solar radiation.
So on days like today not only is the sun producing a lot of direct solar radiation but those big puffy (and white) clouds are also reflecting a lot of that solar radiation towards the sensor.
Of course you also see really low readings as well when the direct solar radiation is blocked by one of those clouds.