The Mt Palomar observatory. A monumental building on a 5,500ft mountain. The glass used for its vast telescope’s mirror took a year to cool down and more than a year to prepare for installation back in the late 1930s. This picture was taken as a very rare (and violent!) thunderstorm broke overhead. It had been forecast to be the hottest day of the year so far but the so-called Californian ‘monsoon’ had other ideas