Sunny frost
Currently in Christchurch it is -4C, sunny and brisk. Due to our recent renovations, we have an old bath sitting in the garden, with 15cm of water in it. It's frozen solid.
EDIT: After Dave kicked the bathwater, we found the ice was only actually an inch thick. It could still withstand a 15kg dog though. Lots of 15's today Casey... whats up with that?