<p>The invention provides a hazard warning sign (10) and a method of warning of a hazard at predetermined ambient temperatures. The sign (10) comprises a first surface (18) and a second surface (12a-12g). The visual contrast between the first surface (18) and the second surface (12a- 12g) reversibly changes at a predetermined ambient temperature. The visual contrast can increase at the onset of the predetermined temperature. One of the first surface (18) and the second surface (12a- 12g) can comprise an ink exhibiting a change in visual contrast at the predetermined ambient temperature. The ink can reversibly change between transparent or translucent and a magenta colour at the predetermined temperature. The sign (10) can be used for warning of frost and icy conditions below the predetermined temperature.</p>
