Snape whirled at the sound of Apparation so close to his own - he hadn't expected to be followed to this deserted place, a random spot in the midst of trackless, treeless tundra.
He sagged with physical relief at the sight of Dumbledore. "I'm Snape," he said quickly, recognising that entirely-too-polite tone, rushing to explain, "Voldemort was at the party, Polyjuiced. Like him." Only when Dumbledore was aware of the danger, did he cut the Petrificus. But he kept his wand raised, curses on the tip of his tongue, in case the impostor decided to strike.