The UK faces a three-week freeze with temperatures plunging as cold as -15C because of a rare North Pole phenomenon that triggered the 2010 big freeze and nationwide white-out. The worst cold spell of winter, with widespread snow, ice and travel chaos, threatens from February 11. The North Pole's high-altitude air has suddenly warmed up and is set to shunt cold low-level Arctic air south to Scotland. However, the whole of the UK is set for a bitter spell of freezing temperatures. The event, known as a Sudden Stratospheric Warming (SSW) usually chills Scotland for two weeks or longer and sees widespread snow. It has not occurred for four years, official records show. The warning points to the Arctic phenomenon in its February to April forecast being briefed to the Cabinet Office, transport bosses, councils and emergency services. An SSW often allows cold air to flood Britain from the east and is forecasting "colder and drier conditions" from the end of the week for up to three weeks,