On Halloween in the UK, There is gonna be whats called 'Rain Blood' & basically it is rain mixed with red dust from the Sahara Desert, Causing a mixture with the precipitation forming 'Red' raindrops.
Article: http://www.gadling.com/2012/10/23/blood-rain-predicted-to-fall-in-the-united-kingdom-this-hallowee/
Article: http://www.gadling.com/2012/10/23/blood-rain-predicted-to-fall-in-the-united-kingdom-this-hallowee/