First, let's define our terms: according to the National Weather Service, a white Christmas happens when there is one inch or more of snow on the ground on the morning of the holiday.

It need not snow Dec. 25 to fit the Weather Service's definition of a white Christmas: There just needs to be at least one inch of snow on the ground. A trace amount of snow also does not count. On average, about 38% of the Lower 48 has an inch of snow on the ground on Christmas Day, according to 21 years of data compiled by NOAA.

As 25 December approaches, many are left wondering if we might wake up to a picturesque, snowy scene on Christmas morning. According to the Met Office, a white Christmas is officially defined by the observation of a single snowflake falling anywhere in the UK within the 24 hours of 25 December.