The warmest January for 15 years is being followed by extreme cold weather.
Northern England, Scotland and parts of Ireland and Northern Ireland have already been hit by heavy snow - and forecasters say it's moving south.
More than 30 schools were closed in Aberdeenshire in Scotland on Monday because of problems caused by the weather, such as blocked roads.
Experts say that Wednesday and Thursday could be particularly chilly - so remember to wrap up warm.
The weather is also causing problems for some footie clubs.
Middlesbrough and Hartlepool Utd have asked fans to clear snow from their pitches so their matches can go ahead this week.
The snow can be lots of fun, but people are also being warned to take care after a man died in a sledging accident in Northumberland on Sunday.