Winter weather FAQ
by Kirsty McCabe, FRMetS
When will it snow?
Snow in the UK is a (fairly) rare and wondrous thing. No other weather phenomena can provoke the same level of media interest/hyperbolae. But can you spot the meteorological set-ups that will trigger major snowfall events in the UK? And just why is the number 528 so important in snow forecasting?
Why didn’t I get snow?
You’ve probably heard weather presenters talk about a “wintry mix of rain, sleet or snow”. But why is it so hard to forecast which one we’ll get? The problem is that just a few degrees difference in air temperature can determine whether you’ll be throwing snowballs, splashing in puddles or skidding on dangerous black ice.
Why is it so cold?
The number on your thermometer doesn't tell the full story, especially during the winter months. When the wind blows from Arctic or polar regions, it's no surprise that temperatures drop in the UK. But it feels even colder in the wind, an effect referred to as "wind chill" or "feels like" temperatures. In other words this is the temperature we feel on our body taking the wind into account.
Will I have to scrape ice off my car?
Ice on your windscreen can be a real pain in the morning, but it’s not just because temperatures dropped overnight. The amount of moisture in the air is crucial in determining whether or not you’ll be scraping your car.
Will it be a White Christmas?
This is probably the most asked snow question! And the odds have shortened on a White Christmas for 2024, after November’s early taste of winter weather. But what counts as a White Christmas, and how common are they in the UK? Here’s what you need to know about festive snow.
Precipitation fact file
- Rainfall: Rain is the liquid form of precipitation – it may have started life as an ice crystal, or a liquid droplet. Raindrops can be up to 6 mm in diameter, but anything less than 0.5 mm in diameter is classed as drizzle.
- Snow: Snow is formed by tiny ice crystals that stick together to become a snowflake. Once heavy enough, and if it doesn’t melt below the cloud, it falls to the ground as snow.
- Sleet: In the UK, sleet mainly refers to a mixture of snow and rain or snow that partially melts as it falls below the cloud. However, in some countries it refers to ice pellets.
- Ice Pellets: Small translucent balls of ice, smaller than hailstones. They form as snowflakes melt into rain and then re-freeze as they fall through colder air. This results in a grainy snow pellet encased in ice.
- Freezing rain: Freezing rain is rain that falls onto a surface with a temperature below freezing. This causes it to freeze on impact with the frozen surfaces, such as trees, cars, and roads, forming a clear coating or glaze of ice.
- Hail: Hailstones are pieces of ice that form in vigorous convective clouds. During winter, hail mainly affects western parts of Britain as incoming showers off the relatively warm Atlantic ocean tend to die out once they move across the colder land.
- Graupel: Graupel is soft snow or soft hail that looks like tiny polystyrene balls. These snow pellets are partially melted and refrozen snowflakes, where supercooled water droplets have added an opaque layer of ice or rime.
- Diamond dust: Tiny ice crystals falling from an apparently cloudless sky (often at night). It usually happens when a temperature inversion is present above the ground – so instead of temperature dropping with height the air aloft is actually warmer than the air at the ground. Warmer air can hold more water vapour so when it mixes with the colder air at the surface it transfers this extra moisture which freezes into ice crystals if the temperature is low enough. If it's not cold enough you would get plain old fog.
