Wintry weather this week - what can we expect?

Taff Trail Snow Credit: Robert Griffiths

After the disappointment of not having a White Christmas, many of us are hoping for a dusting of the white stuff! Some parts of Wales have been lucky and got just that - but most of Wales has missed out on proper snowfall - so far! Statistically speaking, you are more likely to be building snowmen at this time of year than December, so don't panic, there is still plenty of time! In fact, by the weekend we could really be in line for some snow, but I'll keep you posted on that nearer the time!

What we do have to contend with almost everywhere are some of the more unpleasant aspects of winter. Ice and frost we are all used to, but as temperatures plummet many areas are also likely to see freezing fog patches too. Have a look at the forecast below for more details!

Be warned! If you've just listened to my forecast you'll know that temperatures tonight could drop to minus 7 in more sheltered areas. In anyone's book that it pretty cold. But aside from the fact that you are going to need several extra layers in bed tonight, it also brings the possibility of freezing fog.

The question is - what is freezing fog?

Freezing Fog Credit: The Met Office

Well here comes the science bit:

Freezing fog forms in the same way as ‘normal’ fog and is typical at this time of the year. It tends to form under clear skies and calm conditions. The cooling of land overnight under clear skies means that any heat radiates back into space cooling the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation of water vapour into millions of tiny water droplets to occur and fog in to form.

When temperatures are well below freezing, (as they will be tonight by the way!) the tiny water droplets suspended in the air are made up of supercooled water droplets – which remain liquid even though the temperature is below freezing. This occurs because the liquid needs a surface to freeze upon, such as a dust or pollution particles. However if there are not enough of these particles about then the water can stay as a liquid.

However droplets from freezing fog can freeze to surfaces on contact, forming rime. Often confused with frost, rime is a rough white deposit formed of feathery ice crystals. It can often be seen on vertical surfaces exposed to the wind – like lamp posts and pylons – as the supercooled water droplets freeze on contact as they drift past.

Freezing fog can make for a pretty picture, but also fairly treacherous conditions.

You have been warned!

Snowman Credit: Robert Griffiths

And don't forget the possibility of "proper" snow for some parts of Wales by the end of the week! Keep an eye out for more details!