Weather: The uncertain science of forecasting snow

By Cecilia Daly
BBC News NI Weather Presenter

  • Published
snow covering a road
Image caption,
Why is snow so difficult to forecast?

I've been doing this job for more years than I care to admit.

But, when it comes to forecasting the possibility of snow, I still get a chill.

And here is why.

How cold does it need to be?

You may have heard of the phrase "six degrees of separation" - the idea that all people are six, or fewer, social connections away from everyone else.

But the relationship between rain and snow is much closer than just one or two degrees.

The perfect air temperature for snow to fall is 0C or below.

For snow to lie, the ground temperature also needs to be 0C or below.

Different surfaces cool at different rates. Gardens will tend to be colder than pavements and pavements are colder than main roads.

Image caption,
It looked picture-perfect on the Glenshane Pass early on Monday, but motorists may have been less than impressed

Can it be too cold to snow?

Yes. The perfect temperature for snow is about freezing or below, but there needs to be some moisture in the air.

Warm air can hold more moisture than cold air.

So if the air is very cold, there will not be enough moisture to support clouds, never mind snow.

If you live in a very cold climate, it would actually need to "warm up" for it to snow.

What other factors come into play?

You also have to consider location, height, as well as both intensity and persistence of precipitation (a long word that encompasses anything that comes out of clouds).

As we live on an island, location will have a direct influence on the air temperature.

In winter, coastal sites tend to be milder and hill sites colder.

Rural and town sites also have a temperature difference of a few degrees at any one time.

Newry will be warmer than Katesbridge despite only being about 15 miles away, for example, and the same with Omagh in comparison to Castlederg.

The influence of the sea and salt in the air can make the difference of a degree or two.

This is why it can rain in Bangor on the coast but, in Lisburn, it can be snowing.

Image caption,
Sheep in the hills get a raw deal

You might ask why it can be raining in the city centre but snowing in north Belfast.

The air temperature tends to drop with height, on average by 3C per 1,000ft (300m).

If the temperature in the city is 3C, it is likely to be about 0C on the top of Divis Mountain.

If it is 1C or 2C in the city, it will definitely mean conditions are right for snow on the high ground surrounding the Belfast bowl, for example in Hannahstown.

If the snow/rain starts quite light but then becomes heavier, this, too, will have an impact.

The layer of air through which the rain is falling will cool and, if temperatures are at that crucial range between 1C and 3C, this can lead to the snow level dropping from 1,000ft at the top of the hill to 500ft halfway down.

Sheep might live at 1,000ft but lots of people in Northern Ireland live at about 500ft.

And then there is the question of timing...

If we have a spell of precipitation of about three to four hours, this will also cool the air and encourage the rain to turn to snow when at the critical temperature range. Again, the snow level will drop.

The persistence of the rain/snow will also increase the chance of it lying. Puddles of rain and wet roads/pavements will equate to a covering of snow.

This leads to impact. Snow causes much more disruption than rain.

If we had 5mm of rain overnight, you would wake up to wet roads, wet pavements and a few big puddles.

But if temperatures drop to 0C or below, 5mm of rain will end up equating to 5cm of snow.

Waking up in Belfast on a Monday morning to 5cm of snow is a completely different scenario to getting splashed by a car driving through a puddle.

So what does it all mean?

Going back to the impact, the various bodies responsible for keeping our roads safe do not have an easy job.

In Canada, temperatures drop so low that there is no argument over whether it is going to snow, given that the moisture is present.

But in Northern Ireland, we don't often get temperatures as low as that and so the rain/snow mix makes it very difficult to salt roads.

The grit can be diluted or washed away, making it ineffective, and our road network is such that once there is traffic on the roads, it is almost too late to do anything due to restricted access.

It is far from straightforward. Forecasting temperature to one degree of accuracy for different locations is at least a few years away.