Northerly wind to bring us a chill this week
So far this autumn, the weather has brought many of us sunshine and pleasant warmth for the time of year.
However, in the next few days, this is set to change as a northerly wind brings colder air from midweek onwards.
The reason for this is the position of the jet stream – the fast-moving ribbon of air five miles above our heads that determines the weather that we experience at the surface.
During the past week, it has been weak and split in two to the west of the UK, with one branch running north towards Iceland and the other running south towards the Azores.
As a result of this, the weather set up has been quite static, with low pressure to the south west of the UK and high pressure to the north east. This has given us a mild south to south easterly wind – bathing us in warm air.
From midweek onwards, this is going to change, with the jet stream picking up speed and diving southwards over us, bringing a cold northerly wind and the chilliest air we’ve seen so far this autumn.
So, following highs of 16-21C on Monday, Thursday will see temperatures in the range of 7-13C, which below average for this time of year.
The nights are going to be cold as well, with a touch of frost likely in more rural areas, where temperatures will dip to 0-3C. Towns and cities will feel the morning chill as well, with lows of 3-6C.
Colder in the wind
As well as having to contend with low temperatures, a brisk northerly wind will make it feel even colder – especially for eastern parts of the UK.
The wind will also push bands of showers into northern and eastern areas, which are likely to be wintry over the hills and mountains of Scotland as the freezing-level lowers to around 800 metres for a time.
Winter already brewing over Siberia
Whilst looking at current snow cover around the world, I noticed something interesting and fairly significant across Siberia.
The current extent of Arctic sea-ice and snow cover over Siberia (top image below) are both substantially greater than at the same time last year (bottom image below).
What significance does this have on the forthcoming winter? Well, it is too early to attempt any detail, but it is likely to influence the intensity and extent of cold Siberia air.
High albedo and cooling effect
Arctic sea-ice and fresh snow cover have a high albedo which is very effective at reflecting incoming solar energy from the sun back into space, giving a net cooling effect.
Also, because the nights are now getting longer in the northern hemisphere, there are more hours of the day when energy is escaping the earth’s atmosphere than coming into it.
This process of energy escaping back into space is much more effective over ground covered in snow and ice, thus increasing the amount of cooling taking place.
So, potentially, the seeds are being sown for some very cold air to form over quite a large part of Siberia quite early on in the season compared to last year.
Whilst there are no signs of this potentially very cold air arriving on our shores soon, it will be one to watch as winter draws ever closer.
Snow and ice cover images courtesy of NOAA