Over recent weeks, we have spoken about the very strong jet stream across the Atlantic, driving areas of intense low pressure towards the UK. This has bought spells of very wet, windy but relatively mild conditions to the country.
As many of you would have noticed, although the wind and heavy rain has eased, there is now a colder feel to the weather, both by day and night. But what has caused this change in the weather?
Once again the change is down to the jet stream. It has weakened and its track has moved further south, keeping the deep low pressure systems away from our shores. However, now the UK is to the north of the jet stream we are on its cold side, and this has allowed colder weather to feed in across the country.
So what does this mean for us?
As we look ahead into the weekend and next week, the cold weather looks likely to continue. Daytime temperatures will be near or below average and there will be some frosty nights, as temperatures fall below freezing in many areas. We’ll see some sunny spells around and there will also be showers or longer spells of precipitation in places, giving a mixture of rain, hail, sleet or snow, which may settle in some areas.
Because of the threat of wintry weather over the coming days, we encourage everyone to keep up to date with the latest forecasts and national severe weather warnings and to stay weather aware this winter by following the Met Office on Twitter, Facebook, Google+ and YouTube for the latest weather information. You can find information about how to prepare for every aspect of the winter season at Get Ready for Winter.
As we head towards the latter part of the month, we can see some indications that milder weather may return, but there is considerable uncertainty about this so far in advance.
Sorry Met’ but you are doing it again. That is speaking of the jet stream as if it were the ‘be all and end all’.
The jet stream is a ‘product’ of the thermal gradient at the Polar/Ferrel cell boundary.
All winds are the result of pressure differences and the associated pressure gradient force.
On the surface of the Earth the surface pressure differences are produced initially by uneven temperatures driven by uneven solar insolation due to incidence and absorption across the globe. Tele-connected pressure differences across thousands of miles determine the motion of air masses and relative motion of the resultant air mass boundaries.
The position and zonal/meridional nature of the Northern Hemisphere jet stream is retrievable through the Arctic Oscillation (AO) pressure anomaly.
A highly positive AO pressure anomaly results in high pressure in the mid latitudes containing the cold air returning within the Polar convective cell. This results in a strongly zonal circumpolar vortex (jet stream) and cold air being confined to the Arctic. A high positive AO anomaly accompanies a stable polar vortex and circumpolar vortex.
Conversely a strongly negative AO results in higher pressure in the Arctic which allows cold air to stream out of the Arctic over the land masses in the Northern Hemisphere. This breaks up the ‘normal’ mid latitude westerlies and brings frigid air into Europe.
A strongly negative AO follows on for weeks after a major stratospheric warming event which disrupts the otherwise stable winter polar vortex.
The position then of the Polar/Ferrel cell boundary and resultant jet stream is one of snaking up and down the hemisphere in a highly meridional pattern.
Records of the Rhine freezing in Europe show that all 14 documented events were at times of low solar activity.
Can I remind you that 99.99% of energy on Earth is sunlight. The solar variation, documented at 0.1% renders this variation as 10 times larger than the 0.01% of energy that is not sunlight. Solar energy drives all the weather patterns and the hydrological cycle.
Solar activity and the portion directly absorbed in the upper atmosphere is in decline as a result of periodic fluctuations involving sun/solar system momentum exchange. The weather patterns prevalent in the previous century due to relatively strong solar activity are not those we see today, nor are they those we will witness in the coming decades.
Reblogged this on HUMAN RIGHTS & THE SIEGE OF BRITAIN POLITICAL JOURNAL.