Penellype's Weather Channel

[Penellype]

Penellype here is, what I suspect to be a very daft question.

Can moonlight create a rainbow effect?

In the garden Monday night, moon in the East and I'm sure that the arch below it was a moonbow or I was delusional after watching too much sci fi. It was a perfect arch just like the daytime but without the colours.

Not a daft question at all. Yes moonlight can create a rainbow, it will just be very faint. https://en.wikipedia.org/wiki/Moonbow

A rainbow is seen when light is refracted and reflected by water drops viewed from the correct angle to the light source. The rainbow itself is an optical illusion and always appears opposite the light, at an angle of 42 degrees to the observer. The strength of the colours depends on the number and size of the water droplets.

There is nothing special about rain - it is simply water, but it is the most common way that a rainbow is seen. With the correct light source and angle you can see "rainbows" in fountains, waterfalls and fog. You can create a rainbow effect with moonlight if it is bright enough or artificial light - it doesn't need to be sunlight, so with a correctly placed light you could see one in the shower.

You can even see a rainbow on the top of a cloud (its called a "glory"). They are usually very faint, but can sometimes be seen from an aeroplane, and occasionally if you are standing on a hilltop looking down on a valley full of fog you can see your shadow on the fog with a rainbow halo round your head (called a Brocken Spectre). https://en.wikipedia.org/wiki/Glory_...al_phenomenon)

You say that the arch you saw was below the moon, which makes me wonder whether you saw something else. A moonbow would be opposite to the moon, not below it.

[bikermike]

Penellype (now someone else has mentioned it, I don't feel so bad about asking... )

what do you think about Metcheck?

[Nicos]

Great few links there Pen!
I found the 'without technology' one particularly interesting!

[Lumpy]

Thank you Penellype. It was such a fine arch with no wispy bits I just assumed that it was a rainbow. It must just have been a very thin cloud.

[Cadalot]

Thanks for the explanation Penellype I could actually understand it!

[Penellype]

Penellype (now someone else has mentioned it, I don't feel so bad about asking... )

what do you think about Metcheck?

I have never used Metcheck, so I can't really comment.

[Penellype]

A gorgeous sunny afternoon here today, Candlemas/Groundhog day. Interesting, considering the forecast for freezing cold and snow next week...

"If Candlemas Day be fair and bright, Winter will have another fight, But if it be dark with clouds and rain, Winter is gone and won't come again"

Or, as the Americans say, on groundhog day if Punxsutawny Phil sees his shadow when he comes out of his burrow there will be 6 more weeks of winter.

Looks like one up for the old sayings this year then.

[Penellype]

Models are now unanimous that we are due at least a week of cold weather starting this weekend. The pattern that is developing is an unusual one, with cold air feeding in from the east below a Scandinavian high, but also cold air trying to come in from the north west via a low pressure area over Greenland with a high below it in the atlantic. This means that when fronts come in from the west, because the jet stream is diving south over Spain, even the more westerly air is cold. You can see this cold sandwich effect with deep blue colours both sides of us on Monday's upper air temperature chart (these temperatures are at the same level in the atmosphere as the ones on the ensemble chart below it):





The short term models are seeing the default warmer and more normal westerly pattern returning next weekend (as on the ensemble chart), but that is by no means certain. The longer range models are going all-out for a cold month, with the blocking in the north intensifying and spreading over Greenland too. This would lock us into a cold pattern right through into March.

Longer range models must always be taken with a pinch of salt. However, the GFS model is consistently showing a sudden stratospheric warming for the middle of February (it did this in January but nothing came of it). If this were to happen it could reinforce the northern blocking about 2 weeks later. It therefore would not surprise me to see us having a very cold February and a cold or very cold March.

Much can change, but certainly this winter is evolving very differently from any of the last 4 winters.

[Penellype]

Plenty of warnings of ice and snow tonight, tomorrow and Wednesday as a front moves into the cold air that is now over the country: https://www.metoffice.gov.uk/public/weather/warnings.

Snow is always hard to forecast and some of the precipitation on this front may fall as sleet or rain. There is a complication in that the front is weakening all the time, so as it moves east the rain/sleet/snow may become patchy. However most of the models have it pepping up a bit again as it passes over East Anglia and the south east. Keep an eye on the radar at https://www.netweather.tv/live-weather/radar and turn the weather type on at the top so that the snow areas show in pink to see what is approaching your area.

As I mentioned in my last post, the air following the front is still cold, so we can expect frosty nights and cold days for the rest of the week. There may be a slightly milder night in places on Thursday as a small warm sector of a low passes through but this will quickly be followed by colder air again.

The models have backed off the idea of a prolonged easterly wind for the moment, but the jet stream is still aligned north west - south east, so the air coming in off the atlantic originates near Greenland, and is cold. Things may get less cold after the weekend, but that is far too far ahead for any confidence.

[Thelma Sanders]

The weather forums are buzzing with the idea of a Sudden Stratospheric Warming pulling in cold from the East
https://twitter.com/bbcweather/statu...89736903118848
Very interesting isn't it Penellype?

[1Bee]

I sledged the boys to school on Tuesday. It was utterly perfect.

[Penellype]

The weather forums are buzzing with the idea of a Sudden Stratospheric Warming pulling in cold from the East
https://twitter.com/bbcweather/statu...89736903118848
Very interesting isn't it Penellype?

Yes, it is very interesting. However, a sudden stratospheric warming is not a GUARANTEE of cold weather. As I understand it, the warming in the stratosphere over the pole results in an increased likelihood of high pressure over the pole about 2 weeks later. This is known as northern blocking.

The effect on us of northern blocking depends crucially on exactly where it is. If it is over Greenland this puts low pressure over us, and the jet stream runs to the south, pulling cold air down from the north pole. This would be cold and at times snowy. If the high pressure is over Scandinavia this would pull the wind in from the east, which at this time of year is bitterly cold. This is the sort of thing that leads to a big freeze, with either dry and frosty weather or cold and snowy (especially in the east) depending on how far south and west the high extends.

However, there are other positions for the blocking, and if it sits itself just to our east we can find ourselves on the warm side of the high, with southerly or south westerly winds and a taste of early spring.

I tried to find out which years had sudden stratospheric warmings and what the temperatures were like the following months. I am not sure I have them all, but this is what I found (temperatures are from the record of Central England Temperatures for the given month):

Often quoted SSW December 1984. Jan 0.8C, Feb 2.1, Mar 4.7, all significantly colder than average.
apparently 4 other SSWs in the late 80s, but I can't find dates. Some very cold months, notably Feb 1986 (-1.1)and Jan 1987 (0.8) but as I don't know when the warmings were its hard to say if these were related. 1989, which did have a SSW (but I don't know when) had significantly warmer than average Jan (6.1), Feb (5.9) and Mar (7.5).
Strangely there were no SSWs between 1990 and 1998.
winter 1998-9 "2 SSWs this winter" Jan 1999 5.5C, Feb 5.3, Mar 7.4, all on the warm side
2001-2 "2 SSWs this year" Jan 2002 5.5C, Feb 7.0, Mar 7.6 - all significantly warmer than average.
SSW Jan 2003. Feb 2003 3.9 (around average) Mar 7.5 (warm)
SSW Jan 2006. Feb 2006 3.7 (on the cold side), Mar 4.9 (cold)
SSW Feb 2007. Mar 2007 7.2 (on the warm side), Apr 11.2 (very warm)
SSW Feb 2008. Mar 2008 6.1 (around average), Apr 7.9 (around average)
SSW Jan 2009. Feb 2009 4.1 (average), Mar 7.0 (on the warm side)
SSW Feb 2010. Mar 2010 6.1 (around average), Apr 8.8 (on the warm side)
SSW Jan 2013. Feb 2013 3.2 (cold), Mar 2.7 (extremely cold)
There has not been a designated SSW since 2013.

To be honest, unless I am missing something, I can't see a very strong link with cold a month or 2 later. Everyone jumps on 1984-5 and 2013 and says look what happened next, but what about 2002 and 2007? Most of the years come out around average after a SSW.

I may be missing something. The sample is not very big, and there could be other factors in play making cold more likely (the current La Nina, solar minimum and an easterly QBO spring to mind). For the record, 1984-5 was a La Nina winter, 2013 was ENSO neutral and 2006 was El Nino. Of the warm years 1989 was La Nina and 2002 and 2007 were ENSO neutral. I don't have accurate data to hand on the state of the QBO or the solar cycle for these years.

We shall see what happens, as you say it will be very interesting.

I should perhaps add that an average month can conceal a very warm first half followed by a very cold 2nd half or vice versa. However the nature of northern blocking is that once it forms it is extremely hard to shift, and you would expect the overall pattern to remain fairly fixed for a month or 2 at least.

[Penellype]

A less cold night tonight, but wet and windy in the southern half of the country as a low passes through, followed by a return to colder conditions.

The coming week looks unsettled with low pressure systems moving in from the atlantic, bringing wind and rain or snow at times. Towards the end of the week the pressure seems likely to rise, and this seems to be where the fun starts. Whether this is due to the SSW explained in my previous post or not I don't know, but there is a wide range of possible outcomes by day 10 (20th Feb).

The GFS has us in a bitterly cold easterly wind, with snow showers blowing in off the north sea:



The Canadian GEM model has the high pressure blocking much further east, putting us in a warm, wet and windy south westerly flow:



The ECMWF is more restrained with the blocking, leaving us in similar, if perhaps slightly drier conditions to those we have now:



As I say, it critically depends on the position of any blocking high, and at the moment it seems there is all to play for.

[KevinM67]

C'mon Penellype, give us a break !!!

Yes, I hold you wholly responsible.

[Penellype]

C'mon Penellype, give us a break !!!

Yes, I hold you wholly responsible.

Please don't shoot the messenger!

Still loads of uncertainty about what will happen starting next weekend. I think it is fair to say that the models haven't a clue, although any consensus that there is tends towards the colder side. It is still winter after all!