Haar Haar

Figure 1

Apologies for the title, I just couldn’t resist it, because its such an awful dreich day down the east coast this morning with temperatures close to 5°C and extensive low stratus and haar (fig 1). The air temperatures correspond closely to the currently observed sea surface temperatures I mentioned in the article about negative North Sea SST anomalies earlier this morning. It’s mixed fortunes weather wise, because meanwhile on the other side of the North Sea in Denmark its blues skies all the way although it’s still quite fresh for early April. Here’s a chart of the relative humidities at 08 UTC this morning (fig 2) to see just how soggy we are on windward side of the North Sea.

Figure 2

Warm and sticky…

Figure 1

A very warm and sticky day across much of the country today, except for parts of northern Scotland where dewpoints were much lower, with relative humidities as low as 35% to the lee of the mountains.

Figure 2

The next five days look cyclonic, cool, breezy and rather wet at times almost everywhere, in the latest run of the GFS model, before things start to warm up from the west from T+120 (fig 3).

Figure 3 – Courtesy of OGIMET

Cooler but not fresher

It maybe cooler than it has been, but it’s certainly not fresher, in fact it feels very humid this afternoon, with extensive stratus affecting all windward coasts exposed to the westerly flow at the moment. It’s hard to believe that we are now in the cooler air and behind the weak cold fronts that have just cleared the country, but then again dewpoints are still 15 or 16°C in many places. Here’s the 14 UTC charts of relative humidity across southern areas (fig 1).

Figure 1

It’s certainly a bit fresher further north, and low Quirin has brought winds that are very close to gale force this afternoon across the north of Scotland. At height, the Cairngorms have seen storm force winds, with gusts to 79 mph and temperatures only around 2°C (fig 2).

Figure 2

Cooler for some, warmer for others

Figure 1

It maybe a lot cooler for some first thing this morning in comparison with yesterday morning, more notably in the west and southwest of the country, but for other places, particularly in the east it’s much warmer than it was this time yesterday. The chart above (fig 1) is the 24 hour temperature difference, the chart below (fig 2) is the 24 hour differences in humidity.

It’s a lot more difficult to see whats going on with these humidity changes, the North Sea, Denmark, and much of Scandinavia are a lot more humid than yesterday, but lower humidities are evident in the southwest of Ireland, the far southwest and central parts of England.

Figure 2

The use of ‘fresher’ is obviously subjective, and is often bandied about by weather presenters without too much thought, other than to overly dramatise changes. The one big thing that I know is missing this morning in our part of Devon, is the sunshine of the last week (fig 3), it’s rather cloudy, temperature wise it’s certainly cooler than yesterday, but still fairly warm with temperatures around 18°C.

Figure 3

Low humidities over Scotland

One of my Scottish correspondents has noticed some quite low humidities across the country this afternoon. Humidities at Strathallen, just to the south of Perth, have been falling all day, and at 15 UTC the humidity was down to 24.7%, with a temperature of 17.4°C and dewpoint of -2.9°C (fig 1). The surface wind has not really backed from the E or NE’ly it’s been all day, so I can’t put it down directly to descending air from the mountains to the north.

Figure 1

The lowest humidities that I can see from the available SYNOP reports are over the mountains, and at Aonach Mor the humidity fell to just 11.8% at 09 UTC this morning (fig 2), with a temperature of 4.4°C and a dewpoint of -22.8°C (fig 3).

Figure 2
Figure 3

6 April 2017 – Low cloud and the BBC forecast

Figure 1 – Courtesy of the BBC

It was plausible that the SC that was sat over the Irish Sea late yesterday would migrate SE overnight in the Northwesterly flow to cover most of the southwest of England but not to the extent shown on the 09 BST forecast frame (fig 1), compare it with reality in the shape of the 1015 BST visible satellite image (fig 2), and you can see just how the Met Office model has overdone the amount of low cloud this morning. The Met Office NWP model does very well with fast-moving storms such as Doris, but it seems to have big problems in forecasting amounts of low cloud in anticyclonic situations such as this.

Figure 2 – Courtesy of the Met Office

One other criticism that I have of the weather forecast on Spotlight Southwest and that’s the humidity displayed in a caption at the top of the forecast graphics (see fig 1). It’s ridiculous that any single value can have any significance for anywhere in the region, and gives the wrong impression about humidity, relative humidity varies immensely. Why not just replace the humidity caption by occasionally displaying a colour contoured relative humidity chart? This would at least show the viewer how much humidity varies across the region (fig 3) and be a lot more educational and scientific in the process. In passing, the 64% forecast for 09 BST might have been just a little over optimistic!

Figure 3

The 1⋅6% humidity at Altnaharra…

2016-12-03_154343
Figure 1 – Courtesy of the BBC

I came across this news story whilst reading a book called “Since records began” by Paul Simons. On researching the event on Google I found this news article about it on the BBC website (fig 1). There’s no doubt that the 18th of February 2003 was a mild day – the maximum reaching 11.5°C at Altnaharra making it was the warmest place in the UK. This was in no doubt due to a foehn effect in the strong southerly flow over the mountains to the south.

But it wasn’t the temperature that was the remarkable thing as Paul Simon’s mistakenly suggests in his book, but the relative humidity. In the 0952 UTC SYNOP observation from the automatic weather station [AWS] the dew point was an astonishing -39.9°C with a relative humidity of just 1.6%. The Met Office doubt that this was correct  and say that the relative humidity was closer to 8%. I’m not sure how they can be so certain about this, it maybe because they have access to one minute data from the AWS, but even so, a humidity as low as 8% must still rank as one of the lowest reported in the UK at a low-level station.

I love how Andy Yeatman sidesteps any embarrassment over this apparent mistake, I think in the news item almost trying to suggest that the wet bulb reservoir may have been frozen and the wet bulb had dried out, but if that was the case then the wet bulb temperature and hence the dewpoint, would have been much higher and not lower. Here’s the thermograph for that period (fig 2).

thermograph-for-altnaharra-1146-16-02-2003-1800-19-02-2003
Figure 2

And here are the hourly observations for Altnaharra for that day (fig 3). The winds do look a little strange and were flitting around a bit around 08 UTC.

2016-12-06_155947
Figure 3

The 12 UTC chart (fig 4) is dotted with low dew point observations across Scotland that day, notice the -11°C at Cairnwell and the -13°C at Kinloss.

synops-1200-utc-on-tue-18-feb-2003
Figure 4

Foehn in the Po valley

The cold air that’s been pushing south over Europe in recent days has also brought with it some very low dew points and relative humidities [RH] to the Po valley in Northern Italy this afternoon. I have seen a RH of just 3% which looks spurious, but there are three stations with a RH of just 10%. This is all courtesy of the foehn wind of course that’s blowing down from the Alps, temperatures on the other side of the mountains have remained well below freezing all day in Germany and Switzerland.

For instance it’s been a wonderful day to get the washing dry at Brescia in Lombardy, the temperature there at 15 UTC was a very reasonable 7.5°C, the dew point -22.2°C and the RH just 10%.

BBC weather forecast and humidities

IMG_2757
Spotlight Evening Weather – 21 June 2016

There’s a new feature that’s been added to the forecast graphics by the Spotlight weather team in the southwest recently. They now occasionally add a label at the top of one of the forecast frames that displays the humidity, the image above is from last night’s broadcast. I think they introduced the new feature after receiving suggestions from viewers that humidity forecasts might be of interest. well here’s a suggestion from another viewer – forget it.

When I was an outstation assistant, which for me invariably meant at one of the (then) many RAF stations up and down the country, we always had an outside line that the public could phone for forecasts and advice. For some bizarre reason I still remember the clipboard and the form that we had to place a tick in to record the enquiry, but I digress. One of the most commonest questions we received from the public was “I’ve just got this new barometer and I would like to set it up but don’t know what the pressure is”. Of course, you asked where they lived, and then asked the forecaster for a sea level pressure for that location, the forecaster would then proceed to draw a few isobars on the latest British Isles fax chart from Bracknell and hey presto one happy customer. The point of my inane rambling is: humidity just like pressure is very dependent on where you happen to be.

Take a look at this mornings 03 UTC actual chart that matches Bee Tucker’s forecast chart. Ignoring the fact that the forecast humidity of 65% is hopelessly wrong, it would be unusual that a single humidity value would be totally representative of the humidity across the whole of the southwest of England, except of course if the entire region was covered in fog, low stratus and rain, then the humidity would be close to 100%, which occasionally does happen in this part of the world, but of what use a forecast of 100% is debatable.

Relative Humidity for Wed, 22 Jun 2016 at 0300 UTC
Relative Humidity for Wed, 22 Jun 2016 at 03hr UTC

The Spotlight weather team also does a very similar thing with pressure, and to be fair if the gradient is slack, they can get away with this for setting up a hall barometer, but not humidity.  I’ve a feeling that last night they forgot to edit the humidity label in the program that they use to generate the graphics, if they had perhaps used 95% they would have been very close! Below is a chart of low humidities from a day in April of this year which underlines the impossibility of using a single value especially during the day – Plymouth 24%, Exeter 29%, Truro 65% and Hugh Town 73%. The only way that I could see this working was if they produced a table of specific locations such as Plymouth, Exeter, Truro with the forecast humidity for each place, but how viewers would find that interesting and of use is another question.

Relative Humidity for Wed, 20 Apr 2016 at 1500 UTC
Relative Humidity for Wed, 20 Apr 2016 at 15hr UTC

 

Muggy air

Relative Humidity [%] for Mon, 6 Jun 2016 at 1500 UTC
Relative Humidity [%] for Mon, 6 Jun 2016 at 1500 UTC (excuse the contouring)

Inland by day I wouldn’t say that it’s been that muggy today, in fact if this was back in the old days of manual weather observations (SYNOPs), then at many sites across the UK observers would have had to add the Beaufort letter ‘y’ to their observations to signify dry air (<60% relative humidity) this afternoon. Dewpoints were certainly higher in the west especially over Ireland (16-17°C) but for the bulk of the country 10 to 13°C more or less covered it.

Relative Humidity [%] for Mon, 6 Jun 2016 at 1500 UTC
Relative Humidity [%] for Mon, 6 Jun 2016 at 1500 UTC

Of course overnight things do get a little bit sticky as the humidity rises. Here are the humidities in the early hours of this morning across the country.

Relative Humidity [%] for Mon, 6 Jun 2016 at 0300 UTC
Relative Humidity [%] for Mon, 6 Jun 2016 at 0300 UTC (excuse the contouring)
I think the meteogram for Exeter during the past week might be fairly typical of overnight humidity levels across a lot of the country. There used to be something called a comfort index, which might be a useful way of quantifying how difficult it is to sleep – without air conditioning of course.

03839 Exeter Airport - United Kingdom  31 AMSL 30 May-5 June 2016
03839 Exeter Airport – United Kingdom 31 AMSL 30 May-5 June 2016

If you think it’s muggy here then spare a thought for areas around the Gulf of Mexico as Tropical Storm Colin tracks towards Florida, dew points in coastal regions there are in the range 20 to 27.8°C (Nassau).

Dew Point [°C] for Mon, 6 Jun 2016 at 1200 UTC
Dew Point [°C] for Mon, 6 Jun 2016 at 1200 UTC