Monthly Archives: December 2012

2012

December 2012 – Weather Summary

December was a month where the weather pattern swung between clear, bright, sunny spells to overcast, wet and windy conditions, each lasting for several days at a time until the winds arrived at the end of the month.

December started with clear skies, light winds and sunny spells resulting in 5 air frosts in the first 7 days. The clouds gathered from 8th onwards with increasing winds and rain culminating in very dull, overcast, wet and windy conditions on 13th and 14th. Clear skies and sunny periods returned for next 3 days with a further 2 air frosts, only for the very overcast, wet and windy conditions to return for 19th -22nd. During these 4 days, there was no measurable sunshine and 28.6mm of rain fell on the 22nd, both new records for this station. Clearer weather returned from the 23rd with sunshine and showers with the winds gaining strength over the following days culminating in gales on 28th and 29th. On the 28th winds gusted to 80mph (Force 12) with an average wind speed of 48mph (Force 9), the wind run for the day being 661.5 mileThe wind gust and wind run were records for this station. Windy conditions with sunshine and showers then continued until the end of the month.

While temperatures were relatively high during the wet and overcast periods, the clearer spells resulted in lowering the overall temperature,  reducing the monthly (1 minute) average temperature to 5.4°C making December the coldest month on record.

Rainfall was a little above average for the month for the first time this year at 117.8mm which fell on 27 rain days.

The wind speed averaged 11.9mph with a wind run of 8829.9 miles and a high gust of 80mph, the predominant direction being South. The sun shone for a total of 50.3 hours during the month. There were a total of 21 days when it shone for more than 30 minutes , the sunniest being the 4th with 4.3 hours of sunshine.

Decemebr 2012 weather statistics

Temperature (°C):
Mean (1 minute)  5.4                         –    Station recordlowest ever mothly avearage temperature
Mean (min+max)   5.2                        –    Station recordlowest ever monthly average temperature
Mean Minimum     2.9
Mean Maximum     7.6
Minimum          -1.7             10 December
Maximum          11.1           28 December
Highest Minimum  7.2        23 December
Lowest Maximum   3.9        1 December
Air frosts       7

Rainfall (mm):
Total for month  117.8
Wettest day      28.6            22 December     –    Station record
High rain rate   46.2            22 December
Rain days        27
Dry days         4

Wind (mph):
Highest Gust     80.0            28 December    –    Station record
Average Speed    11.9
Wind Run         8829.9 miles
Gale days        2                                              –    Station record – most recorded in any 1 month

Pressure (mb):
Maximum          1032.41     10 December
Minimum            973.05      14 December

Days with snow falling                   0
Days with snow lying at 0900        0

Total hours of sunshine             50.3

November 2012 – Weather Summary

Where October announced the onset of winter, November confirmed it’s arrival with below average temperatures, frosts and the first snow on the high ground.

The month started where October had left off with some sleet and snow showers during the first 4 days and even a thunder storm during the night of the 2nd. The first snows on Ben Mhor were seen on the 3rd. Although the temperatures rose a little after that, they remained below average for this time of year giving a chill edge to the wind at times.

While the rest of the UK experienced heavy rainfall towards the end of the month, here the skies cleared and the winds faded from the 24th onwards resulting in some very bright, sunny and calm days. As a consequence, temperatures were low resulting in frosts at night and when clouds did develop, they lay heavy at times during the final days of the month.

Despite the flooding that occurred elsewhere in the UK, the rainfall for November was again below average, this time by some 21 mm with 108.4 mm falling on 28 rain days.

November was a chill month at times with the average temperature (1 min) only reaching 8.9°C, almost 2°C below the 10 year average. 11.4°C was the highest temperature recorded on the 13th but temperatures fell below zero on 4 occasions, the lowest being -2.1°C on the 27th.

The winds were lighter than previous years, this being due in part to the exceptionally calm spell during the final days of the month which saw most of the clearer skies  and frosts.  The average wind speed was 10.1 mph with a wind run of 7235.5 miles.  The highest gust of 52 mph was recorded on the 20th and the predominant wind direction was SW.

The sun shone for a total of 43.1 hours during the month. There were 23 days when the it shone for more than 30 minutes, the sunniest being the 25th with 3.7 hours of sunshine.

October 2012 – Weather Summary

October brought the month on month record breaking sequence to end, but it did bring the first taste of winter with the first frosts, below average temperatures and the first, albeit brief, sleet/snow.

The northerly airflow experienced this October kept the average temperature  down to 7.8°C, some 3°C below the 10 year average. The clearer skies  experienced with these northerly airflows also gave rise to the first frosts, there being 3 air frosts in total with the lowest  temperature  being -1.0°C on the 26th. The highest temperature recorded during the month was 13.6°C on the 21st.

October did continue the trend of below average rainfall with 64.8mm falling on 23 rain days.  This amount represents  a mere 50% of the expected rainfall for this time of year.

A slow moving high pressure system over the UK brought a rest bite from the wind and we experienced several days of calm, almost windless days between 17th– 22nd. Clearer skies during this period also brought the first ground frost on the 20th.

The predominant wind direction was NW, a substantial factor in the below average temperatures. The average wind speed was 7.2mph with the highest gust being 38mph on the 30th. The wind run was 5381.4 miles.

The sun shone for a total of 83.8 hours during October. There were 24 days when the sun shone for more than 30 minutes, the suniest being the 14th  with a total of 6.7 hours.

September 2012 – Weather Summary

September will be remembered for the wind. Continuing this years trend of records being broken every month, the highest ever daily wind run and highest daily average wind speed was recorded during this month.

Throughout September, the wind was the predominant feature, there being only one day when the highest wind gust did not exceed 20 mph. The record for the highest daily wind run was broken twice during the month with 587.2 miles being recorded on 25th. This day also broke the record for  the highest daily average wind speed which was recorded at 27.9 mph. Despite all this, September’s wind run of 9093.8 miles nor the average windspeed of 12.7 mph did not exceed  that of November 2011 (9383.4 miles & 13.0 mph) and there were no gale days. The predominant wind direction was WSW and the highest wind gust for the month was 54 mph also recorded on 25th.

September also continued the trend of below average rainfall with only 62.8mm falling on a total of 24 rain days. This is some 35.8mm (circa 30%) below the 10 year average.

Average temperatures were slightly below the norm with a mean of 11.6°C. The highest temperature was 16.5°C on the 2nd and the lowest falling to 1.8°C on 20th.

The total hours of sunshine for September was 75.5 hours with the sun shinning for more than 0.5 hours on 22 days.

 

Another Windy Day!

Published September 26, 2012

September is proving to be a month noted for the wind with the highest wind run being recorded for the second time.

With the record being broken on the 13th of this month, stood for just 12 days,  being superceeded today (26th) with a wind run of 587.2 miles.

The average wind speed for the day was 27.9mph with the highest gust being recorded at 54mph

 

The Windiest Day

Published September 14, 2012

The 13th September 2012 was the windiest day to be recorded at this station with a  windrun of 516.6 miles for the 24 hour period ending today (14th September) at 0900GMT, beating the previous record by 0.9 miles.

Despite the winds only peaking at 48mph during the early evening, steady Force 5 to 7 winds during the day which then persisted overnight accounted for the high windrun.

 

August 2012 – Weather Summary

Published September 1, 2012

August was in many ways “back to normal” with a return to a southerly airstream  and its accompanying moisture and seasonal winds. Despite this, August still managed to continue the record breaking sequence, being the month with the highest average temperatures and highest minimum temperature ever recorded at this station.

Average temperatures for the month were slightly higher than the 10 year average (1 min) at 14.5°C which in itself is a record for this station. There was also new records for the highest monthly mean minimum and monthly mean maximum temperatures, these being 11.5°C and 17.6°C respectively. The record for the highest minimum temperature was also broken with a temperature of 15.1°C being recorded on the 12th. There were 5 days when the temperature exceeded 20°C.

With the return of southerly winds, rainfall was higher than previous months with 94.0 mm falling on 19 rain days. Unlike previous months, this is a return to something like the expected monthly average for August, it still falling short of that average by 1.4mm. Although only slight, statistically it still means that for this year, every month has had below average rainfall.

The dominant wind direction was SSW bringing with it more blustery conditions, especially towards the end of the month. The highest gust was recorded at 44 mph on the 25th with a monthly average wind speed of 8.5 mph. The wind run was 6320.1 miles, which exceeded last Augusts total by some 1300 miles

With the installation of our solar radiation censor, we are now able to measure hours of sunshine, this being the first full month. A total of 120.4 hours of sunshine were recorded and there were 25 days when the sun shone for more than 0.5 hours, the maximum being 14.1 hours on the 3rd.

 

Temperature record!

Published August 13, 2012

A new record was achieved overnight – the highest minimum temperature we have recorded since our records began in December 2010.

The temperature did not drop below 15.1′C for our weather day yesterday (the 24 hours from 9am 12th August to 9am 13th August)

Highest minimum temperature record

 

*New Weather Statistics pages*

Published August 9, 2012

One of the endless fascinations of collecting weather data is being able to look back over the records when asking questions like “what was it like this time last year?” and “what was the driest month last year or since we started?”.

We particularly liked the look of the pages by Murry Conarroe of Wildwood Weather who has made available scripts for producing colourful summaries from the data extracted from Cumulus (the weather software that we use) and the Saratoga Weather AJAX/PHP template set that we use for our website.

With just a little adjustment for our own particular set-up the pages were up and running in just a short time. For each of Temperature, Rain, Wind and Wind Run there are three different summaries available – Daily Detail – Monthly Summary – Seasonal Summary. See the pages here: http://www.southuist-weather.info/wxtempdetail.php

As we only started our weather records here in mid-December 2010 i have omitted that part-month from the displays. The summaries are updated automatically on a daily basis just after the end of our weather day which is 9am GMT/UTC

The new pages available

The amazing thing, to me, in this ever more commercial world is that these were all freely available! The templates and scripts from Saratoga Weather are available to download from: http://saratoga-weather.org/wxtemplates/install.php and work with the following weather software: Cumulus, Weather Display, WeatherLink and others. Again the scripts from Murry were provided free, downloadable from:  http://weather.wildwoodnaturist.com/downloadsCVW.php  and the most excellent Cumulus software, constantly being improved by it’s creator Steve in Sanday, Orkney, that our weather station is connected to is available to download for free from: http://sandaysoft.com/downloads 

What else can we say but “thanks guys for your many hours developing these things, it’s much appreciated!”

Measuring Sunshine – Not Such an Easy Task!

Published August 2, 2012

As with all these things, nothing is ever quite so simple as it first appears. When attempting to measure sunshine, like all things you should always ask the question “what are you measuring and how are you measuring it?”

“Current solar radiation” (technically known as “Global Solar Radiation”) is a measure of the intensity of the sun’s radiation reaching a horizontal surface, in this case, our sensor. This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky. The solar radiation reading gives a measure of the amount of solar radiation hitting the sensor at any given time, expressed in Watts / sq.m (W/m²).

That’s the easy bit! Now we have to convert that into sunshine. As in all scientific measurements, there are standards. In this case there are 2, Bright Sunshine and Sunshine,  and both are commonplace.

First, Bright Sunshine as it is the simplest to understand,  the definition being :-

Bright sunshine is considered to be occurring when the solar radiation level exceeds 100 W/m².

From this we can easily work out how many hours of Bright Sunshine occur daily from the readings obtained. The problem is that even on the dullest of days there is still a likelihood that some Bright Sunshinewill be recorded as the threshold of 100 W/m² is quite low, and it stretches the imagination to the limits to consider this to be “sunny”.

Then there is Sunshine and this is not so straight forward. Sunshine is based on a complex algorithm know as the Ryan-Stolzenbach formula. This is a mathematical algorithm that calculates the theoretical maximum solar radiation throughout the day on a day by day basis. Once integrated into the software, it is further modified to calculate the theoretical value for your exact location based upon longitude and latitude.Sunshine will then occur when the solar radiation reading equals or exceeds 75% of that theoretical value.

As ever, the theory is easy to understand but the practicalities of measuring then come into play. Setting the sunshine threshold to 75% – no problem. Next step is to keep the readings from the sensor within the boundaries of the theoretical maximum and near the maximum on bright, clear days.  But differences occur due to atmospheric conditions. In city locations there is less penetration of the suns rays due to pollution etc., while here in the Western Isles there is very little in the way of pollution, so the suns strength, and hence the readings obtained will be different. In software this compensation is achieved by adjusting the “transmission factor” of the atmosphere. Even then, just waiting for that clear, bright sunny day can take a while and the constant anguish over whether the readings are correct leads to the continual “adjustment” of the settings.  All these things takes their toll.

Then there is the wee problem of dawn and dusk. Even when the sun is shining, the radiation levels will be low at these times.  Equally, the algorithm may calculate a value of “zero” while there is still some light which will result in some sunshine being recorded. To compensate for this, there is a minimum threshold which can be set, under which no sunshine will be recorded.  So, if this is set to zero or a very low value, sunshine will be recorded for up to 2 hours a day even though it will be almost dark. Conversely set too high and some sunshine will be lost. Striking a balance is proving a little tricky!

In conclusion Bright Sunshine is the simplest and would give the most consistent results across all stations, although the results would be almost meaningless in terms of what most people would consider to be sunshine. The Sunshine measurement is more scientific and would give better results, but there are several arbitrary settings that could influence consistency between stations. This is probably why the data we provide to other weather agencies such as the Met Office is provided in its raw form i.e.,  Watts/sq m. They can then do their own calculations.

Even so,  it still begs the question “what are they measuring and how are they measuring it?”

As for us, we shall record both, but display only Sunshine. We are still endeavouring to calibrate the algorithm and adjust the thresholds, so this is very much “work in progress”. As to how well we do, you will be able to make up your own minds once we are able to display the graphs showing the theoretical and actual solar radiation measurements in the next release of software.

July 2012 – Weather Summary

Published August 1, 2012

After the recent run of record breaking months, it was back to normality of a kind in July.

Most noticeable once again was the rainfall, 17 rain days producing just 36.8mm with 11.6mm of that falling in the last 12 hours of the month. Without that downpour, July would have been the driest month on record, but was still 22.7mm below the 10 year mean.  This continues the trend of every month this year being below the 10 year average.

The average temperature (1 min mean) for July was 13.0’C, slightly lower than the average of 13.9’C despite there being 3 days over 20’C. This is a rare enough event in itself, the only other days to exceed 20’C  on record being 5 in May 2012.

The dominant wind direction once again had a northerly element, it being  NNW, but this was the same as July 2011. The highest gust was measured at 39mph and the average wind speed was 8.1mph, slightly calmer than last July. The wind run was 6006.2 miles.

As an aside, the Eden Winter Snow Index  (EDWI) for the year 2011/12 ended on 31st June.  The EWSI snow year runs from 1st July to 31st June and measures the amount of snow during that year. The EWSI is calculated by adding together the snow depth in centimetres for all days with snow-lying at 0900 GMT.

The Snow Index for  2011/12 at Askernish was 4.

Weather Station Upgrade – Solar Radiation Sensor Added

Published July 30, 2012

The weather station has now been upgraded to include a solar radiation sensor. This will allow the measurement of, amongst other things, evapotranspiration and hours of sunshine.

At present, the current value of solar radiation is all that can be displayed on the home page as a measurement in Watts/sq meter until our software is upgraded to a build which is currently undergoing  Beta testing but due for release shortly.

Until then, we have developed a  “sunshine” page in the main menu.  This shows in tabular form evapotranspiration for the current day, the hours of sunshine today so far, the total hours of sunshine for the previous day and the current theoretical solar radiation value. This table is updated every 10 minutes.

The dial shows the “real time” value of solar radiation being measured at the station. If you hover the mouse over the dial, it will show a current theoretical solar radiation value, the “real time” solar radiation and a percentage value.

The percentage value is by how much the real time solar radiation exceeds or falls short of the theoretical value. For the “geeks” amongst us, this is important as the calculation as to when the sun is shining or not is deemed to be when the real time value exceeds 75% of the current theoretical value.

For the uninitiated, evapotranspiration  is a term used to describe the sum of evaporation and plant transpiration from the Earth’s land surface to the atmosphere, ie how quickly the land surface is drying. This is used mainly for agricultural and irrigation purposes.

I hope the explanation helps but it will be easier to display when the new software build becomes available.

June 2012 – Weather Summary

Published July 1, 2012

June continued the recent trend of becoming a “record breaker”, with the lowest wind speed and wind run ever recorded at this staion.

The month continued where April  and May left off,  in a northerly airstream which brought brighter, clearer, drier weather if a little cooler than is usual for this time of year. This continued until the 24th when the wind swung southerly  for 6 days and with it came moist air resulting in the majority of this months rain fall along with low cloud and mist. The month ended with the winds returning to the north once again.

Rain fall for the month was 42.8mm which is still 20.9mm below the 10 year mean for June and this years rainfall is now 162.2mm below the average for the year to date.  Temperatures were unspectacular with the average temperature (1 min mean) reaching 11.7’C, comparable with June 2011 but still 0.8’C below the 10 year mean.

The dominant wind direction was NE with an average wind speed of 6.5mph and a wind run of 4660.7 miles, making this month the calmest since records began.

May 2012 – Weather Summary

Published June 1, 2012

Despite the dominant wind direction again being NNE, May will be remembered for it’s sunshine, below average rain fall and a spell of record breaking high temperatures.

The month began with a chill northerly wind but dry and sunny with an overnight low of -0.6’C on the 6th.  Under clear skies, temperatures were slow to rise, that is until the 22nd when they began to increase rapidly resulting in 5 consecutive days over 20’C. The highest temperature reached was 27.2’C on the 25th, a record for this station. As the previous record high had only been 19.8’C set on 30th July 2011, all other high temperature related records were also broken. (see previous posting – Record Highs).  This late spell of warm weather also raised the average temperature (1 min mean) for the month to 10.4’C  which is the 10 year mean for May.

May was also noted for it’s clear skies resulting in copious amounts of sunshine, sometimes 17 hours a day and below average rainfall. The total rainfall for the month was 29.6mm which is 38.9mm below the 10 year average. This means that the total rain fall to date for 2012 is 272.6mm, 141.2mm below the 10 year average for January to May.

Compared to May 2011, this month could not be more different, it having 28 rain days compared with 11 this, which resulted in 147.2mm of rain falling.

The predominant wind direction was again NNE. Winds were a little lighter than last year with the average wind speed being 8.3mph and a total wind run of 6194.5 miles.

March 2012 – Weather Summary

Of all the months, it is March that is showing the most deviation from the 10 year norm (2000 – 2010), a trend continuing on from last year.

Most pronounced was the months rainfall with only 19 rain days producing 32.8mm of rain, 10.4 mm of this falling in 1 day. This is a full 48.4mm below the 10 year average! The month ended with no measurable rain recorded on 11 consecutive days, an all time record in itself at our station and set to continue into April.

The temperature was also higher than the 10 year norm by 1.8′C, the mean temperature (1 minute) being 8.6′C. Although there was 1 air frost, the mean minimum was also 1.2′C higher than last year.

Wind and wind direction was as expected with the monthly average wind speed being 10.2mph, the predominant wind direction being SW. The wind run was 7568.7 miles, slightly up on last year.