Frosty morning stats

cheezums

My memory of school 20+ years ago is that "most" mornings from December to February were frosty. I'm wondering if my memory is accurate or is this just selective.

Is there any historical weather data around for "frosty mornings" per year over time? Might need a bit of creative data analysis.

10-10-20

I had the same thought 3 days ago. I lived 4km closer to the sea than I do now, I'm higher up now though and I rarely see frost.

My perception of the 80's was that once or twice a week there was a frost which had to be scraped off the Mini and our hands would be red-cold when we got into school, so much so that we had to jump around and shake ourselves to warm up.

I think that the question was asked here about 3 years ago and somebody was able to find an answer. How definate that was - I'm unsure of. :)

Oneiric 3

Incomplete list and ongoing project, but the average date on which some of our inland Met Eireann stations will see their first (autumn) frost. Data based on the relatively small sample size of 5 years (2016-2020), but representative of recent/current trend.

I shall update this table to include some of the more coastal stations in due course.

Data from Met Eireann.

pauldry

So far 3.1c is the lowest in Sligo on 15/10 and 2.9c in Finner also on this date.

10-10-20

https://www.boards.ie/discussion/comment/118145590#Comment_118145590

I'd love to see 1980 to 1985. It might be a symptom of the Reynauds, but I swear I couldn't feel my hands on most days until small break even thought the radiators were on.

Oneiric 3

https://www.boards.ie/discussion/comment/118149516#Comment_118149516

Whereabouts were you roughly based 10-10-20?

10-10-20

2km either side of Stillorgan, effectively.

Danno

normals_supplement_1_ANNUAL_2010.qxd (met.ie)

Has data for frosts/ground frosts in Casement and Valentia.

I remember a yellow colour covered book produced in the 80s (I'm guessing) that had great detail on the Irish climate and was full of maps, tables and statistics. There was alot of 1951->1980 data in it. I remember seeing two maps relating to frost, one showing average first date of frost and the other showing average last date. In my area, southern Laois the frost season was basically 23rd October to May 23rd. My memory of the last frost might be a wee bit out though.

The first frost map showed parts of the southwest coast not getting their's until January 10th. I must hunt down that book again!

Supercell

Heres a few pages from my treasured (circa 1983) "Climate Of Dublin" book, honestly i'd imagine the frequencies are less these days.

First frost date:

Amount of air frosts:

Amount of grass frosts:

Tae laidir

Thanks for sharing.

Treasured indeed.

An excellent baseline for comparing current values.

Billcarson

Remember listening to a climatoligist on the radio around 85/86 ,predicting we would see less frosts ( cold spells and snow) during the following decades.

pauldry

Thanks Supercell wish there was more information available like this online.

Oneiric 3

Added more stations to the table I did above. (Average first Autumn frost day over the previous 5 years)

As far as I can tell, here has been no air frost recorded at any of the stations so far this Autumn, and here we now are in mid-November. Horrible, horrible year with the pattern that set up in mid-July still refusing to give way.

@10-10-20, I looked at Casement (nearest met station to you) data and the mid, more so than the early 80s, were notably frosty in that part of the world:

All data above from Met Eireann and thanks to Danno for sharing his.

Edit, updated table further to include Dunsany, Markee Castle & Mt. Dillon. Mace Hd, Malin Hd and Roches Pt unworthy of entering as they frequently recorded no air frosts at all during the Autumn & Winter.

Further Edit: Contrary to my earlier statement, an air frost was recorded on the 2nd of this month at both Mt. Dillon and Athenry.

10-10-20

Very interesting Oneiric and all the other posters.

So the my perception is correct - the trend-line demonstrates it well - the occurrence of frosty nights (where the air-temperature is below 0.0?) has lessened from an average of ~44 nights per year to ~35 today. The severity of these frosts has probably also lessened, so even if we have a "frosty night" recorded in the stats, the resulting frost (if any) may not have become a ground-frost or could have melted or been imperceivable as a frost by 8am.

Indeed, the trend-line is pulled up heavily by the two years of 2009 and 2010, implying that they are outliers in the data. If these were normalised somewhat, the trend for 2020 would be around the 30 night frost mark, I guess.

Oneiric 3

Another thing to keep in mind 10/20, is that frosty nights and mornings may not indicate air temps below freezing. I've seen some very white frosty nights and mornings where air temps did not fall below +1.0c. Clearly, ground temps were far lower during these events than those of the air.

Oneiric 3

https://www.boards.ie/discussion/comment/118159933#Comment_118159933

In this graph, I used a 10 year running mean air frost count for Casement Aerodrome to help smooth out those outliers you spoke of. What is clear is not only is a sort of cyclical trend, but also the long-term downward one; and just based off these trends alone, it would appear that we are now entering into another prolonged period of seeing less air frosts than normal.

10-10-20

The NAO was quite negative in the period of Apr '08 to Aug '11.

https://www.ncdc.noaa.gov/teleconnections/nao/

I don't fully understand the NAO index, but I presume it's a critical factor in the cyclical nature of the data-set?

pauldry

Very good info Oneric. Interesting how late Cork gets a frost. Markree had a 0.2c this Autumn but Sligo was 3c that day in mid October. In Sligo we had 4c another morning. Most nights 8 or 9c this season. 13c this evening but that's due to drop to 10c.

Looking at fantasy charts there are a few slack areas of High Pressure in the offing so there could be some frosts on the clear nights.

sryanbruen

https://www.boards.ie/discussion/comment/118161772#Comment_118161772

The NAO is an index that is reflecting the strength of the Icelandic Low/Azores High which are two of the driving forces in our day-to-day weather.

A positive NAO index reflects a strong Icelandic Low/Azores High and thus, there is an intensification in westerlies across Europe usually associated with mild weather for us as air comes directly from the Atlantic/tropical Atlantic rather than the continent. Sometimes, the Azores High can be strong but displaced into the Atlantic rather than ridging to our southeast which can bring cold zonality or winds from a northwesterly direction with air being sourced from Greenland or the North Atlantic so this can bring cooler conditions despite being a positive NAO. However, often times with such setups, winds are too strong to bring much in the way of air frost. Another way that cold weather or cooler than average conditions can come from positive NAO including the risk of frost is via inversions from winter time anticyclones. Inversions are where temperature increase with height - normally it is the opposite. These happen under anticyclones or areas of high pressure in winter time with light winds and fog forms on the surface. General light winds and clear skies can cause frost. Inversions have become far less common over time but they were not uncommon back when smog was a big issue in places like Dublin.

A negative NAO index reflects either a weak Icelandic Low/Azores High combination or on occasion, a complete reversal with high pressure around Iceland and low pressure around the Azores. The likelihood of northerly or easterly winds from such a pattern is increased as the North Atlantic gradient between the pole and tropics is reduced. Northerly and easterly winds are what deliver cold air to Ireland with northerlies bringing arctic maritime air and easterlies bringing polar continental air. As a result, frost risk is highest when the NAO is negative but as mentioned with the case of inversions and high pressure, even a positive NAO can bring frost on occasion though this depends on clear skies. Sometimes, the negative NAO may also be western based where low pressure from the south undercuts the blocking over Greenland as the anticyclone is too far away from us to influence our weather and is over towards North America. We tend to be milder and wetter in this regime, in spite of the negative NAO.

Excuse my very amateur drawings to illustrate everything I've mentioned above. Hope this clears things up.

Yes it was a massive player in how 2010 was so frosty. That year seen a sheer persistence of Greenland blocking and a lack of westerly winds with notably cold winters at both ends of the year. There have been a lack of negative NAO winters in modern times, 2010 was an exception (majorly so). It is of no doubt that this has played a huge part in the reduction of frosts.

Oneiric 3

Can't edit post above to include Newport on the table but its first post summer frost will, on average, occur on Dec 23rd, with the median date being Dec 15th.