*** The official frost hollow thread ***

For Italy the first area I can think of is the high plain in north eastern Italy, in the altopiano di Asiago or the near area of Marcesina just NE of it. 

That area is generally relatively cold because of the altitude and the open plain favour the night cooling by radiative processes. For example in the area of Asiago (about 1000 m of altitude) lows of about -20°C are not so exceptional during winter cold waves. 
The coldest place of the area seems to be the dolina di Campoluzzo, hollow at altitude of about 1768 m, where the coldest recorded temperatures are close to -40°C. Anyway, consider that the area is a karst high plain, so hollows are spread all over the area. 

Obviously, looking for a frost hollow at higher altitude the temperatures can be even lower. In this case the frost hollow is probably Busa Fradusta, a frost hollow in the area of the Pale di San Martino (Dolomites) at a relatively modest altitude of 2607 m, where the temperature seems to reached the temperature of -49.6°C in February 2013. 

With these data I was intentionally vague because there aren't official weather station in those places (apart some regional weather stations maybe), and they are not necessary located in correspondence with the coldest spots.


As a comparison, the record low measured on the weather station of the Monte Rosa (at about 3500 m of altitude, it's the highest official weather station we have I think) is "just" -34.6°C (period 1951-2016). 

The one named by Rozenn at the beginning of the thread is not so far from here, but surely it was at a quite high elevation already.

As for my region, there are a couple of places in the Po plain or in the low altitude areas between the hills in Langhe or Monferrato which could actually be described as frost hollow. It's hard to tell which is the coldest low-altitude place of the region because those spots are quite scattered all over the region, and also because in this moment the website of the regional weather station network is not working.

Taking an old article about the February 2012 cold wave [1], which was actually a quite tough one for here, the coldest temperatures recorded in the plain were:
- Turin center, altitude of about 270 m (reference): -11°C
-Villanova Solaro, altitude of about 270 m: -23.8°C
- Castell'Alfero (altitude 235 m, in the middle of hills) and Nizza Monferrato (altitude 138 m, main center of the Monferrato hills): -21.7°C
- Sezzadio (altitude 127 m): -21.5°C
- Carmagnola (altitude 240 m): -21.4°C

at least a couple of them are probably good candidates as frost hollows. 

Average lows for 'Bournemouth', recorded at Hurn Airport. They are extremely misleading; nearby Swanage has annual lows ranging from 4-14c.

Jan: 1.5c
Feb: 1.2c
Mar: 2.7c
Apr: 3.8c
May: 7.2c
Jun: 9.8c
Jul: 11.9c
Aug: 11.6c
Sep: 9.4c
Oct: 7.1c
Nov: 3.7c
Dec: 1.6c

The northwestern towns in the county close to I 69 are often the coldest. They see a week every year with lows at or below -20 C Rest of the area may never see those low temperatures, and in the lake we rarely drop below -15 C. I happened to be in one of those towns during the 2017 polar vortex and the windchill was -36 C and it made my head spin within minutes.

Aussies are often astounded by the extreme cold nights up on the New England Ranges in Northern NSW.
Glen Innes, at 30*S has 3 separate recording sites, in town at the Post Office, at the Airport, and at the Agricultural Research Station.
All time winter lows are....
At the Post Office: June -8.3c, July -9.4c, Aug -8.4c
At the Ag Research site: June -8.3c, July -7.5c, Aug -7.5c
At the Airport site: June -11.6c, July -12.8c, Aug -12.8c
The Airport is a notorious frost hollow....altitude 1044m. Ag Station 1060m, Post Office 1062m.

Here is my favorite frost hollow in the US: Valles Caldera in New Mexico at 8,470 ft (2,582 m). At the bottom of a treeless volcanic caldera, it has a subarctic climate and averages 40 F diurnal ranges year-round.

Street view:



I've posted climate data for this place before, but here is a more detailed climate box I just made.



This area is capable of some very impressive diurnal ranges on specific days.

December 26, 2011 with -27 F/46 F (-33 C/8 C)



June 21, 2013 with 12 F/80 F (-11 C/27 C)



Here are some shots of the station:

This featureless winter is good for frost hollows as there are lots of radiative nights. Alas, they're often accompanied with mild 850 hPa temps, so no extremely cold lows have been recorded.


Here's an interesting report from theodu12 on Infoclimat. That guy seems to search for the best spots to set up a weather station in the karstic region he lives in between Montpellier and Clermont-Ferrand. Makes for fascinating reads. For example this one on the causse Méjean:







6°C and breezy on a nearby hilltop:





-24°C and still at the bottom of the sinkhole:




Such a temp diff over a few dozen meters of elevation difference could actually catch an uninformed hiker off guard. This makes me want to take a ride to one of these places and take a morning stroll there.

There's a coupla spots in the South Island of NZ that qualify as bone fide 'frost hollow' locations. I've passed through both these spots , but only in Dec, so not in Winter.
Ophir, an old gold mining boom town in Central/Southern Otago qualifies for the description with brutal overnight mins in the winter months.
"New Zealand's second-lowest official temperature of -21.6 °C was recorded at Ophir on 3 July 1995, even though it is not located at a high elevation."

The climate data...but who knows where the source is... MetService NZ is not as user friendly ( or I am too dumb more likely ) as the Aussie BOM for Climate Data Online.

www.stuff.co.nz/life-style/homed/latest/121995960/think-its-cold-spare-a-thought-for-the-town-of-ophir
www.roamingdownunder.com/coldest-town-nz.php

Twizel is the other brutally cold town in the MacKenzie District of Central Canterbury. Although only 447m asl, the town lies at the foot of the highest peaks of the NZ Alps and is renowned for its cold winter min temps. My son and his equally dumb arse mates flew into Christchurch in June from Darwin and hired a campervan to tour the South Island... did I mention in June....the Southern Hemisphere winter at 45*S ?
So they camped in the Van in a Twizel Airport winter night when the temp dropped to near -20c ! Silly bastards are lucky they didn't die from hypothermia !
www.gearshop.co.nz/blogs/news/how-cold-does-it-get-in-nz-this-cold
In fact my son ended up in hospital in Queenstown with cold related illness of which I had to send him money to pay for because the stupid pricks didn't have travel insurance...


www.lauriewinterphotography.com/twizel-hoar-frost
tikitouringnz.blogspot.com/2020/10/winter-in-mackenzie-hoar-frost-part-2.html

Some of my son's photos Twizel, New Zealand June 2015 on a -19c morning.