This a genuine question about why it currently isn't as cold here as I would expect. It is 5C. Is it as simple as adding 1.5C due to climate change. Obviously not that simple. (snip)
Originally Posted by: fairweather
850s have only been around -6C so far today (varying between -5 and -7), and as such it's not going to be especially cold.
Although people have rightly mentioned that the classic cold spells had 850s of similar, or even higher values, bear in mind they already had cold air at the surface - i.e. the thin "boundary layer" near the surface was colder than it is now. It's my experience that it takes -10 or lower to get snow near coastal areas (and even lower once you get past mid-Feb), but after a day or two of it you'll get away with higher 850s.
The other thing to bear in mind is the track of the winds - at the moment they're NE'ly, so coming over quite a long stretch of sea. They'll be more ENE'ly tomorrow, meaning less modification of the boundary layer by the sea, i.e. even with the same 850s you would expect a degree or so to be knocked off.
As for the lack of frost, there are several reasons - the saturated ground will have stored quite a bit of heat, and wet ground takes longer to cool down than dry ground. The airmass itself was still in the process of drying out, the colder Continental air didn't arrive until midnight. There was also some high cloud floating around in the early hours this morning and that would have affected the outgoing longwave radiation - a gin-clear night is best. It also wasn't flat calm, there were gusts of wind through the small hours in your neck of the woods, and even if your own garden was calm, you wouldn't have had to go far up to get some wind.
Note that this all applies to the Kent/Essex area, the calculations and conditions would have been different "up north", for example.
Now that the cold airmass is firmly in place, and with temperatures aloft falling over the next 18 hours or so, conditions will be more conducive for frosts - IF the wind falls light, which it won't!
Some places inland may even see a penetrating frost tomorrow night, where the airmass itself is so cold temperatures fall below zero despite the wind. They're the frosts which are especially damaging to tender plants.
Incidentally, for those who were unaware - Wundermap is a great way of seeing all the PWSs, and you can click on one to see the graph of wind, temperature etc. I'd imagine most of us will have several nearby, and even if you have your own station it's a great way of seeing whether you're in a bit of a frost hollow, for example.
https://www.wunderground.com/wundermap?lat=51.397564&lon=0.900857