BG> Maybe Ascot stays relatively warm, but out here we often
BG> have morning AIR temperatures as low as -3øC during winter

RS> I find that VERY hard to believe.

BG> Be that as it may, it's where the overnight minimum pointer ends up.

I dont believe it, you aint measuring it properly and are in fact
measuring a hybrid air/grass temp which isnt the true air temp at
all. Because you dont have a proper radiation shield and so the
very low sky temp is stuffing up the measurement of the air temp.

Its a very well known effect, the stevenson screen was invented
to fix that problem and the similar one during the day, with the
daytime temp fuck the other way by the solar radiation input.

RS> That is called an 'air frost' and is rare
RS> here, tho it certainly happens every year.

BG> Nothing too techo here at all; no Stevenson screens,
BG> no anemometers, no barometer, only a max/min recording
BG> thermometer, a wet & dry bulb, and my trusty old rain-gauge.

Thats the problem, no radiation shield on the temp.

RS> I find it hard to believe that you see that 'often'
RS> when measured properly in a stevenson screen.

BG> Imagine a high-set house, with the thermometer mounted at eye-level on
BG> the first floor, maybe 12-14 feet in total from the ground, and on a
BG> part of the house which receives NO direct sunlight at ANY time of the
BG> year.  It might not be perfect, but it's certainly adequate for my purposes

Not for measuring overnight air temp. It sees the extremely low sky
temp on a cloudless night, and you aint actually measuring air temp
at all because you dont have a radiation shield to prevent that effect.

Its the same problem as measuring the daytime max when its not in shade.
People dont realise you have to 'shade' from the very low sky temp too.
Coz the thermometer just radiates to that very low temp and that fucks
the measurement of air temp. Put a piece of polyfoam over it, say at a
higher of 10cm above it, so there is no direct line from the bulb to the
sky at all, and you will find the temp doesnt get anywhere near as low.

RS> I bet the official figures dont say that.

BG> That's quite correct.  They sure don't.

RS> And I dont mean the official Bris site either.

BG> Sure, that's at the top of Wickham Terrace, a place not normally
BG> noted for its excessively cold overnight minima.  The other official
BG> sites are now at the airports, Eagle Farm, Archerfield, and Amberley
BG> (of those three sites, we most closely mirror Amberley here).

You will probably find there are others too, the old post office
sites. Particularly down your way between Gold Coast and Bris.

Minimum air temp is actually harder to measure than you might
think too, its a lot more variable than say the max is. Because
in some situations you get cold air pouring out of a valley and
thats a hell of a lot colder than say on the adjacent ridge.
@EOT:

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