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surfaces. I don't know how this charge develops but wonder if
Hi Andrew & All,
I didn't express my thoughts very clearly. As in "This rise of a warmer
air mass above high ground will lead to a compensating upward flow of cooler
air from lower levels". And I left out a lot, such as expansion of the
rising warmed air body and concurrent cooling which as I recall occupies
about four chapters in an introductory Met. book and lets not go there.
So I will take another run at it. The initial state of the model is
sunlight hitting some area of high ground, which is located at the summit of
a long east-facing slope, and this insolation leads to warming of the
overlying air. This will result in an updraft accompanied by a downdraft
around the perimeter of the warmed area and inflow of unheated air in under
the rising air body which will then be warmed; a Stirling engine without
pistons, cylinders or drive shaft. As the sun rises further the insolated
area of the slope increases, increasingly lower elevations of the slope will
be warmed and increasingly lower elevations of cooler air will become drawn
into this circulation.
But such updrafts and air pockets have no bearing on the pooling of
coldest air into frost hollows. This is best visualized by considering the
flow of air in a long straight east-facing ravine which heads in a large
barren at high elevation. Shortly after sundown has reached this ravine the
air will start to cool, become more dense and flow down the ravine to
create, over a period of several hours, a river of cold air. If there are
low areas of ground in undulating terrain beyond the mouth of this model
ravine then one may expect the coolest air to pool in these 'frost pockets'.
The same applies, in a less dramatic way, to any saucer-like depression at
the lower reaches of a slope, however gradual the slope is. Coldest air,
being most dense, will find its way to the frost hollow, displace any warmer
air from it and then stagnate.
These updrafts and air pockets can be quite intense even at mid-day when
flying in light aircraft between terrain that is readily warmed (e.g. dry
cultivated soil) and areas which are less readily warmed (e.g. lakes,
woodland and moist meadows). You rise rapidly over the warmer area and can
drop so rapidly when entering the cool that, with seatbelt not fastened, the
plane roof drops with a bang onto your head.
Meanwhile we may have our first frost tonight in the yard and I trust it
knows how to do it.
And I hope the second and third paragraphs make more sense that the
brief version did.
Yt, DW
----- Original Message -----
From: "Hebda, Andrew J" <Andrew.Hebda@novascotia.ca>
To: <naturens@chebucto.ns.ca>
Sent: Saturday, October 10, 2015 3:05 PM
Subject: RE: Long: Re: [NatureNS] Frost in the morning
>
> Thanks David
>
> So what would the mechanism for the wider-scale "frost-hollow" phenomenon
> be? If denser, colder air is then drawn upward to replace the
> warmer-less-dense air, would that be creating a density gradient with
> colder/most dense air remaining at low levels?
>
> Andrew.
> ________________________________________
> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on
> behalf of David & Alison Webster [dwebster@glinx.com]
> Sent: October-10-15 2:28 PM
> To: naturens@chebucto.ns.ca
> Subject: Long: Re: [NatureNS] Frost in the morning
>
> Hi Paul, Andrew & All, Oct 10, 2015
> That is an interesting question Paul. In common with most natural
> phenomena there are no doubt several forces at work and I will describe
> what
> I suspect to be the major possible mechanism from first principles.
> I don't buy the explanation you suggested Andrew. Sun on high ground
> does not draw heat upward causing cold air to be drawn down.
> Sun on high ground will warm the local air causing it to expand,
> decrease the density of this air mass and it will rise by the same forces
> that cause a hot air balloon to rise. This rise of a warmer air mass above
> high ground will lead to a compensating upward flow of cooler air from
> lower
> levels.
> It will help, I think, to consider some of the effective ways of
> warding
> off a marginal frost because this sheds some insight on mechanisms.
> (A) Nothing is gained by working all night when frost is unlikely so
> the
> first step is deciding when to prepare for action. In the days before
> weather forecasts frost omens were the combination of calm air and absence
> of cloud cover when evening temperatures were marginal. Thus, in the
> absence of lateral air movement, local cooling can be rapid if out
> radiation
> is not compensated by in radiation (from clouds).
> (B) And one time-tested strategy for frost avoidance is putting frost
> sensitive crops on a slope. Even in otherwise calm air the very slow
> downward flow of air under cooling conditions offers some protection. That
> this works also with tree fruits where the sensitive tissue is well above
> ground level suggests that transpiration from plant organs (leaves or
> flowers) is also a cause of cooling; thus airflow displaces this sheath of
> cooled air adjacent to the transpiring plant organs [Stomata will be open
> at
> night and the latent heat of vaporization is about 590 calories per gram
> of
> water at 10o C; enough heat to melt 7 grams of ice at 0o C.] This effect
> of
> air flow also applies to D.
> (C) One ancient way to avoid frost is smudge pots; fires in portable
> containers smothered with anything which will generate smoke and moved as
> necessary so smoke will drift over the crop. The heat generated by these
> fires is insignificant so protection, if any, is provided by back
> radiation
> of heat from the smoke which will be able to absorb some out radiation. A
> modern variant of this uses aerosols generated by spraying liquid onto a
> vibrating surface(I think).
> (D) Because frost develops when objects with the potential to cool
> rapidly are not warmed somewhat by a compensating inflow of warmer air or
> incoming radiation, one very effect