The Frost Report
Everything you always wanted to know about
frost, but were too cold to ask.
Angus White, Architectural Plants, Cooks Farm, Nuthurst, W. Sussex,
RH13 6LH, U.K.
Chamaerops No. 12, published online 23-09-2002
[an error occurred while processing the directive]
Above: Washingtonia filiferas snug in their winter
overcoats of straw. Exposed lea yes may be damaged by frost and
cold but will be rapidly replaced during the follo wing spring &
summer.
Below: Photographedin late summer, these Washingtonias look perfectly
at home in Exeter, Devon.
The stuff of beauty and wonder to many as it lays
its delicate and immaculate crystals upon the bare filigree of branches
on a bright winter's morn or the stuff of fear, loathing and manic
neurosis to the ambitious exotic gardener. Frost or 'Bit Parky',
as it's sometimes known, doesn't really exist; there's no such thing
as cold - it's just lack of heat. Cold doesn't penetrate at all
- it's the heat that leaves. It's the degree to which heat leaves
that's known as frost.
Where The Cold Air Comes From
When high pressure dominates, we expect settled
conditions with light winds and clear skies. In northwest Europe,
high pressure centred over the continent will be spinning in a clockwise
direction, feeding in air with a continental origin - i.e. coming
in over land rather than sea. Land heats up very quickly, so in
the summer this situation will lead to warm conditions but it also
cools down very quickly, so in the winter exactly the same situation
will lead to cold conditions - sometimes, if the air is of Siberian
origin, very cold conditions.
The Sea's Tempering Influence
In the British Isles, this situation is often made
less extreme by the tempering influence of the sea. Unlike the land,
the temperature of the sea remains relatively constant throughout
the year; it takes a long time to heat up and a long time to cool
down. The effect of this on the temperatures experienced here can
be considerable; the more sea between you and the source of cold
air, the more the sea will warm the air. The more sea between you
and the source of the hot air, the more the sea will cool the air.
Thus, in the event of high pressure over Poland feeding in freezing
air from the Russian Steppes, one would expect to experience lower
temperatures in south-east England than in north-east England, solely
because of the greater expanse of North Sea between the north-east
and the continent than the relatively narrow Channel. So - the further
away from the continent you live, the milder the climate.
Well, that's the theory anyway. Areas of the south-west
of Britain are sometimes described as being 'lapped by the warm
gentle waters of the gulf stream'. Expressions like that are pure
tourist hype but it is true that the Atlantic is much warmer at
our latitude than it ought to be and this has a warming influence
on our climate - particularly the further west you go. The meteorological
pedant will, incidentally, point out that at 50 - 60 degrees north
(our latitude) the current that brings this warmer water is called
the North Atlantic Drift and not the Gulf Stream, and also that
this 'warmth' is only relative. Try going for a swim in the sea
off the Isles of Scilly, even in midsummer. You'll freeze your buns
off.
The other source of cold air in the British Isles
is polar. Coming down from directly above us, this tends to affect
Scotland most. Coming in over the relatively warm sea, it's often
less cold than the sort of continental air that the south-east of
England is prone to, but is likely to be carrying more moisture
and therefore can lead to heavy snow falls which are relatively
unusual in the south east.
That's a brief description of where cold air comes
from and if you didn't understand a word of it, it doesn't really
matter. The most important things to understand are how cold air
behaves, the formation of microclimates, their implications for
the gardener and to clear up a few commonly held misunderstandings:
Microclimates
Microclimates are small areas that enjoy (or regret)
a different climate than that which their geographical situation
would suggest.
The so-called Urban Heat Island is the most pronounced;
a large built up area will generate a huge amount of heat - in the
winter, from the sheer number of people and their central heating
boilers and in the summer, from the absorption of the sun's heat
during the day leading to radiation of that stored heat during the
night. The larger this built up area, the greater the 'Heat Island'
effect. Differences of nighttime minimum temperatures of more than
10¾C have been noted between the centre of very large cities and
the opposite situation only a few miles beyond the city in:
The Rural Frost Pocket. Cold air is heavier
than warm air and on still, cold, cloudless nights it will roll
down slopes and collect in pools of cold air where escape is impossible.
Valley bottoms are the most obvious of culprits but frost pockets
can also be created by buildings and trees. Walled gardens on sloping
sites will invariably have a door on the lowest wall. This can be
amazingly useful for getting in and out of the garden - but it's
also there to be left open on cold clear winter nights to allow
the cold air to drain out. No, this is not fanciful nonsense, it's
perfectly true! Victorian gardeners seem to have had a much better
understanding of how cold air behaves than their modern counterparts.
Elevated positions with good frost drainage.
Often exposed to the wind, this is the opposite of the frost pocket.
During still, cold conditions that lead to a temperature of, say,
-4¾C in a sheltered frost pocket, in a garden at 700 feet a.s.l.
at the edge of a nearby south-facing escarpment it might only go
down to +2¾C. Indeed, there was some anecdotal evidence given to
me by the Met. Office that they had an infrared aerial photograph,
taken south of London, showing ground temperatures on Reigate Hill
(800 feet) at +8¾C compared with the surface of the M25 motorway
some 600' below at -4¾C. Even they were sceptical of such figures,
but there's plenty of evidence that topographical variations can
lead to some extraordinary differences.
Evergreen Trees. There's an expression 'radiation
frosts' that one comes across from time to time. It means frosts
that are caused by the loss of heat being radiated from the ground
and is slightly misleading as, to some degree or other, this is
how all frosts are caused. However, it describes the phenomenon
quite well and the process can be slowed down if there's a canopy
of trees to stop all the heat from disappearing into the atmosphere.
Deciduous trees accomplish little in this respect in the winter
(for obvious reasons); so tall growing broad-leafed evergreen trees
are the best. Eucalyptus are quite good but being a little sparse
up top do not compare with the extraordinary insulating qualities
of the Holm Oak (Quercus ilex). A copse of these with their crowns
lifted (lower branches cut off) can make a dramatic difference to
the minimum temperatures recorded in a particular spot.
Near a west or south facing wall. This is
the best known of all. If it's stone or brick and is a wall of a
heated building, then so much better. One could put a wall behind
a chimney breast to good use - it's bound to be a little warmer
than your average wall especially in very cold weather.
Soil
Light sandy soil has a reputation for being warm,
and heavy clay soil has a reputation for being cold. The reason
for this is that well-drained sandy soil warms up faster in the
spring than heavy wet clay, causing the plants to burst into activity
earlier. Calling the soils warm and cold as a result of this is
also misleading. Wet heavy clay warms up slowly but also cools down
slowly; light dry sandy soil warms up fast but also cools down fast.
It's rather like the land and the sea argument and as far as the
survival of plants goes, many are much more likely to survive severe
frost on 'cold' clay than 'warm' sandy soil. You will sometimes
hear weather forecasters referring to frost being likely in 'sheltered
areas' (the expression 'sheltered glens', referring to Scotland
is often heard) and areas on 'light soil'. This is well borne out
by the susceptibility of certain areas just inland from Bournemouth
to early frost - Hum airport for example. It is said that in the
long cold winter of 1963 the soil at Woburn Sands in Bedfordshire
(very sandy soil) was frozen to a depth of 3.5 feet, whereas heavy
clay soil was frozen to a depth of only 1 foot at the most. The
consequences of this will not be lost on growers of exotic plants.
It's only fair to add that some plants need well drained soil in
order to survive but root hardy herbaceous plants are much more
likely to survive on sticky old clay.
Cloud And Wind
Something that everyone will have noticed is how
the presence of cloud stops the temperature dropping too much. During
cold weather, if there is cloud cover, temperatures at night will
hardly drop below what they are during the day. If, on the other
hand, there is no cloud cover you will hear the weather man using
spine chilling expressions such as 'drop like a stone', 'severe'
and 'penetrating'.
Wind will also have a slightly warming effect on
potentially low temperatures. As long as the wind is blowing, temperatures
will become more constant over a given area i.e. places that are
usually prone to frost will be no more prone to it than anywhere
else. Wind will always have been blowing over the sea before it
gets to you (if you're in the British Isles) and will have picked
up a certain amount of heat in the process.
On the subject of 'wind chill', I remain agnostic.
Certainly an animal will perceive it to be very cold if there is
a wind added to freezing temperatures. Whether this is the case
if you're a plant, I'm not so sure, although wind will cause a greater
rate of transpiration (losing moisture from the leaves) and if the
ground is frozen, the plant finds it hard to bring up enough moisture
from the roots and dies of dehydration.
And Finally
You may like to ponder on this: Nature is in balance,
most of the time. Hot here, cold there. You are as likely to get
a bitterly cold winter as you are to get a blisteringly hot summer,
and who knows, they're rare enough.
References: For more information on weather and
climate get in touch with the Meteorological Office, London Road,
Bracknell, Berkshire RGI2 2SZ Tel :0344 856038.
Ask for a copy of 'The Met Office Publications'.
This is a free booklet; it's a bit out of date but will give you
an idea of the amazing range of information available.
Readers Comments:
(No comments yet. Be the first to add a comment to
this article!)
|
