Having been prodded once too often by this, here is a draft of a
climate FAQ (including remarks about USDA zones). Comments
welcome.


Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:
Tel.: +44 1223 334761 Fax: +44 1223 334679







This FAQ is dedicated to all of uk.rec.gardening's fans, both in the USA
and UK, who have difficulty in understanding how the other's climate can
be so different.


Q: Well, why are they so different?

A: The first and main reason is that the USA has a strongly continental
temperate climate, excluding Hawaii, the Gulf of Mexico coast and the
Pacific North West (including the southern Alaskan coast), and the UK a
strongly maritime temperate one.


Q: But the USA has lots of coastline! Isn't that maritime?

A: No. In a coastal continental climate, the marine influence extends
very little inland (often only a mile or two). Cambridge has as close
to a continental climate as anywhere in the UK, is 70 miles from the
nearest coast, and is still has a strongly maritime climate.


Q: What are the differences between continental and maritime climates?

A: In the former, the seasons and even the days are typically very
definite, with summers reliably much hotter than the winters and the
days hotter than the nights. Similarly, spring and autumn are definite
seasons, and typically short. In the latter, it all depends where the
air movement is coming from.


Q: Why do we have a maritime climate over all of the UK, but California
doesn't?

A: Recent research indicates that the Rocky Mountains cause a stream of
air to flow south from Canada, across the Caribbean and Atlantic, and
cause our prevailing westerly to south-westerly winds, which keep us
from freezing.


Q: But aren't we warmed by the Gulf Stream?

A: Yes. The Gulf Stream (and North Atlantic Drift) cause the boundary
between the warm southern water and the polar (Arctic) water to be
somewhere to the north of Iceland, so the wind blows over warm ocean.
18,000 years ago, the movement was in the other direction, the boundary
was level with Lisbon and the UK was covered with glaciers and tundra in
the south. The change happened within about 50 years, incidentally.


Q: So, if global warming causes the change to reverse, we are in DEAD
trouble?

A: Yes.


Q: Is the 'temperate' relevant?

A: Yes, if you are comparing the USA with Africa, India or Brazil, or the
UK with Hawaii or the Canary Islands.


Q: What is the second reason for the differences?

A: The populated or 'gardening' parts of the USA lie south of 49 degrees
north (from about 30 degrees, excluding Hawaii) and those of the UK lie
north of 50 degrees (up to about 58 degrees). For reference, the Tropic
of Cancer lies at 23 degrees north and the Arctic Circle at 69 degrees.


Q: Which does this matter, if we are kept warm by the sea breeze?

A: Mainly the light levels. In the south of England, the sun gets to 19
degrees above the horizon in midwinter and the day is 8 hours long; in
Aberdeen, it is more 12 degrees and 6 hours. Even in the south of
England, the insolation (amount of sunlight per square metre) in the
winter is 10% of that in the summer.


Q: What sort of temperatures does the UK have?

A: The average daytime peak temperature of the hottest month is in the
range 18-22 degrees Celcius, and the average nighttime low of the
coldest month is in the range 0-6 degrees Celcius, both depending on
location. A very uniform climate, but global standards.


Q: In what way depending on location?

A: Few people in the UK live above about 300 metres, and almost nobody
above 600 metres, so height is largely irrelevant. The temperatures in
summer grade quite strongly south to north, and in winter increase
east-north-east to west-south-west (sic), though with a definite coastal
warming effect.


Q: So this is why Inverewe in the north west of Scotland can grow palm
trees but Ipswich in the south east of England can't?

A: Yes.


Q: And is the wind off the ocean why we have so much rain?

A: Actually, we don't get a lot of rain by global standards, except in
the far north west. Most places get between 50 cm and 100 cms, with the
wettest inhabited places getting 200; the difference between the UK and
much of the USA is that the annual evaporation is only about 20-30 cm
almost entirely in summer.


Q: How is the rain distributed?

A: It is pretty well constant over the year, despite what it feels like,
and increases east-south-east to west-north-west. There is, of course,
quite a strong tendency for the prevailing winds to dump the water on
the first high ground they come across, which is why the Hebrides are
justly notorious for heavy rain.


Q: How humid is the UK, relatively?

A: In terms of absolute humidity (water vapour pressure), not at all.
In terms of relative humidity as measured by metereologists, it is high
but not unusual (e.g. it is comparable to Houston, Texas). In terms of
gardening, it is sky high.


Q: That doesn't seem to make sense. Why is that?

A: In the winter, the temperature is usually just above freezing, and
it is common for there to be effectively no water carrying capacity in the
air, so that things that are wet stay wet. Often for weeks at a stretch,
which causes rust and rot. At higher temperatures, the same relative
humidity implies a larger margin for evaporation.


Q: Is this related to the heavy dewfalls?

A: In the USA, most dewfalls are caused by the ground radiating into
space and becoming cold. In the UK, it is equally common for the air to
do that, causing a condensing atmosphere (supersaturated, where it is
carrying more water vapour than it can hold); this causes condensation
even on tools and plants under cover, summer and winter. I don't know
what the average dewfall is, but it would not surprise me if it were not
20 cm a year or more.


Q: So does this mean that we can grow a lot of tender plants?

A: Well, yes and no. The problem in the UK is that we do get some
frost, which damages plant cells, and then there is often a long period
of cool, dark, high humidity, which doesn't allow the plant to regrow
the damaged tissue but does allow bacteria and fungi to thrive. See
below.


Q: Your mentioned wind. How do ours compare with hurricanes?

A: It depends where you are. The amount of wind corresponds roughly
to the rainfall. The south-east of England gets only weak winds, but
the Hebrides have about 50 says a year with gale force winds. At their
peak, they are only 1.4 as destructive as a minor hurricane, but EVERY
place in the Hebrides gets hit for that number of days EVERY year.
Also, UK winds are typically very gusty, and cause more damage than
steady winds of the same speed.


Q: Why do we ever get very cold conditions if the wind comes from the
west?

A: It doesn't always come from the west. If an anticyclone (high
pressure area) settles in the wrong place, it can cause the air to come
down from the Arctic (causing cold conditions, as in the winter of
1962/3) or from France (causing heat waves in summer). This rarely lasts
for longer than a week, but that does happen.


Q: What are USDA hardiness zones?

A: The USA Department of Agriculture has defined zones of hardiness,
mainly for application to woody plants, corresponding to the average
extreme low temperature encountered in a year. The relevant ones, and
which parts of the UK fall into them, a

7a 5-10 Fahrenheit (mountains in the Scottish Highlands)
7b 10-15 Fahrenheit (the Scottish Highlands, but only inland)
8a 15-20 Fahrenheit (most of the colder inland parts of the UK)
8b 20-25 Fahrenheit (most of the warmer inland parts of the UK)
9a 25-30 Fahrenheit (the far west and south, and much of the coast)
9b 30-35 Fahrenheit (just perhaps parts of Torquay)


Q: Are they relevant to the UK?

A: Not really. USDA zone 8 includes much of France (e.g. Toulouse) and
much of Texas (e.g. Dallas and Austin), which can grow plants that we
don't have a hope of growing. Our hardiness criteria are simply
incommensurate. This all comes back to the continental versus maritime
climate differences. And, of course, the USDA ratings of plants are the
USDA zone at which they will grow, when grown in the USA.


Q: Why is this?

A: Lots of reasons. Here are some.

In continental climates, USDA zone 8b is not normally associated
with the soil freezing; in the UK, it is. Many plants are much
more sensitive to their roots freezing than their tops freezing
(and, obviously, all herbaceous plants are).

In colder continental climates, the winter sets in, the soil
freezes, snow falls, and the soil below the frozen band dries out.
The dessication is actually what kills many plants, but it can
also protect those that hate waterlogging (e.g. Passiflora
incarnata, hardy to zone 6a in the USA and tender in the UK).

There are also plants (e.g. Albizia julibrissin, Daphne genkwa)
that abosolutely must have enough summer heat to ripen their
year's growth to survive the winter. These are rated at USDA
zones 6b and 5b, but are seriously tender in the UK.

The nightmare conditions in the UK are a warm February, a week of
bitter frost in early March, and then a fortnight of cold, wet,
dark conditions. The first causes plants to start growing, the
frost kills the new growth and the miserable conditions allow the
bacteria and fungi to thrive but don't allow the plants to regrow.
This sort of thing really does happen.

And, of course, many plants just hate waterlogging, which is what
we get all winter and every winter (no evaporation, remember?)


Q: But surely they can tell us something?

A: Of course. If a plant is rated at a zone above where you live, it
almost certainly won't grow outside in the UK. But you can't guess from
the USDA zone rating of a plant in the range 6-9 whether it will be
hardy in the UK, though the lower a number the more likely it is to be.


Q: Are there any better zoning systems?

A: A controversial question! The traditional one in the UK is the
latitude at which a plant is found in the wild, which is quite rightly
deprecated by the USDA zone camp. But it does work extremely well for
almost all annuals and food crops, because latitude is strongly
correlated with the cumulative amount of heat and light the plant gets
in the growing season.