Thank all respondents for corrections, comments and compliments!

I have checked all the comments, and added almost all of them, plus
adding a fair amount of new material and removing some bias where I
could see it. This is likely to be the last version I post for now,
and more comments will go only into the 'final' version.

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.a1: Well, why are they so different?

A: The first and main reason is that North America, excluding Hawaii,
the Gulf of Mexico coast and the Pacific North West (from Portland
through to Canada to Anchorage), has a strongly continental temperate
climate; and the UK has a strongly maritime temperate one.


Q.a2: Are any of those comparable to the UK?

A: There is a small stretch of coast around Portland, Seattle and
Vancouver that is comparable to southern England, but the sea and air
currents make the (almost uninhabited) Canadian and Alaskan coast quite
a lot colder in winter than most of Scotland. The Gulf of Mexico coast
and Hawaii are very much hotter than the UK, of course.


Q.a3: 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), and the weather can be
very non-maritime when the wind blows off the land. For comparison,
Bedfordshire has as close to a continental climate as anywhere in the
UK, is 90 kilometres from the nearest coast, and is still has a strongly
maritime climate.


Q.a4: 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, which is why our weather forecasts are all
about cyclones, anticyclones and warm and cold fronts.


Q.a5: 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. The sea and air currents around California tend to be
more aligned with the coast, and the latter are weaker.


Q.b1: 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 sea movement was in the other direction, causing
the water temperature boundary to be level with Lisbon, so the UK was
covered with glaciers (with tundra in the south). The change happened
within about 50 years, incidentally.


Q.b2: So, if global warming causes the change to reverse, we are in
dead trouble?

A: Yes.


Q.b3: Is the 'temperate' relevant?

A: Yes, if you are comparing the USA with Africa, India or Brazil, or
the UK with Hawaii, Fiji or the West Indies.


Q.b4: What other places are similar to the UK?

A: Much of coastal north-western Europe (including Brittany, Normandy,
the Low Countries, Germany's North Sea coast, Denmark and, of course,
Ireland), the Pacific North West coast of Canada and the USA, and most
of the lower ground of New Zealand south of Rotorua. There are quite a
lot of other places that are close enough that we can grow many of their
plants, including several other parts of Europe, much of Japan, the tip
of South Africa, New England and a few places in South America.


Q.c1: What is the second reason for the differences between the UK and
USA?

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, excluding the Scilly
Isles). For comparison, the Tropic of Cancer lies at 23 degrees north
and the Arctic Circle at 69 degrees.


Q.c2: Which does this matter, if we are kept warm by the sea breeze?

A: Mainly the light level. 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 like 12 degrees and 6 hours. The often heavy cloud
cover in the winter makes the situation worse. Even in the south of
England, the insolation (average amount of sunlight per square metre) in
the winter is 10% of that in the summer.


Q.c3: Isn't the UK also very cloudy?

A: It can be, but the most common conditions are light cloud to
continuous cloud cover that hides the sun but not its direction. Except
when raining heavily, of course. It is definitely a significant factor
in lowering the light level, summer and winter, and probably reduces
it on average by 30-50%.

The available cloud cover measurements seem to be in a variety of
peculiar measurements that relate more to the appearance of the cloud
than the amount of light it lets through. Can any meteorologist
reading this help with some average absorption figures here?


Q.c4: Is this also why we lose our tans in the winter?

A: Yes, and why children can develop rickets if they don't get enough
vitamin D; during our gloomier winters, the ultraviolet level in the UK
is close to zero. The low light level is also implicated in SAD
(Seasonal Affective Disorder), a sort of depression common in the winter
in the darker countries.


Q.d1: 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 Celsius, and the average nighttime low of the
coldest month is in the range 0-6 degrees Celsius, both depending on
location. A very uniform climate, by global standards.


Q.d2: 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 decrease quite strongly south to north, and in winter decrease
west-south-west to east-north-east (sic), though with a definite coastal
warming effect.


Q.d3: 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.d4: What is the temperature range in a single location?

A: Annually, about 9-13 degrees Celsius and, daily, about 3-10 degrees
Celsius,with the larger ranges in the places with lowest rainfall. It
isn't unheard of for the noon or midnight temperatures on June 21st
to be lower than those on December 21st in some parts of the UK.


Q.e1: 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 populated places get between 50 cm and 100
cms, with the wettest populated places getting 200 cms. The average for
the UK is slightly above a metre, but the heaviest rain occurs on the
almost upopulated high ground.


Q.e2: 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. In the west, the
winter is slightly wetter than the summer, but only by about 25%. 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.e3: But what about our water shortages?

A: These are almost entirely mismanagement, caused by the fact that we
have minimal storage capacity, because the rain is so uniform. The
official definition of a drought in the UK is something like 6 weeks
with only a couple of centimetres of rain. Tell that to people from
central Africa, or even much of the USA, and they will fall about
laughing.


Q.f1: How humid is the UK, relatively?

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


Q.f2: 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, which causes rust, rot and so
on. This applies particularly at ground level and in the middle of
plants. It is common for the dew (and rain) not to evaporate off grass,
even in bright winter sunshine, for weeks or months at a stretch. At
higher temperatures (e.g. the southern USA), the same relative humidity
implies a larger margin for evaporation.


Q.f3: What is the actual rate of evaporation?

A: The annual evaporation is 30-50 cms from a potential evaporation of
35-80 cms (i.e. the amount that would evaporate off a permanently damp
surface), with the higher figures in the south, and is almost entirely
in summer. The closeness of the actual and potential figures shows how
damp the climate is.


Q.f4: Does this also cause the growth of algae?

A: Very much so. The slippery algae on paths are really water plants,
but will grow in conditions where the humidity stays close to 100%.
They are also killed by ultraviolet light, which is why they grow only
in shaded places in the summer but often grow on south-facing and
exposed paths in the winter.


Q.f5: Is this related to the heavy dewfalls?

A: In drier climates, most dewfalls are caused by the ground radiating
into space and becoming cold, so the water vapour in the air condenses
onto it. In the UK, it is equally common for the air to chill down,
causing a condensing (supersaturated) atmosphere, where it is carrying
more water vapour than it can hold; this causes condensation even on
tools and plants under cover, summer and winter.

Average dewfall data does not seem to be available, but it could well be
20 cm a year or more. Can any meteorologist reading this help with some
average dewfall figures here?


Q.g1: You 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, which is
why the rare strong wind is so destructive, but the Hebrides have about
50 days 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 with gales 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.g2: 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 (for us, that is), it can
cause the air to come down from Scandinavia (causing cold conditions in
winter, as in 1962/3) or from France (causing heat waves in summer).
This rarely lasts for longer than a week, but it does happen.


Q.g3: We are supposed to be able to grow a lot of tender plants in the
UK. Is this true?

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 cold, dark, high humidity conditions, which don't allow the plant to
regrow the damaged tissue but do allow bacteria and fungi to attack the
wound and harm the plant further. This is also why waterlogging is more
of a hardiness problem than frost in the UK.


Q.g4: What are the worst conditions the UK produces?

A: 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 damages
the new growth and the miserable conditions allow the plant to become
attacked by bacteria and fungi. This sort of thing really does happen,
and causes considerable destruction when it does.


Q.g5: Do we get a lot of late frosts?

A: Yes, but rarely severe ones, or we wouldn't be able to grow most of
the plants that we do. Because the conditions for causing late frosts
(i.e. air flow from the Arctic or Scandinavia) rarely last long, we
don't often get the conditions that kill a lot of plants. However, cold
snaps can happen at any time of year, and the colder parts of the UK
get occasional frosts even in summer.


Q.h1: 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.h2: 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.h3: 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 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 tender in the UK.

And, of course, many plants just hate waterlogging, which is what
our heavier soils get all winter and every winter, and all soils
get whenever it is raining in the winter (no evaporation, remember?)


Q.h4: 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.
Those rated at 5 or below won't usually die from frost in the UK, but
might well from waterlogging.


Q.h5: Why do they work in the USA, if they don't in the UK?

A: It's not that they wouldn't work in the UK, but that they can't be
used to compare the USA (or even more central Europe) and the UK. They
work in the USA because there is a strong correlation between the
extreme low temperatures and the average temperature of the coldest
month (and hence whether the soil freezes) as well as many other
conditions that affect winter hardiness. There is no such correlation
between the USA and UK.


Q.j1: Are there any better zoning systems?

A: A controversial question! Perhaps the best for our purpose is the
Sunset scheme, but simple latitude has its uses, and there is the
traditional UK "hardy", "half hardy" and "tender" classification. But
the last describes only plants, and does not categorise places in the
UK.


Q.j2: What are the Sunset zone scheme?

A: These are much more relevant than the USDA ones, but very limited.
The Sunset New Western Gardening Book, a California publication, defines
some zones that describe the gardening conditions in the western states
of the contiguous USA, taking both temperatures and rainfall into
account. Zone 5 is Portland Sound etc. (rather like the West Country),
zone 4 is the higher country around it, and zone 3 is higher and colder
again. The others have little relevance to the UK.


Q.j3: How should we interpret the Sunset zones?

A: A plant rated for both Sunset zones 4 and 5 should be hardy in the
warmer areas of the UK and one rated for all of 3-5 should be hardy over
most of the UK. Beware of ones rated for zones 1-3 but not 4 and 5,
because they will probably dislike warm, wet winters. Ones rated for
zones 6 upwards only will rarely survive outside in the UK.


Q.j4: What about latitude?

A: Another traditional one is simple latitude at which a plant is found
in the wild, which can be compared with that of the UK, 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.


Q.j5: What about the hardy, half hardy and tender classification?

A: In the UK, plants are classified as "hardy" or "half hardy",
effectively according to whether they can overwinter outside at Kew
without any protection or with protection only from frost. A "tender"
plant will not overwinter outside even at Kew. More recently (perhaps
quarter to half a century back!), the term "frost hardy" was added to
mean the ability to survive a light frost (say, down to -5 Celsius).
Such a classification is usually fairly reliable for use in the UK, as
far as it goes.

In article ,
Nick Maclaren writes:
The south-east of England gets only weak winds, which is
why the rare strong wind is so destructive,

You seem to be contradicting yourself here. I suggest replacing "only"
by "mostly". Also "light" is the usual adjective, rather than "weak".
--
John Hall
"Three o'clock is always too late or too early
for anything you want to do."
Jean-Paul Sartre (1905-1980)

On 27 Feb 2003 20:31:08 GMT, (Nick Maclaren) wrote:

For comparison,
Bedfordshire has as close to a continental climate as anywhere in the
UK, is 90 kilometres from the nearest coast, and is still has a strongly
maritime climate.

Typo?

..... [ it] still has a strongly maritime climate.

Sterling work BTW.

Hussein