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Climate FAQ

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 vaguely comparable to southern England, but the sea and air currents make the (almost uninhabited) Alaskan coast quite a lot colder in winter than most of Scotland. The almost uninhabited Canadian coast between the two may be comparable. 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, such as around San Francisco, 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 it 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: The Rocky Mountains cause a stream of air to flow south from Canada, across the Caribbean and Atlantic, and causes 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, or even just stop, 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. Tropical climates are very different, as are polar ones.


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

A: Most 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.b5: How does Alaska compare?

A: The southern coastline is fairly comparable to northern Scotland in summer, but significantly colder in winter. The Yukon valley is drier and vastly colder in winter but its overall average temperatures for June, July and August are comparable to Scotland. Its average daytime temperatures for those months are comparable to the south of England, which is what matters for vegetables.


Q.b6: What sort of growing season does the UK get?

A: Based on soil temperatures 30 cm down, the MAFF (Ministry of Agriculture, Fisheries and Food) figures for pasture are between 200 and 250 days for most of the UK, but extending to more than 300 in the West Country and below 150 as you get up into Scotland. The expected variation is plus or minus 20 days, and plus or minus 40 is not rare. The USA seems to use the definition of the time from the last spring frost to the first autumn one, but that is largely irrelevant to the UK.


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 (excluding the Scilly Isles) up to about 58 degrees. For comparison, the Tropic of Cancer lies at 23.5 degrees north and the Arctic Circle at 66.5 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 about 15 degrees above the horizon in midwinter and the day is 8 hours long; up near Aberdeen, it is less than 10 degrees and 6 hours. 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, and the annual total is only 70-75% of that of Maine (perhaps the darkest state in the contiguous USA).


Q.c3: Isn't much of that because the UK is so 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. It is definitely a significant factor in lowering the light level, especially in the winter. On average, in the south of England, only about 30% of the sun's radiation reaches the ground in summer and 20% in the winter.


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.c5: How does this affect the growing season?

A: Plants that can grow in low light levels and can take light frost (e.g. lawn grass, hardy brassicas) will usually grow all year round, though very slowly in the winter, but ones that need high light levels (probably including dwarf and runner beans, squashes, sweetcorn etc.) may be able to grow for only 3 months. That is the reason that farmers have to grow short-season varieties of food crops in the UK - not because of the frosts.


Q.c6: Isn't this just a matter of day length?

A: No. Day length is very important to many plants, but the light levels are even more so. The day length in late September is still 12 hours, but the amount of light is perhaps 40% of that in June, which handicaps plants that rely for ripening on fairly strong autumn sunlight. This doesn't just affect vegetables and fruits (e.g. medlars and grapes), but also woody plants that need to ripen their new growth for the winter. And, of course, the lower sun angles mean that plants are more likely to be shaded by buildings, trees, etc.


Q.c7: Do you have any figures?

A: Kew gets a total of about 3.3 Gjoules/m^2 a year, with a maximum average of about 18 Mjoules/m^2 a day, 360 watts/m^2 at noon and a minimum average of about 1.7 Mjoules/m^2 a day and 65 watts/m^2 at noon. There are about 5 months with averages above 10 Mjoules/m^2 a day. Even a hot summer's day rarely exceeds 600 watts/m^2, and a gloomy winter's one may be below 20 watts/m^2 even at noon. The monthly averages are very variable, and are often a factor of two apart in consecutive years.


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 range of temperatures, by global standards. But note these are long-term averages, and the actual temperatures depend a lot on the atmospheric weather patterns at the time, though temperatures outside the range -15 to 30 Celsius are rare anywhere in the UK at any time (except for the peaks of Scottish mountains).


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 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 in some parts of the UK for the noon or midnight temperatures on June 21st to be lower than those on December 21st. Daily variations depend as much on air movements as the time of day.


Q.d5: I keep reading about planting after all risk of frost has passed. When is that?

A: In much of the UK, when you see the first flying pigs. The expected date of the last frost varies from early April in the West Country to sometime in June in mid-Scotland, and it is extremely variable year to year. In 40 years in Cambridge (in the south of England!), it has varied from January to the second half of June. The date of the first autumn frost is only slightly more predictable. Outside the very warmest areas, you simply have to take a risk.


Q.e1: 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 unpopulated 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 meteorologists, 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 not far above freezing, and it is common for there to be effectively no water carrying capacity in the air; so 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 normal for the dew (and rain) not to evaporate off grass, even in bright winter sunshine, for many weeks 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 (including transpiration from plants) is 30-50 cms from a potential evaporation of 35-80 cms (i.e. the amount that would evaporate off a permanently wet 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: What about dewfall, ground mist etc.?

A: In dry 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, most of the apparent dewfall and mist is caused by water diffusing up from the wet soil rather than coming down from above. As the air chills down overnight, it is fairly common for it to become condensing (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.


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 mainly light winds, which is why the rare strong wind is so destructive, but the Hebrides have about 50 days a year with gale force winds. Even at their peak, they are only a quarter 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 air 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. These are now usually split into halves. The relevant ones, and which parts of the UK fall into them, are:

7a 0-5 Fahrenheit (mountains in the Scottish Highlands)

7b 5-10 Fahrenheit (the Scottish Highlands, but only inland)

8a 10-15 Fahrenheit (most of the colder inland parts of the UK)

8b 25-20 Fahrenheit (most of the warmer inland parts of the UK)

9a 20-25 Fahrenheit (the far west and south, and much of the coast)

9b 25-30 Fahrenheit (perhaps parts of Torquay and the Scilly Islands)

The figures are based on the long-term averages that were current in the late 1980s; from the early 1990s, most of the UK has had a series of very mild winters (effectively moving everywhere up half a zone), but nobody knows if that will continue. Some tables of USDA zones (e.g. the Swedish Fuschia Society) use figures 5 degrees Fahrenheit higher.


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 very tender in the UK).

There are also plants (e.g. Albizia julibrissin, Daphne genkwa) that must have enough summer and autumn 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 rains steadily 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 6 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 much of 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.


Q.j2: What is the Sunset zone scheme?

A: This is much more relevant than the USDA one, 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. Ones rated for only zone 5 will be very risky outside the warmest areas 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; this 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, and even more the amount of autumn sunlight.


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.


Nick Maclaren
October 2009

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