Climate of the British Isles

The British Isles has a maritime climate, characterized by mild winters and relatively cool summers, which is a consequence of its proximity to the sea. This is because water has a much larger heat capacity than materials making up the land. As a consequence, it takes more heat energy to raise the temperature of water one degree, and there is more heat energy to give up when the water cools by one degree, when compared with rock and soil. Consequently bodies of water warm up and cool down more slowly than adjoining land. The nearby sea thus prevents coastal areas becoming as cold in the winter as inland areas and also helps maintain temperatures well into the autumn.

In contrast, inland areas on the great landmasses at the same latitude have a more extreme climate, with very cold winters and hot summers; the features of a continental climate. Whereas most of the British Isles lowland is normally above freezing for most of the winter, average mid-winter temperatures for Moscow and Hudson Bay (both continental climate situations) are nearer — 15°C.

The North Atlantic Drift, the ocean current flowing from the Gulf of Mexico towards Norway, dominates the climate of the British Isles (see Figure 2.5). The effect of the warm water, and the prevailing southwesterly winds blowing over it, is particularly influential in the winter. It creates mild conditions compared with places in similar latitudes, such as Labrador and the Russian coast well to the north of Vladivostok, which are frozen in the winter.

The mixing of this warm moist air stream and the cold air masses over the rest of the Atlantic leads to the formation of a succession of depressions. These regularly pass over the British Isles bringing the characteristic unsettled weather; with clouds and rain where cold air meets the moist warm air in the slowly swirling air mass. Furthermore, the moist air is also cooled as it is forced to rise over the hills to the west of the islands giving rise to orographic rain. In both instances clouds form when the dew point is reached (see p41). This leads to much higher rainfall levels in the west and north compared with the south and east of the British Isles. In contrast, a rain shadow is created on the opposite side of the hills because, once the air has lost water vapour and falls to lower warmer levels, there is less likelihood of the dew point being reached again (see Figure 2.6). Depressions are also associated with windier weather.

The sequence of depressions (low-pressure areas) is displaced from time to time by the development of high-pressure areas (anti-cyclones). These usually bring periods of settled drier weather. In the summer these are associated with hotter weather with air drawn in from the hot European land mass or North Africa. In the winter, clear cold weather occurs as air is drawn in from the very cold, dry continental landmass. In the spring, these anti-cyclones often lead to radiation frosts, which are damaging to young plants and top fruit blossom.

The growing season

The outdoor growing season is considered to be the time when temperatures are high enough for plant growth. Temperate plants usually start growing when the daily mean temperatures are above 6°C. Spring in the southwest of the British Isles usually begins in March, but there is nearly a two-month difference between its start in this area and the northeast (see Figure 2.7).

Cloud Formation British Isles
(a)

air cools as it

rises; clouds form

as dew point is reached

'V rain shadow

(b)

Figure 2.6 Cloud formation and rainfall caused by (a) fronts and (b) higher ground (orographic rain). Note: warm air caused to rise over cold air or higher ground forms cloud when the air reaches the dew point of the air mass.

Figure 2.6 Cloud formation and rainfall caused by (a) fronts and (b) higher ground (orographic rain). Note: warm air caused to rise over cold air or higher ground forms cloud when the air reaches the dew point of the air mass.

Growing Season British Isles

In contrast, as temperatures drop below 6°C the growing season draws to a close. This occurs in the autumn, but in the southwest of England and the west of Ireland this does not occur until December, and on the coast in those areas there can be 365 growing days per year. Within the general picture there are variations of growth periods related to altitude, aspect, frost pockets, proximity of heat stores, shelter and shade: the so-called local climates and microclimates (see p37). However, for most of mainland UK, the potential growing season spans between eight and nine months. Examples are given in Table 2.1.

Although this length of growing period will be a straightforward guide to grass growing days and the corresponding need for mowing, many other plants will stop growing as they complete their life cycle well before low temperatures affect them. Furthermore, there are plants whose growing season is defined differently. For example, most plants introduced from tropical or sub-tropical areas do not start growing until a mean daily temperature of 10°C is experienced. More significantly, they are restricted by their intolerance to cold so for many their outdoor season runs from the last frost of spring to the first frost of autumn.

Proximity to the sea not only increases the length of the growing season, but also reduces its intensity, i.e. the extent to which temperatures

Table 2.1 Length of growing season in the British Isles

Area

Length of growing season in days*

Time of year

start

finish

S-W Ireland

320

Feb 15

Jan 7

Cornwall

320

Feb 15

Jan 7

Isle of Wight

300

March 1

Jan 1

Anglesey

275

March 1

Dec 15

South Wales

270

March 15

Dec 15

East Lancs

270

March 15

Dec 1

East Kent

265

March 15

Nov 28

N. Ireland

265

March 15

Nov 28

Lincolnshire

255

March 21

Nov 25

Warwickshire

250

March 21

Nov 22

West Scotland

250

March 21

Nov 20

East Scotland

240

March 28

Nov 15

N-E Scotland

235

April 1

Nov 10

  • Length of season is given for lower land in the area; reduce by 15 days for each 100 m rise into the hills (approximately 5 days per 100 feet).
  • Length of season is given for lower land in the area; reduce by 15 days for each 100 m rise into the hills (approximately 5 days per 100 feet).

exceed the minimum for growth. Although inland areas have a shorter season they become much warmer more quickly before cooling down more rapidly in the autumn. The differences in intensity can be expressed in terms of accumulated temperature units.

Accumulated temperature units (ATUs) are an attempt to relate plant growth and development to temperature and to the duration of each temperature. There is an assumption that the rate of plant growth and development increases with temperature. This is successful over the normal range of temperatures that affect most crops. On the basis that most temperate plants begin to grow at temperatures above 6°C the simplest method accredits each day with the number of degrees above the base line of 6°C and accumulates them (note that negative values are not included). A second method calculates ATUs from weather records on a monthly rather than daily basis. Examples are given in Table 2.2.

Methods such as these can be used to predict likely harvest dates from different sowing dates. Growers may also use such information to calculate the sowing date required to achieve a desired harvest date. In the production of crops for the freezing industry, it has been possible to smooth out the supply to the factory by this method. For example, a steady supply of peas over six weeks can be organized by using the local weather statistics to calculate when a range of early to late varieties of peas (i.e. with different harvest ATUs) should be sown.

More accurate methods, such as the Ontario Units, use day and night temperatures in the calculation. These have been used to study the growth

Table 2.2 Examples of Accumulated Temperature Units (ATUs) calculated on (a) a daily basis and (b) monthly basis

a) Accumulated Temperature Units (ATUs) calculated on a daily basis with a base line of 6°C

Date March

Average temp. (°C)

Temperature units in day-degrees

ATUs in day-degrees

1

6

(6 -

6 = 0)

1 X

0 = 0

0

2

7

(7 -

6 = 1)

1 X

1 = 1

0 + 1 = 1

3

7

(7 -

6 = 1)

1 X

1 = 1

1 + 1 = 2

4

5

(5 -

6 = '0')

1 X

(0) = 0

2 + 0 = 2

5

8

(8 -

6 = 2)

1 X

2 = 2

2 + 2 = 4

6

7

(7 -

1 = 1)

1 X

1 = 1

4 + 1 = 5

7

8

(8 -

6 = 2)

1 X

2 = 2

5 + 2 = 7

b) Accumulated Temperature Units (ATUs) calculated on a monthly basis

Month

Average temperature

Temperature units in day degrees

ATUs in day-degrees

February

5

( 5 -

6 = '0')

28

X 0 = 0

0

March

7

(7 -

6 = 1)

31

X 1 = 31

31

April

8

(8 -

6 = 2)

30

X 2 = 60

91

May

11

(11

- 6 = 5)

31

X 5 = 155

246

June

13

(13

- 6 = 7)

30

X 7 = 210

456

July

14

(14

- 6 = 8)

31

X 8 = 248

704

This method provides a basis for comparing the growing potential of different areas (see Table 2.3 and Figure 2.7).

This method provides a basis for comparing the growing potential of different areas (see Table 2.3 and Figure 2.7).

Table 2.3 Accumulated Heat Units for different places in Europe

Location

Accumulated Heat Units (AHUs)

May to June

July to Sept

Total

Edinburgh

300

700

1000

Glasgow

250

650

900

Belfast

300

700

1000

Manchester

425

875

1300

Norwich

430

950

1380

Birmingham

450

900

1350

Amsterdam

480

980

1460

Swansea

450

900

1350

London

470

950

1420

Littlehampton

450

950

1400

Channel Isles

480

970

1450

Paris

550

1100

1650

Bordeaux

600

1200

1800

Marseilles

800

1500

2300

of tropical crops such as sunflowers, tomatoes and sweet corn grown in a temperate area. Using this approach the extent to which bush tomatoes could be grown in Southern England for an expected yield of 50 tonnes per hectare in nine years out of ten could be mapped (see Figure 2.8).

The accumulated heat unit concept can also be used to estimate greenhouse heating requirements by measuring the extent to which the outside temperature falls below a base or control temperature, called 'degrees of cold'. In January, a greenhouse maintained at 18°C at Littlehampton on the coast in Sussex accumulates, on average, 420 cold-degrees compared with 430 for the Figure 2.8 Use of Ontario Units to determine the likely success of bush same structure inland at Kew, near London. For tomato crops. a hectare of glass this difference of 10 cold-

degrees C is the equivalent of burning an extra 5000 litres of oil. This provides a useful means of assessing possible horticultural sites when other data such as solar heating and wind speed are all brought together. Other methods based on this concept enable growers to calculate when different varieties of rhubarb will start growing and the energy requirements for chill stores and refrigeration units.

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  • Patrizio Esposito
    Which climate dominates british isles?
    3 years ago
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    Which crops grow well in the british isles climate?
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    What is the climate of the british isles called?
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  • james
    What is the average in the british isles?
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  • destiny
    What flows to the british lsles and creates a relatively mild climate for land at such high latitude?
    2 years ago
  • sarah
    What is the climat of britih isles?
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  • Rose
    What latitude has a monthly mean of 6.1 to 6.6 degrees c?
    1 year ago
  • ALLEN
    What is the weather of the british isles?
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