The major elements of climate are common to a large region of a country but the local climate can be specific to a very limited area. Plants are dependent upon the climate they experience locally, which is set within the general climate of the country.
Britain’s climate is dominated by the proximity of the Atlantic ocean. As an island on the west side of the Continent of Europe, within the latitudes where the earth’s rotation causes winds to blow predominantly from the west, Britain receives most of its weather from the Atlantic and has what is called a maritime climate.
The major elements of Britain’s weather originate as depressions in the southern Atlantic or Caribbean and are blown northeast across the ocean. Coming from the south the air is relatively mild but never really hot; also passing over water, it becomes moist.
The Gulf Stream or North Atlantic Drift is the main current flowing across the Atlantic ocean and follows the same direction. It is a warm current and has a warming influence on our seas, which makes the country much milder than areas such as Newfoundland at the same latitude but on the western side of the ocean.
When this warm moist air meets the shore, it is forced to rise over the land. This causes it to cool and the moisture condenses into cloud, bringing rain.
As the topography of the British Isles is higher along the western side, most of the rain falls along this western seaboard, with steadily reducing amounts reaching the east. Running from the northwest to the southeast, there is a steady decrease in total rainfall, from over 250cm (l00in) per annum to around 50cm (20in). This is remarkably constant for Ireland, Wales, Scotland and England, with the capital cities of each country situated in the driest zones.
Because of this rainfall gradient across the country, generally, there is also less cloud in the sky over the eastern half, giving more sunlight. The wind coming in from the ocean is slowed down as it crosses land. Sunlight and less wind makes the eastern half of the country warmer during the summer period than the west. Clear skies and still nights, however, give more frost and lower winter temperatures.
The other major influence on our climate is the Continent of Europe to the east. In summer this can bring in hot dry air, but in winter it can cause cold arctic conditions to run across from the east, or down from the north. These conditions are felt most strongly in Kent and along the eastern coast. Kent receives both the strongest impact from continental Europe and the weakest influence of the North Atlantic, giving it the most continental climate found in Britain.
There is relatively little effect of latitude on the climate because the main influences are from the west and the east. Close to the western coast, right up to beyond Ullapool in northwest Scotland, the climate is mild with few frosts. Further east, the temperature can drop to -18C (0 degrees F) from Kent to Aberdeen.
As two very different climatic influences are involved, it is possible for the temperature to fluctuate widely within very short periods. A mild January day of around 10 degrees C can be succeeded by a night frost of as much or even more below freezing. This switchback nature to the climate can cause plants from more staid continental ones to start into growth too soon and be caught by frost. Late spring and early autumn frosts are a feature of Britain’s climate.
The impact of the lower rainfall and higher temperatures and sunlight in the eastern half is that there is a potential excess of water use by plants over the rainfall received. Part of this deficit is made up from water stored in the soil during the winter, but in dry years, it will cause plants to slow down growth and ripen wood earlier. Plants which come from more continental climates do better in the east, where the extra sunlight and drier summer helps them to ripen the wood.
Where your garden in Britain is situated will affect the type of climate you receive, how much rain falls, the extremes of frosts recorded and the distribution of sunlight.
Local or Micro Climates
The local climate is dependent upon the national climate but influenced by local features, such as the topography, aspect and planting.
The differences between light soils, which warm up quickly and cool down as fast, and heavy soils, which do neither quickly, were discussed. These differences strongly influence the local environment, although they are insignificant on the national scale.
Local topography has a very pronounced impact on the climate. There will be quite marked differences between the south, east, west and north sides of a hill. The south side will receive more sunlight and be sheltered from cold north winds. The east side will get the early sunlight in the morning, with the increased chance of damage by unseasonal frosts; it will be exposed to north and east winds and sheltered from the west wind, so having less rainfall.
The west side will receive the full force of the west wind, complete with an extra ration of rain, but will be milder overall than the other sides. The north side will be permanently in the shade, and much colder. It will receive the worst of cold northerly winds; growth will start later in the year but damage from late spring frosts is less likely, although early autumn ones may damage plants if the growth has not ripened in time.
The sides of a hill will tend to be warmer than either the windier top or the bottom. This is because as air cools down, it becomes heavier and will drain down the sides to collect in a frost pocket lower down. Flat land tends to be open and exposed, with no natural air drainage to remove cold air.
The different aspects will receive different amounts of rainfall and sunlight, so that the relative humidity will be different. This can affect the growth of some plants.
The wind will vary with the aspect. The most obvious effect of wind is to cause the abrasion and breakage of plant parts; the main effect, however, is in reducing the rate of growth due to putting the plant under moisture stress. In Britain, this can result in significant losses of yield, and effects ranging up to 20 per cent reduction in yield have been recorded in crops such as potatoes.
Local planting will influence the climate. A screen of trees can reduce the wind speed and also shade the garden, giving a less frosty and more humid environment. If on the east side, they will slow down the rate at which the air warms up in the morning, reducing the damage caused by unseasonal frosts.
By drying out the soil, they can cause other plants to slow down their vegetative growth in late summer, leading to better ripening of the wood and possibly a higher set of flowers for next year.