The earth is enveloped by a vast ocean of air, called the atmosphere, at the bottom of which lives man. The total depth is about 100 miles, but at great heights the air is very thin, so that half the weight of the atmosphere is contained in the lowest 3.5 miles. This lower part is in a state of constant turmoil, the varied effect of which are recognised as weather.
Weather is something one has to live with, and, in the British Isles at least, it is a universal topic of conversation.
In everyday language weather means such qualities as wet, or fine, warm or cold, and until perhaps a hundred years ago these descriptive terms were adequate for most purposes. Since the growth of industry, however, the weather factor has become more significant economically and weather study has been put on an organised and scientific footing. Instead of descriptive terms, standardised terms are used. These apply chiefly to those factors, which are measured by instruments and specified numerically, like temperature and rainfall, but less precisely to some other factors like cloudiness and fog. The branch of science concerned with weather study is called meteorology.
Most countries maintain official meteorological services. As part of their functions these services have set up networks of weather observing stations at which regular observations are made and recorded according to internationally agreed procedures, at fixed hours of each day. This ensures that the weather at any station can be truly compared with that at any other.
Weather stations are of various kinds. Many provide observations used in weather forecasting, while others are concerned only with a general picture of the day’s weather. All reports are scrutinised and the stations inspected and the weather instruments checked by the controlling meteorological service.
Weather reports from the vast ocean areas are supplied by many hundred of merchant ships of all nations. These ships however, are not always at sea, and when they are they constantly change position, so that reports from a particular ocean area vary in number from day to day. For the past twenty years a small fixed network has been provided by ships, especially equipped for comprehensive meteorological observing and occupying fixed positions in the North Atlantic and North Pacific. These are called ocean weather ships, and they report four or eight times daily as do the major land station’s.
Weather can be simply measured by observing and recording temperature, rainfall, pressure, humidity, sunshine, wind and cloudiness. It is also possible to identify and name different types of clouds, which are associated with different patterns of weather. Commonly observed cloud types include cirrus, cumulus, cumulonimbus and stratus.
Meteorologists still use simple ground-based instruments to measure the various elements of the weather, including thermometers, rain gauges and barometers. However, to make really accurate weather forecasts it is useful to know what the current weather is like over a large geographical area. Weather radar and satellite photography can offer the meteorologist a snapshot of the weather in a single image across an entire continent. Radar uses microwaves to scan for raindrops. Wherever it is raining the raindrops bounce the signal and by listening to the returning pulse, the radar can compute the location and intensity of the rain. Weather satellites allow meteorologists to track the path and development of weather systems. Satellites don't just "look" in the visible part of the spectrum. They can also measure the temperature of the ground and the clouds by "seeing" in infrared. Some satellites even measure the amount of water vapour in the atmosphere.
The climate of the British Isles is influenced by the movement of all the major air masses, including maritime tropical (mT), maritime polar (mP), maritime Arctic (mA), continental polar (cP) and continental tropical (cT) air. The British Isles, residing in the mid-latitudes, lies in the path of air mass convergence between warm tropical airflow and cold polar airflow. At this convergence zone, the lighter warm air rises over the heavier cold air, producing the typical weather phenomena associated with this regime, fronts, depressions, and rain. As a consequence, the British climate can be very changeable, and all types of weather may be experienced within a single day. In general however, the British climate is relatively mild for its latitude, since it is influenced by the Gulf Stream. Warm maritime tropical (mT) air retains much of its heat because it is in contact with the Gulf Stream, a warm ocean current that originates in the Gulf of Mexico and travels northeast across the Atlantic. In contrast, Newfoundland, on the west side of the North Atlantic, at a similar latitude to the British Isles, can be up to 10°C colder in winter.
Although the British Isles makes up only a relatively small geographical area of the Earth's surface, differences in climate across the region do exist. The climate of the western half of the British Isles is dominated by maritime tropical and maritime polar air, whilst the climate further east is often influenced by more continental regimes. In addition to the major air masses, the strength of sunlight received at different locations in the British Isles, which varies according to latitude, also has an influence on climate. Consequently, the climate of the British Isles can be divided into four quarters. The northwest quarter, including Northern Ireland, the Lake District and the Western Isles of Scotland, is characterised by mild winters (average 6°C) and cool summers (average 15°C), whilst the northeast of Scotland has cold winters (average 3°C) and cool summers. The southwest quarter, containing the Irish Republic, Wales and Southwest England, experiences mild winters and warm summers (average 17°C), whilst the Midlands and Southeast England have cold winters and warm summers. In general, the western half of the British Isles experiences a more maritime climate during winter, whilst the east receives influence from the cold air streams from the continent. In summer, climatic differences are dominated more by latitude.
The greater influence of maritime air in the western half of the British Isles means that it receives considerably more rainfall than in the east. The generally higher ground in the west forces incoming Atlantic air to rise, further enhancing precipitation. Parts of highland Scotland, for example, can receive over 250 cm or 100 inches of precipitation per year. The east, by contrast, is lower and flatter, lying in the rain shadow of the Welsh Mountains, the Lake District and the Scottish Highlands, and is consequently much drier. Some parts of the Southeast of England may receive only 50 cm or 20 inches of precipitation per year.