SQA National 5 Geography

Weather is the day-to-day state of the atmosphere: temperature, precipitation, wind, cloud and pressure. Within the United Kingdom it varies from place to place because four factors act on it: latitude, relief, aspect and distance from the sea.

Latitude

Latitude is how far a place is north or south of the equator. The UK stretches from about 50°N in the south of England to about 60°N in Shetland, and the further north a place is, the cooler it is on average.

The reason is the angle of the sun. In the south the sun is higher in the sky, so its rays hit the ground at a steeper angle and are concentrated on a smaller area. In the north the sun is lower, so the same amount of energy is spread over a larger area and each square metre receives less heat. Rays at a low angle also pass through a thicker layer of atmosphere, where more of the energy is absorbed or reflected before it reaches the ground. The difference is greatest in winter, when Shetland has only a few hours of daylight and southern England has noticeably more.

Relief

Relief is the height and shape of the land. It affects both temperature and rainfall.

Relief and temperature

Air cools as it rises, and the usual figure is a fall of about 1°C for every 100 metres of height gained. A summit at 900 metres should therefore be about 9°C cooler than a village at sea level nearby. Upland areas such as the Cairngorms are colder, windier and snowier than the lowlands around them for this reason, and snow lies on the high tops for much longer.

Worked example: a weather station at 150 metres records 12°C. The temperature at a summit at 1,050 metres is estimated like this.

Relief rainfall

Most of the winds that reach the UK blow from the south-west and carry moist air from the Atlantic. When this air meets high ground such as the Scottish Highlands, the Lake District or the Welsh mountains, it is forced to rise. As it rises it cools, the water vapour it holds condenses into cloud, and rain falls on the windward slopes and the summits.

By the time the air has crossed the hills and begins to descend, it has lost much of its moisture. It warms as it sinks, so clouds break up and little rain falls. The sheltered side of high ground is called a rain shadow. This is why the west of Britain is much wetter than the east. Fort William, in the western Highlands, receives far more rain in a year than Aberdeen, on the east coast.

Other types of rainfall

Rain in the UK comes from three processes, and each depends on air being forced upwards and cooled.

• Relief rainfall: air is forced up over hills and mountains, as described above.
• Frontal rainfall: warm air meets cold air at a front, and the lighter warm air is forced to rise over the colder, denser air. This brings the long spells of rain associated with depressions.
• Convectional rainfall: the sun heats the ground strongly, the air above it warms and rises, then cools and condenses into tall clouds. It produces short, heavy showers and thunderstorms, mainly on hot summer afternoons and more often in the south and east of England than in the north and west.

Aspect

Aspect is the direction a slope faces. In the northern hemisphere the sun is in the southern half of the sky, so south-facing slopes receive more direct sunlight and are warmer, drier and have a longer growing season. North-facing slopes are in shade for much of the day, stay colder and hold frost and snow for longer.

Aspect also matters for exposure to wind. A slope facing the prevailing south-westerly winds takes the full force of storms and receives more rain, while a slope facing away from them is more sheltered. In upland Scotland, settlements and farms are often on south-facing slopes, where it is warmer and the land is drier. Ski areas do the opposite and use north-facing slopes, because the snow stays on them for longer.

Distance from the sea

The sea heats up and cools down much more slowly than land. This changes the temperature of places depending on how close they are to it.

• In winter the sea is warmer than the land, so coastal places have milder winters, with fewer frosts and less snow.
• In summer the sea is cooler than the land, so coastal places have cooler summers.
• Places far from the sea have a larger range of temperature, with hotter summers and colder winters.
• Coastal places are also more likely to be windy and cloudy, and get more rain where the wind comes from the sea.

A climate that is moderated by the sea is called maritime, and one that is far from the sea and has a greater range of temperature is called continental. The whole of the UK is maritime by world standards, but the effect is stronger in the west. Parts of the west of Scotland are mild enough for palm trees to grow in gardens, such as at Inverewe, because the sea is kept warm by the North Atlantic Drift. This ocean current carries warm water from the Gulf of Mexico across the Atlantic and warms the air that blows onto western coasts. In contrast, places inland in the east of England, such as Cambridgeshire, can be much colder on a winter night.

Putting the factors together

The factors rarely act alone, so an explanation of a place's weather should combine them. A village in the Highlands on a south-facing slope at 300 metres, close to the west coast, is cooler than a village in Kent because of latitude and height. It is also wetter because it lies in the path of moist south-westerly winds and rain falls as the air rises over the mountains, and it is milder than an inland site at the same height because of the sea and the North Atlantic Drift.