A number of different processes form a complex mix of energy, water and moving air to produce our everyday weather and long-term climate.
As the Earth's surface warms, energy is emitted back into the atmosphere in a similar way that the hob of an electric cooker radiates heat. But if that's all that happened, the Earth's surface would be frozen, with an average temperature of around -18 °C - too cold to support life.
Instead, gases in the Earth's atmosphere absorb some of the outgoing energy and return part of it to the Earth's surface. These gases (water vapour, carbon dioxide, methane, nitrous oxide, ozone and some others) act like a blanket by trapping some of the heat. The greater the concentration of these atmospheric gases, the more effectively they return energy back to the Earth's surface, trapping even more heat and warming the Earth rather like a greenhouse. That's why this process is known as the greenhouse effect.
At any one time, the atmosphere contains many travelling weather systems with variable winds. When these winds are averaged over many years a well-defined pattern of large-scale 'cells' of circulation appears. These cells help to explain some of the different climate zones across the world.
The largest cells (named Hadley cells, after English meteorologist George Hadley) extend from the Equator to
30-40° latitude. Here, warm, water-laden air rises, condensing to form a broken line of thunderstorms, sustaining the world's tropical rainforests. From the tops of these storms, air flows towards higher latitudes where it sinks to produce high pressure regions with hot, dry air - the world's deserts.
Out-flowing air from these higher latitudes forms the trade winds that blow towards the Equator over the ocean.
At the opposite extreme, the smallest and weakest cells are the polar cells extending from 60-70° latitude to the poles. Here, the air is very dry and stable - Antarctica is the driest continent on Earth. The cold air sinks and flows away from the poles.
In between, in the mid-latitudes - where the UK is located - warm, moist air from the subtropics meets cold, dry air from high latitudes bringing unsettled wet weather typical of the temperate zones.
When water falls as precipitation, it may fall back in the oceans, lakes or rivers or it may end up on land. The oceans hold about 97% of the Earth's water, while the remaining 3% is the freshwater so essential for life. About 78% of freshwater is frozen in the ice-sheets of Antarctica and Greenland; 21% is stored in sediments and rocks below the Earth's surface; and less than 1% falls as precipitation and is found in rivers, lakes and streams.
Eventually, nearly all of the water that falls on land finds its way back to the ocean affecting the temperature, saltiness and density of different ocean regions. Colder, saltier water sinks in the oceans while warmer, less salty water rises. This overturning of the oceans creates warm and cold currents in different parts of the world and plays a significant part in determining the climate.
Weather is the temperature, wind and precipitation (rain, hail, sleet and snow) that we experience every day. Weather systems are constantly circulating within the Earth's atmosphere so what you see today may be different tomorrow.
An approximate 23° tilt in the Earth's axis also causes the atmospheric circulation cells to shift and the seasons to change. Yet weather follows identifiable patterns in different regions and over time. This is known as climate.
Changeable conditions are a feature of the British weather - a topic that is often used to break the ice in conversations. Nowadays, however, we need to consider what may happen around the world over the next century, not just the next few days.
Last updated: 26 September 2013