Atmosphere and climate

• Global atmospheric circulation transfers heat from the equator around the globe in three cells known as the tri-circular model, creating a global pattern of atmospheric pressure and winds.
• The whole system of global atmospheric circulation is driven by the equator, which is the hottest part of the Earth.
• Air rises at the equator, leading to low pressure and rainfall.
• When the air reaches the edge of the atmosphere, it cannot go any further and so it travels to the north and south.
• The air becomes colder and denser, and falls, creating high pressure and dry conditions at around 30° north and south of the equator.
• Large cells of air are created in this way.
• Air rises again at around 60° north and south and descends again around 90° north and south.
• Global atmospheric circulation creates winds across the planet and leads to areas of high rainfall, like the tropical rainforests, and areas of dry air, like deserts.
• The first cell in global atmospheric circulation is called the Hadley cell.
• At the equator, the ground is intensely heated by the sun, causing the air to rise which creates a low-pressure zone on the Earth's surface.
• As the air rises, it cools and forms thick cumulonimbus (storm) clouds.
• The air continues to rise up to the upper atmosphere, and the following then happens: The air separates and starts to move both north and south towards the poles.
• When the air reaches about 30° north and south, it cools and sinks towards the ground forming the subtropical high-pressure zone.
• As the air sinks, it becomes warmer and drier.
• The Hadley cell is complete when the air completes the cycle and flows back towards the equator as the trade winds.
• The Hadley cell is a region of little cloud and low rainfall, where deserts are found.
• The boundary between the warm and cold air is called the polar front.
• The Polar high then flows towards the lower latitudes.
• The polar front accounts for a great deal of the unstable weather experienced in these latitudes.
• The warmer air from the tropics is lighter than the dense, cold polar air and so it rises as the two air masses meet.
• The Polar cell At the poles, air is cooled and sinks towards the ground forming high pressure, this known as the Polar high.
• In the southern hemisphere, the winds flow to the left and are called the southeast trade winds.
• In the northern hemisphere, the winds flow to the right and are called northeast trade winds.
• These winds pick up moisture as they travel over the oceans.
• This uplift of air causes low pressure at the surface and the unstable weather conditions that are associated with the mid-latitude depressions.
• The Coriolis force, due to the spinning of the Earth, deflects movement of particles and wind.