Global circulation patterns

What Is global circulation?

Global atmospheric circulation refers to the large-scale movement of air that redistributes heat and moisture around the planet. While weather systems constantly shift, when we average these patterns over many years, a consistent global flow emerges. This circulation is crucial for balancing temperatures between the equator and the poles, preventing extreme heat at the equator and excessive cold at the poles.

Why does it happen? Differential heating

The main driver of global circulation is the uneven heating of Earth’s surface. The sun’s energy is distributed differently due to the planet’s tilt, curvature, and atmospheric conditions. This creates temperature differences between the equator (which receives more direct sunlight) and the poles (which receive less). These differences set up a natural ‘air conditioning’ system, moving warm air poleward and cold air equatorward.

The three-cell model

In each hemisphere, the atmosphere is organised into three main circulation cells that extend through the troposphere (the lowest layer of the atmosphere, up to 10–15 km high):

Hadley cells:

• Extends from the equator to about 30–40° latitude.
• Trade winds blow toward the equator, where they rise in the Inter-Tropical Convergence Zone (ITCZ), forming thunderstorms.
• Air then flows to higher latitudes, sinking over subtropical regions, creating high-pressure regions over the subtropical oceans and deserts like the Sahara.

Ferrel cells:

• Found between 30–40° and 60–70° latitude.
• Air converges at low altitudes to rise at the boundaries between warm subtropical air and cool polar air.
• The Ferrel cell acts like a gear between the Hadley and Polar cells, with surface winds generally coming from the west or southwest in the northern hemisphere (influenced by the Coriolis effect).

Polar cells:

• The smallest and weakest, stretching from 60–70° latitude to the poles.
• Air sinks at the poles and flows toward lower latitudes at the surface.

The Coriolis effect and prevailing winds

Earth’s rotation causes the Coriolis effect, which deflects moving air to the right in the northern hemisphere and to the left in the southern hemisphere. This effect shapes the direction of prevailing winds in each cell and explains phenomena like trade winds and westerlies. It also influences the movement of air around areas of low pressure.

Jet streams and weather patterns

Jet streams—fast-moving ribbons of air high in the atmosphere—are closely linked to the boundaries between these cells. For example, the polar front jet stream forms where warm, moist air from the tropics, fed north by the surface winds of the Ferrel cell, meets cold, dry air moving south in the Polar cell. This boundary, known as the polar front, is a hotspot for unsettled weather, especially in regions like the UK.

The position of the jet stream is key to local weather:

• In summer, it usually sits north of the UK, leading to more settled weather.
• In winter, it shifts south, increasing the risk of storms and even snow if arctic air moves in.

Jet streams can flow in a straight west-to-east path (zonal flow) or meander north and south (meridional flow). When the flow is meridional, weather systems can stall, causing prolonged periods of rain or wind.

Weather (or low pressure) systems bearing rain and unsettled conditions move across the Atlantic on a regular basis. The jet stream guides these systems, so its position is important for UK weather.

The interplay of the Hadley, Ferrel, and Polar cells, combined with the Coriolis effect and jet streams, creates a global conveyor belt that transfers energy from the tropics to the poles. This system shapes prevailing winds and the distribution of deserts and rainforests around the world.

Jet streams and weather patterns

Jet streams—fast-moving ribbons of air high in the atmosphere—are closely linked to the boundaries between these cells. For example, the polar front jet stream forms where warm, moist air from the tropics, fed north by the surface winds of the Ferrel cell, meets cold, dry air moving south in the Polar cell. This boundary, known as the polar front, is a hotspot for unsettled weather, especially in regions like the UK.

The position of the jet stream is key to local weather:

• In summer, it usually sits north of the UK, leading to more settled weather.
• In winter, it shifts south, increasing the risk of storms and even snow if arctic air moves in.

Jet streams can flow in a straight west-to-east path (zonal flow) or meander north and south (meridional flow). When the flow is meridional, weather systems can stall, causing prolonged periods of rain or wind.

Weather (or low pressure) systems bearing rain and unsettled conditions move across the Atlantic on a regular basis. The jet stream guides these systems, so its position

Jet streams and weather patterns

Jet streams—fast-moving ribbons of air high in the atmosphere—are closely linked to the boundaries between these cells. For example, the polar front jet stream forms where warm, moist air from the tropics, fed north by the surface winds of the Ferrel cell, meets cold, dry air moving south in the Polar cell. This boundary, known as the polar front, is a hotspot for unsettled weather, especially in regions like the UK.

The position of the jet stream is key to local weather:

• In summer, it usually sits north of the UK, leading to more settled weather.
• In winter, it shifts south, increasing the risk of storms and even snow if arctic air moves in.

Jet streams can flow in a straight west-to-east path (zonal flow) or meander north and south (meridional flow). When the flow is meridional, weather systems can stall, causing prolonged periods of rain or wind.

Weather (or low pressure) systems bearing rain and unsettled conditions move across the Atlantic on a regular basis. The jet stream guides these systems, so its position is important for UK weather.

The interplay of the Hadley, Ferrel, and Polar cells, combined with the Coriolis effect and jet streams, creates a global conveyor belt that transfers energy from the tropics to the poles. This system shapes prevailing winds and the distribution of deserts and rainforests around the world.

Hadley cell

The largest cells extend from the equator to between 30 and 40 degrees north and south, and are named Hadley cells, after English meteorologist George Hadley.

Within the Hadley cells, the trade winds blow towards the equator, then ascend near the equator as a broken line of thunderstorms, which forms the Inter-Tropical-Convergence Zone (ITCZ). From the tops of these storms, the air flows towards higher latitudes, where it sinks to produce high-pressure regions over the subtropical oceans and the world's hot deserts, such as the Sahara desert in North Africa.

Ferrel cell

In the middle cells, which are known as the Ferrel cells, air converges at low altitudes to ascend along the boundaries between cool polar air and the warm subtropical air that generally occurs between 60 and 70 degrees north and south. This often occurs around the latitude of the UK which gives us our unsettled weather. The circulation within the Ferrel cell is complicated by a return flow of air at high altitudes towards the tropics, where it joins sinking air from the Hadley cell.

The Ferrel cell moves in the opposite direction to the two other cells (Hadley cell and Polar cell) and acts rather like a gear. In this cell the surface wind would flow from a southerly direction in the northern hemisphere. However, the spin of the Earth induces an apparent motion to the right in the northern hemisphere and left in the southern hemisphere. This deflection is caused by the Coriolis effect and leads to the prevailing westerly and south-westerly winds often experienced over the UK.

Polar cell

The smallest and weakest cells are the Polar cells, which extend from between 60 and 70 degrees north and south, to the poles. Air in these cells sinks over the highest latitudes and flows out towards the lower latitudes at the surface.

The Coriolis effect, winds and UK weather

Now we know about the Hadley, Ferrel and Polar cells, let’s take a look at how all that translates to what we see at the Earth’s surface. As a result of the Earth’s spin, each cell has prevailing winds associated with it, and we also have jet streams, all influenced by something called the Coriolis effect. This explains why air moves in a certain direction around an area of low pressure, and why trade winds exist. It also gives us an idea of why we see certain weather in and around the UK.

Warm moist air from the tropics gets fed north by the surface winds of the Ferrel cell. This then meets cool dry air moving south in the Polar cell. The polar front forms where these two contrasting air mass meet, leading to ascending air and low pressure at the surface, often around the latitude of the UK.

The polar front jet stream drives this area of unstable atmosphere. The UK and many other countries in Europe often experience unsettled weather, which comes from travelling areas of low pressure which form when moist air rises along the polar front.

Weather (or low pressure) systems bearing rain and unsettled conditions move across the Atlantic on a regular basis. The jet stream guides these systems, so its position is important for UK weather.

In summer, the normal position of the jet stream is to be to be north of the UK - dragging those weather systems away from our shores to give us relatively settled weather.

Normally the jet stream runs fairly directly from west to east and pushes weather systems through quite quickly. However, sometimes the steering flow of the jet stream can meander (a bit like a river), curving north and south as it heads east across the Atlantic. This is called a meridional flow, with the more linear west to east flow being called a zonal flow.

During a meridional flow areas of low pressure can become stuck over the UK leading to prolonged periods of rain and strong winds. During the winter the polar front jet stream moves further south leading to a greater risk of unsettled weather, and even snow if cold arctic air masses move south over the UK.

The continued effect of the three circulation cells (Hadley cell, Ferrel cell and Polar cell), combined with the influence of the Coriolis effect results in the global circulation. The net effect is to transfer energy from the tropics towards the poles in a gigantic conveyor belt.