Setting the global picture: how air moves around the earth

“To understand what’s going to happen next in your sailing spot, you need to know what’s happening around the world.”

When planning a sailing trip, particularly in the relatively enclosed Adriatic, knowing where the wind comes from, where it might go next and what will be the drivers is vital. Most of the winds we experience aren't just random gusts. They are part of a much larger earth’s atmospheric circulation.

Figure 1. Idealised depiction (at equinox) of large-scale atmospheric circulation on Earth, Wikipedia

This post explores how the global engine of wind and weather works, and why it matters every time you raise your sails...

What sets the atmosphere in motion?

Uneven heating of the earth surface is at the origin of the air motion in the atmosphere. The equator gets far more sunlight and therefore heat than the poles. This temperature imbalance sets air moving:

• Nature tries to balance things out by moving air from warm to cold areas,
• Light warm air rises at the equator, generating low pressure and being dragged towards the poles in altitude. This result in the two Hadley air circulation cells on each sides of the equator.
• Heavy cold air sinks at the poles, generating high pressure and being dragged towards the equator at the surface of earth. This results in the two polar air circulation cells. 
• As the air in the polar cells and the Hadley cells are moving in opposite direction, "mid-latitude cells" are generated at their interface, called the "mid-latitude cells" in each hemisphere around the 30° latitudes of both hemisphere.
• Overall, it results in three cells of global air motion in each hemisphere, as depicted on figure 1 above.

But earth is rotating, and that adds a twist to the air circulation, the Coriolis effect.

The Coriolis effect: a deflection game

Because earth spins from the east to the west, moving air as described above doesn’t go straight from the equator to poles and back. It gets deflected by the earth rotation. The deflection is always considered in relation with the direction of the wind, which differs in the different cells, as explain in the section above.

• To the right in the northern hemisphere. 
• To the left in the southern hemisphere.

The Coriolis effect curves wind flow rather than going directly from high to low pressure. Overall, it affects the groups "air motion cells" differently:

1. Hadley Cell (0°–30° latitude)

• As describe above, hot and moist air rises at the equator, moves poleward aloft, then sinks around 30° latitudes, and returns to the equator as the trade winds. In short, at the surface of the sea this component of air motion heads towards the equator in both hemispheres.
• But it is deflected to the right by the Coriolis effect resulting in:
• the original air motion from the north to the south in the northern hemisphere being deflected to the right, resulting in the northeasterly trades wind in the northern hemisphere,
• the original air motion from the south to the north in the southern hemisphere being deflected to the right, resulting in the southeasterly trades wind in the southern hemisphere.
• These are reliable, steady winds, ideal for ocean crossings.

2. Ferrel Cell (30°–60° latitude)

• In the Ferrel cells, air sinks around 30° latitudes, then flows poleward at the surface of earth, meets cold polar air around 60° latitudes and rises again.
• Being deflected to the right by the Coriolis effect, it results in the westerly surface winds:
• blowing from the southwest in the northern hemisphere,
• blowing from the northwest in the southern hemisphere.

This is the zone of the Mediterranean and Adriatic. Weather here is dynamic, with storms, calm spells, and shifting winds.

3. Polar Cell (60°–90° latitude)

• In the polar cells, cold air sinks at the poles and flows towards the equator until reaching the 60°latitudes where it rises again before returning to the pole.
• It’s deflected by the Coriolis effect resulting in polar easterlies winds.

Less relevant to Adriatic sailing, but key to understanding polar outbreaks and winter highs.

Intercell boundaries: latitude zones between cells and jet streams

At the intersection of the cells described above, two phenomenons are appearing. At the higher altitude level, jet streams are forming, driven by temperature gradient, shaped by Coriolis effect, driving the overall motion of air masses. At the earth surface it results in specific weather patterns expressed at these boundary areas, to take into account for weather forecasting. 

Jet streams

Jet streams are high altitude circulation of air at the junction between air cells described above. They are narrow bands of very fast air high in the atmosphere (up to 400 km/h):

• Blowing west to east,
• Helping steer weather systems,
• Having meandering paths called Rossby waves creating troughs (cold dips) and ridges (warm bulges).

Jet streams are present between:

• Hadley and Ferrel cells (~30° latitude), where they are slower and weaker because of a lower gradient of temperature. The Coriolis effect being weaker at higher latitude results in more meandering of the jet stream.
• Ferrel and Polar cells (~60° latitude), where they are stronger because of the stronger pattern between cold polar air and warm intertropical warmer air. Stronger Coriolis effect closer to the equator results in more stable and less meandering jet streams on these latitudes.

Special zones between cells

These boundary areas shape the global wind belts.

- Intertropical convergence zone (ITCZ), along the equator:

• Where the trade winds meet,
• Hot, humid, unstable air,
• Thunderstorms, light winds, and squalls, not fun for sailors!

- Horse latitudes (~30° N/S)

• Where air sinks between the Hadley and Ferrel cells,
• High pressure, clear skies, little wind,
• Notorious calm zones for sailing ships, hence the name.

- Polar front (~60° N/S)

• Where Ferrel and polar cells meet,
• Stormy weather, frequent low-pressure systems,
• Source of many weather systems in Europe.