Why does weather move west to east? How many times have you wondered about this question? Did you come up with an explanation?
The truth is that the west-to-east flow of weather has significant effects on Earth’s climate and atmospheric patterns.
But, why does weather system move from west to east? Let’s talk about some of the underlying scientific concepts that govern it.
The Earth’s rotation creates the Coriolis effect, which causes weather systems to travel from west to east.
Why Does Weather Move West to East?
You may have heard a lot about weather systems moving west to east. But, can the weather move east to west? Well, it can, but it is not that common.
It moves from west to east because of the following reasons:
The Earth’s Rotation
Our planet Earth is a fast-spinning globe, rotating around a tilted axis at about 1000 miles per hour.
The direction of the spin, from west to east, pulls the atmosphere and the weather along with it.
And ultimately, this produces an effect that influences the direction taken by major global weather systems.
Fact: Because the Earth is a sphere, the rotation speed at the equator is significantly faster than at the poles.
The Coriolis Effect
The Coriolis Effect is a strange phenomenon, described as an inertial or fictitious force, created because parts of Earth rotate at different speeds.
It makes airborne objects, such as planes and air currents appear to move through the atmosphere. They move following the surface curve rather than in the straight line they are traveling.
How Does It Affect the West-to-East Movement?
The Effect forces weather systems that have developed over warm tropical waters to deflect and turn away from the equator.
Depending on which side of it they formed, either right into the Northern Hemisphere or left into the Southern.
The Coriolis Effect also influences the direction weather systems spin around their low-pressure center.
- It is anticlockwise in the north to form hurricanes
- It is clockwise in the south forming cyclones
Global Wind Patterns
The surface winds that make up the global wind pattern, or ‘general circulation’ form three distinctive wind belts:
- The Polar Easterlies at 60-90 degrees latitude
- The Prevailing Westerlies at 30-60 degrees latitude
- The Tropical Easterlies called the trade winds at 0-30 degrees latitude.
Bands of clouds and thunderstorms form and encircle the globe.
Here, the easterly trade winds of both hemispheres meet in the Intertropical Convergence Zone (ITCZ) near the equator.
How Does It Affect the Weather Systems?
Under the hot sun and over warm shallow seas, the air gathers moisture.
The increased humidity makes the air buoyant and it expands and rises. It cools higher in the atmosphere to eventually release moisture as rain.
The ITCZ shifts according to the seasons.
Its position either just north or just south of the thermal equator changes the direction of the trade winds as they come under the influence of the Coriolis Effect.
When the ITCZ is north of the Equator, the southeast trade wind changes to a southwest wind.
Jet Streams
There are four main jet streams across the globe:
- Two at polar regions
- Two close to the equator
They exist because of the difference in temperatures between the Earth’s hemispheres.
One passes over the UK. It is a rapidly moving ribbon of air that forms at the edge of where the cold air meets warm in the atmosphere.
It is cold along its northern edge but warm along the southern.
Fact: Jet streams blow west to east and bring weather but being heat sensitive, exact positions shift.
Atmospheric Circulation and the Hadley Cells
The global movement of air is driven by temperature and its close relationship with air density and pressure and nature’s attempts to make them all equal.
There are three air systems making up the Hadley Cells, cycling air in each hemisphere, the small Polar Cells and the mid-latitude Hadley and Ferrel Cells.
The Polar Cells
Unlike the mid-latitude cells, the polar cells are relatively dry and cool.
However, there is still sufficient heat and moisture for a simple system of looping thermal currents to rise at the 60th parallel and move poleward before deflecting east.
They create a region of dry high pressure so that as air descends it pushes the air its replacing towards the 60th parallel.
The Hadley and Ferrel Cells
In the tropics, the Hadley and Ferrel cells cycle opposite one another. The air currents flow towards their respective poles along the top of the troposphere.
It is some 6 to 9 miles above the Earth’s surface before dropping down.
It drops down at the sub-tropics and returns to the equator by filling the area of low pressure left by the warm air rising in front of it at the ITCZ.
Some air is lost to the polar cells and the currents of the Ferrel which are weaker than the Hadley, can’t support a closed loop.
An Important Consideration
The cells are disrupted as the top layers of the two cells collide.
The Coriolis Effect determines whether the current deflects north or south to become either part of the westerly trade winds or the tropical easterlies.
The Trade Winds
Temperature differences create pressure differences and this triggers the movement of air.
Therefore, wherever there are winds, they always blow from areas of high air pressure too low. Around seven pressure belts are circling the Earth,
- Two Subpolar lows
- Two Subtropical highs
- Two Polar highs
- The Equatorial low
The Role of Trade Winds in Weather Movement
The trade winds (or the easterlies) form in the Equatorial low and are strong permanent west-to-east winds.
They flow from the mid-latitudes of the equator to come in from either:
- The northwest in the northern hemisphere
- The southeast in the southern, strengthening in winter.
- They bring tropical storms with them.
They developed over the warm equatorial waters of:
- Pacific
- Atlantic
- Indian Oceans
And they dump water on the landmasses of:
- North America
- Southeast Asia
- Madagascar
- East Africa
Historically, the reliable trade winds enabled exploration which eventually led to global trading.
Fact: The trade winds are responsible for delivering Saharan rich in phosphate and nitrate to South America, the Caribbean and southeast North America.
The Prevailing Westerlies
The prevailing westerlies are known as the anti-trades.
Like all weather, because of the spin of the Earth and the Coriolis Effect, the air currents of prevailing westerlies move west to east .
However, they begin in the higher horse latitudes, the result of subtropical high pressure meeting a subpolar low. Also, they tend to stay closer to the poles.
Fact: The prevailing westerlies blow strongest in the Southern Hemisphere where landmasses much less impede them.
Ocean Currents
On land, winds shape landscapes.
At sea, they drive ocean currents but there are only two types, the warm ocean currents leaving for the poles from the equator and the cold coming back.
Ocean currents transport warm waters from the east to the west near the equator and cold waters from the west to the east in higher latitudes.
And this impacts the movement of weather patterns in a similar direction.
More about the Role of Ocean Currents
There are many local coastal currents where the impact of the wind and waves is determined by land features and the slope of the continental shelf.
Because the Earth spins on its axis west to east, in the open ocean, the wind pushes water around in circles in ocean-wide gyres.
There are five and they are the jet streams of the oceans moving enormous volumes of water in different directions.
For instance:
| Gyre | Direction |
| North Atlantic | Clockwise |
| South Atlantic | Counterclockwise |
| North Pacific | Clockwise |
| South Pacific | Counterclockwise |
| Indian Ocean | Counterclockwise |
The direction an ocean current takes is determined by how the Coriolis Effect influences the wind pushing it.
Fact: The wind deflected into the northern hemisphere drives the Gulf Stream gyre clockwise bringing warm tropical air into Europe.
Climate Influences
As the cool air is warmed at the tropics, it expands to support water suspended and carried as vapor.
It is collected from the upper levels of the warm surface water as the cool air current is warmed by passing over it.
In the cold altitudes it condenses into water droplets that form clouds.
The Impact of Climate on Weather Movement
As long as the temperature remains fairly constant, the clouds can travel on the wind over the oceans for thousands of miles.
This mechanism distributes vital fresh water across the planet.
But, it relies on the right atmospheric conditions for accurate delivery, sadly something less guaranteed with climate change.
Takeaway
Why does weather move west to east? Essentially, the west-to-east movement of weather is the result of the earth’s rotation and the Coriolis effect.
It is important to understand the factors contributing to this phenomenon.
It helps us understand the intricate relationship between the planet’s atmosphere and the earth’s rotational forces.
