What Force Drives The Wind?

Wind is the movement of air from areas of high pressure to areas of low pressure across Earth’s surface. It is an invisible yet powerful force that shapes our weather and climate. But what actually drives the wind? What causes air masses to be set in motion in the first place? In this article, we will explore the major factors that generate wind and drive global air circulation patterns.

Wind is ultimately driven by the uneven distribution of solar energy reaching Earth’s surface. This unequal heating sets convection currents in motion, creates areas of high and low pressure, and induces several forces that cause wind flow. The interaction of these forces produces the global winds that circulate around the planet.

Uneven heating of Earth’s surface

The sun’s rays hit the Earth at an angle, causing some areas like the equator to receive more direct sunlight than other areas like the poles. This uneven heating creates a temperature imbalance across the planet’s surface. As air heats up, the molecules spread apart, decreasing air density and pressure. Warm air at the equator rises and flows toward the poles, while denser, cooler air flows in to take its place. This circulation creates low pressure areas at the equator where air is rising, and high pressure areas around 30 degrees north and south where denser air is sinking. The differences in air pressure drive winds as air flows from high to low pressure regions, attempting to equalize the pressure imbalance. Areas receiving the most intense, direct sunlight develop into low pressure zones, while areas receiving less concentrated sunlight become relative high pressure zones.

Pressure gradients

The atmosphere is made up of high and low pressure systems. High pressure systems have denser, cooler air that sinks and spreads out near the ground surface. Low pressure systems have less dense, warmer air that rises into the atmosphere. The difference in air pressure between two points in the atmosphere is called the pressure gradient.

Air flows from areas of high to low pressure. The greater the difference in pressure, the faster the air flows. Air moves horizontally along the pressure gradient, creating wind. Wind flows from the higher pressure around a high pressure system toward the lower pressure around a low pressure system. The rotation of the Earth also impacts the direction winds flow due to the Coriolis effect.

Pressure gradients explain wind patterns on local and global scales. For example, land breezes occur when land heats up faster than the water during the day, creating an area of lower pressure over land. The higher pressure over the water pushes the air over land, creating a breeze. At night, the opposite occurs, creating a sea breeze as air moves from the higher pressure over land toward the lower pressure over the cooler water.

Coriolis Effect

The Coriolis effect is caused by Earth’s rotation and causes winds and ocean currents to curve. As Earth rotates on its axis, the equator is moving faster than the poles. This is because the equator has farther to travel in the same amount of time. An object moving between the equator and the poles will appear to veer off course due to the difference in velocities. This veering is called the Coriolis effect.

In the Northern Hemisphere, the Coriolis effect deflects winds and currents to the right. In the Southern Hemisphere, it deflects them to the left. The magnitude of the deflection depends on an object’s velocity and latitude. Faster moving objects and objects closer to the poles experience greater deflection. The Coriolis effect helps explain the rotation of cyclones and the circular motion of ocean currents.

The Coriolis effect impacts all large-scale wind patterns on Earth. The trade winds in the tropics and the westerlies in the mid-latitudes curve due to the Coriolis effect. Even smaller scale winds like sea breezes are impacted. The Coriolis effect must be considered when calculating long-range projectile motion over large distances.

Mountain and valley winds

Mountain and valley winds are local winds caused by the temperature differences between mountains and adjacent valleys. During the day, the mountain slopes receive more intense sunlight than the valleys, causing the air over the slopes to warm more rapidly. The warm air over the mountainside becomes less dense and rises upslope. Then, cooler denser air moves up the valley to replace the rising warmer air, creating an upvalley wind.

At night, the slopes rapidly radiate heat and cool down faster than the valleys. The air over the valleys becomes warmer than the air at the same elevation over the slopes. The cooler denser air from the mountainsides moves downslope as a mountain breeze, while the warmer, less dense air from the valleys flows up the valley as a downvalley wind.

The strength of these winds depends on the temperature difference between the mountains and valleys. In calm weather, these winds are generally light. But in some mountainous areas, daytime upvalley winds and nighttime downslope mountain winds can be strong and gusty, especially in clear weather and during strong surface heating and cooling.

Land and sea breezes

Land and sea breezes are local winds that occur due to temperature differences between the land and the sea. During the day, the land heats up more quickly than the sea because land has a lower heat capacity. This causes the air above the land to become warmer and less dense than the air above the sea.

The warmer, less dense air over the land rises, causing a region of lower pressure. The relatively cooler, denser air over the sea then moves in to replace the rising warm air, creating a breeze from the sea to the land called a sea breeze.

At night, the opposite effect happens. The land cools down faster than the sea, so the air above the sea is warmer. This causes the warm air over the sea to rise, and the cooler air over the land moves in to replace it, creating a land breeze blowing from the land out to sea.

The daily alternating cycle of land and sea breezes is most noticeable in coastal areas, particularly in the tropics. The sea breeze usually develops around late morning, peaks in the afternoon, then dissipates at night as the land breeze develops.

Global wind patterns

The uneven heating of the Earth’s surface causes major global wind patterns that influence weather around the world. Some major global wind patterns include:

Trade winds: These prevail between 30° latitude in both hemispheres. They blow from the subtropical high pressure belts towards the equator. In the Northern hemisphere, they blow from the northeast and are called the Northeast trade winds. In the Southern hemisphere, they blow from the southeast and are called the Southeast trade winds.

Westerlies: These are the winds that blow from the subtropical high pressure belts towards the subpolar low pressure belts in the middle latitudes between 30° and 60° latitude in both hemispheres. They generally blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.

Polar easterlies: These winds originate over the polar high pressure regions and blow towards the subpolar low pressure belts. They blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.

Jet streams: These are fast flowing, narrow air currents in the upper troposphere. They form along boundaries between hot and cold air masses and influence large scale weather patterns.

Doldrums: These are the low pressure belts near the equator where the trade winds converge, resulting in very light variable winds. This can create stagnant air that hampers sailing vessels.

Local winds

In addition to the global wind patterns, many localized winds develop due to the geography and temperature variations in specific regions. Some examples of well-known local winds include:

• Santa Ana winds – Dry, warm offshore winds that develop in Southern California and flow from the high desert areas toward the coast.
• Chinook winds – Warm, dry winds on the eastern side of the Rocky Mountains in the United States and Canada.
• Sirocco – Hot, dry, dusty wind that blows northward from the Sahara desert across the Mediterranean area.
• Mistral – Cold, dry wind in France that flows down from the north into the Mediterranean region.
• Bora – Cold gusty wind in the Adriatic coastal regions caused by cold air spilling over high mountains.

These local winds develop due to the interactions between large-scale weather patterns, topography, temperature gradients, and other regional factors. They can have significant impacts on local weather, temperature, precipitation, and wildfire risks in certain areas.

Effect of climate change

Climate change is having profound impacts on wind patterns across the globe. As average global temperatures rise, weather patterns are being disrupted, leading to shifts in wind directions, speeds, and intensity. Several key factors related to climate change are altering winds:

Rising ocean temperatures – Warmer oceans cause more evaporation, adding more moisture to the atmosphere. This extra moisture allows for more intense storms with higher wind speeds.

Changing air circulation – The difference in temperature between the poles and the equator drives major wind patterns. As the poles warm faster than the equator, these circulation patterns are weakening in some areas.

Shifting jet streams – The high-altitude jet streams are shifting out of historical norms, altering wind directions. For example, the northern polar jet stream has swung farther north in recent years.

Loss of sea ice – Diminishing sea ice in the Arctic exposes more open ocean. This allows for more evaporation and is impacting wind patterns in the northern hemisphere.

While climate change impacts many aspects of weather, winds and storm systems are particularly affected. Subtle changes in global temperatures, moisture levels, and circulation patterns can translate into substantial shifts in wind behavior across the world.

Conclusion

Wind is an incredibly complex phenomenon that is caused ultimately by the uneven heating of the Earth’s surface by the Sun. The temperature differences create areas of low and high pressure, causing air to flow from high to low pressure. This sets the air in motion as wind. The rotation of the Earth also impacts global wind patterns through the Coriolis effect. Wind is also heavily influenced by the shapes of landforms like mountains and valleys on local and regional scales. The daily heating cycles over land and water lead to land and sea breezes. Looking at Earth as a whole, the major wind belts circulate air and help transport heat from the equator to the poles. However, climate change is already altering many wind patterns around the world, with consequences yet unknown. While influenced by many complex factors, the fundamental driver of wind is sunlight striking our planet.