Have you ever felt a strong gust of wind and wondered what was behind it?
It isn’t magic and it isn’t some enormous, invisible blower. Every type of wind, ranging from a light breeze through the trees to a huge hurricane, is based on the energy generated by the Sun and the Earth’s rotation. Understanding where wind originates requires understanding air as a fluid in constant motion and responding to differences in temperature and pressure across the entire Earth.
The Sun: The Engine Behind Almost All Wind
In many ways, the primary driver of nearly all wind here on Earth is the Sun. The Sun radiates energy toward our planet. While that seems straightforward enough, it does not radiate that energy evenly. For instance, because the Sun shines down on the Equator more directly than anywhere else, most tropical locations tend to be warmer than those at the Poles. This uneven heating is what causes air to circulate throughout the atmosphere and ultimately generates wind.
Consider a simple illustration of what occurs when you boil a pot of water. When you first start boiling water, the water at the bottom of the pan is warmed by the flame below. That makes it less dense than the surrounding water and causes it to rise. Meanwhile, cooler, denser water will sink beneath it to replace it, and you have established a circulating current. The Earth’s atmosphere operates similarly; however, rather than being confined to a small pot of boiling water, this process takes place on a planetary level.
Temperature Differences and Pressure Changes
Air expands and becomes less dense as it warms. Since this warm, less-dense air has fewer molecules per unit volume than colder, denser air, it rises. The result is a decrease in pressure in the region where the air is rising. Conversely, when air cools, its molecules pack closer together, becoming more dense. Denser air is heavier than less dense air and will therefore sink. As it descends, it exerts downward pressure on the surface of the Earth, increasing the pressure in that location.
Wind exists primarily due to air flowing from high-pressure areas (where the air is densest) to low-pressure areas (where the air is least dense). This process seeks to establish equilibrium among the varying pressures along the Earth’s surface.
Pressure Gradient Forces and Air Flow
The greater the variation in pressure between two regions, the greater the force exerted on the air attempting to travel from one location to another. This force is referred to as the pressure gradient force. To illustrate this concept consider a hill: if a ball were placed atop that hill, it would roll down it quickly due to gravity. Likewise, when there is a steep pressure gradient force applied to an object, that object will move quickly.
A clear example of how a strong pressure gradient can produce strong winds can be seen in the vicinity of a thunderstorm. Within thunderstorms massive amounts of warm air rapidly ascend upward, producing numerous large low-pressure zones close together. Conversely, cool air sinks downward within these storms, creating several large high-pressure zones nearby. Together these conditions cause large variations in pressure over short distances.
Coriolis Effect: An Important Component
Even though Earth does not rotate completely horizontally and vertically (it rotates slightly in each direction), its rotation still produces an additional component affecting how wind blows: the Coriolis effect. On Earth this effect acts upon any moving object (including air and oceanic currents); it appears as a deflection force. Moving objects in the Northern Hemisphere appear to be deflected to the right while moving objects in the Southern Hemisphere appear to be deflected to the left. The Coriolis effect significantly affects larger-scale atmospheric circulation patterns but has little impact upon smaller-scale movements such as those occurring locally.
Although the Coriolis effect itself does not actually generate wind, it does affect how wind moves. The reason why tropical cyclones develop their characteristic circular pattern rather than moving directly into their center of low pressure is entirely due to the Coriolis effect. Without this effect, tropical cyclones would never develop their typical circular shape.
Friction: Another Influence
Every time wind flows across any feature of the Earth’s surface (such as mountains, forests, buildings etc.) it meets resistance. This resistance is referred to as friction. Friction can slow wind down; depending on whether features of the ground provide great obstacles (mountains), moderate obstacles (forests), or few obstacles (oceans or flat plains), friction reduces wind speed. Furthermore, friction changes wind direction; particularly close to the surface.
Above approximately 1000 meters (3300 ft), since friction provided by the surface is essentially non-existent, wind travels almost parallel to lines of equal pressure (isobars) as it reaches equilibrium between the pressure gradient force and Coriolis force; this type of wind is termed geostrophic wind. Close to the ground friction prevents such equilibrium from being achieved; thus wind is directed somewhat diagonally across lines of equal pressure (toward lower pressure).
Examples of Local and Global Wind Systems
By combining these three main components — differential heating of the surface by the sun, pressure gradients and their effects on air flow, Coriolis effect influencing direction of air flow, and friction slowing down air flow — we arrive at numerous examples of both local and global wind systems.
Global Circulation Cells: Large-Scale Atmospheric Circulation
While examples of local wind systems exist everywhere on Earth (from breezes caused by different temperatures of adjacent bodies of water to mountain/valley breezes), global circulation cells operate on a far greater scale. Due to unequal heating from pole-to-pole by sunlight and the Coriolis effect caused by Earth’s rotation, there are three main types of circulation cells found within Earth’s upper atmosphere: Hadley cells, Ferrell cells, and Polar cells. These circulation cells determine major atmospheric circulation patterns such as Trade Winds, Westerlies, and Easterly winds.
Examples of Local Wind Systems:
- Sea Breeze/Land Breeze Examples: A common example of local wind systems are sea breezes. Each afternoon when land receives direct sunlight it begins warming faster than any body of water located near it. As a result, low pressure develops over land while relatively higher pressure develops over cooler bodies of water. Thus, air flows from bodies of water to land providing cooling relief. At night the situation reverses: land cools faster than any body of water, and a land breeze forms.
- Mountain/Valley Breeze Examples: Similarly, each afternoon when mountain slopes receive direct sunlight they become warmer than valleys located at their base. Warm air rises from these slope areas creating a valley breeze. At night when mountain slopes cool down quicker than valleys located at their bases, cool air descends into these valley areas creating a mountain breeze.
Jet Stream Examples: Bands of extremely fast-moving air known as Jet Streams encircle our planet at high altitudes. These are formed by extreme variations in temperature between masses of air at different latitudes combined with the Coriolis effect. As such, Jet Streams function as ‘atmospheric highways’ that can transfer influences from distant weather systems across multiple continents.
Role of Turbulence
Rarely is wind completely smooth and unchanging; usually there are gusts and whirlpools present in addition to other chaotic motions known collectively as turbulence. Several items contribute to turbulence including friction with irregular surfaces; differences in horizontal wind velocity (shearing action); and thermal instability (when parcels of hot air rise irregularly).
Turbulent motion plays a role in distributing heat and moisture; transporting pollutants; and generally mixing fluids (in this case gases) within a liquid (the atmosphere).
What methods are available to measure wind?
Wind speed is commonly determined using an anemometer (which may include a rotating-cup anemometer) and/or wind vane measuring into which direction the wind is blowing. There are also newer technologies available for determining wind at varying heights such as Doppler radar or lidar.
Does wind power electricity generation?
Yes; wind represents a very viable alternative energy source for generating electricity using wind turbines. Wind turbine blades rotate into the passing airflow capturing kinetic energy which is converted into electrical energy. With growing interest in utilizing green-energy sources for meeting future needs, wind turbines are gaining popularity worldwide.
Are there any winds in space?
There are no “winds” in space because there is no atmosphere (a gas) existing outside of Earth’s atmosphere. However, the Sun continuously releases plasma into space known as solar-wind.
Sources
- Pressure, Temperature, and Density: How They Are Related — UCAR Center for Science Education
- The Coriolis Effect — National Oceanic and Atmospheric Administration (NOAA)
- What is wind? — Met Office (UK)
- Atmospheric Pressure — NASA Earth Observatory
- Wind — Wikipedia
