8.1: Why the wind blows

Forces that act on the wind

Because wind is air in motion, it is governed by Newton’s three laws of motion, which are as follows:

Newton's Laws of Motion

• First Law: An object in motion stays in motion, and an object at rest stays at rest, unless acted upon by a net force.
• Second Law: The net force exerted on an object is equal to the object’s mass times its acceleration: \(\sum F = m*a\)
• Third Law: Every action has an equal and opposite reaction. (We won’t need it here.)

Assuming that the mass of the atmosphere stays constant, the net force on the air is proportional to the air’s acceleration. In short, forces acting on air accelerate is to cause wind. There are three types of acceleration that we’ll cover, and three forces that accelerate the wind are:

• Pressure Gradient Force: Speeds up the wind.
• Coriolis Force: Changes the wind’s direction.
• Frictional Force: Slows the wind down.

Pressure Gradient Force

Surface air pressures can vary horizontally between two locations on the Earth's surface. This results in a difference in the number of molecules between the two locations, creating an imbalance. Natural systems tend to seek equilibrium, so air travels from one location to another until the two have the same air pressure. This movement of air results from a force called the Pressure Gradient Force. Here’s what you need to know about the pressure gradient force:

• The Pressure Gradient Force arises from a pressure imbalance between two locations.
• The Pressure Gradient Force moves air from areas of high pressure to areas of low pressure.
• The strength of the Pressure Gradient Force is determined by the ratio of the pressure difference between a high-pressure area and a low-pressure area, and by the distance between them.

A good demonstration of how the pressure gradient force works can be made by connecting two identical tanks: one filled with water and the other only half full (Figure \(\PageIndex{1}\)). A connector near the bottom allows water to flow between the tanks. Because there is a difference in the water level, a pressure difference exists near the bottom between the two tanks, and water will flow from the fuller tank to the less full tank until the two tanks have the same water level (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{1}\): Imagine two tanks are filled with unequal amounts of water and are connected to each other near the bottom. The full tank will have higher pressure near the base, while the half-full tank will have lower pressure. Net water movement occurs from the higher-pressure region to the lower-pressure tank. (CC BY 4.0; Alicia Mullens)

Figure \(\PageIndex{2}\): If both the tanks have equal levels of water, they have equal pressure, and net water movement ceases. (CC BY 4.0; Alicia Mullens)

Let's consider how a similar situation can occur in the atmosphere. Figure \(\PageIndex{3}\) is a simplified weather map, consisting of three isobars that are each 50 km apart, and have a difference of 4 mb from the other. A parcel of air, represented by a red dot, is located at point A, and lies on the 1012 mb isobar.

Figure \(\PageIndex{3}\): A simplified surface weather map shows three horizontal isobars separated by 50 km from each other. The 1008 mb pressure isobar is to the north, the 1012 mb isobar is in the middle, and the 1016 mb isobar is to the south. Point A lies on the 1012 mb isobar, Point B lies to the south of the 1016 mb isobar, while Point C lies to the north of the 1008 mb isobar. (CC BY 4.0; Alicia Mullens)

1. Based on the information in Figure \(\PageIndex{3}\), higher pressure is on the _____ side of this map.
   1. north
   2. south
2. If a parcel of air is released from rest at point A and allowed to travel on its own, it will travel towards _________ due to the pressure gradient force.
   1. point B
   2. point C