7.2: 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:

• 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.
• Friction: 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 over the two locations, and thus 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 occurs due to a force called the Pressure Gradient Force. Here’s what you need to know about the pressure gradient force:

• The Pressure Gradient Force exists due to 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 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 \(\PageIndex{1}\): If two tanks with unequal amounts of water in them are connected, water will flow from the fuller tank to the less full 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 stops. (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 situated to the north, the 1012 mb isobar is located in the middle, while the 1016 mb isobar is located to the south of the map. 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)

3.  Using the information in Figure \(\PageIndex{3}\), higher pressure is located on the ____ side.
   1. North
   2. South

4.  If released from rest at point A, and allowed to travel on its own, a parcel of air will travel towards _____ due to the pressure gradient force.
   1. Point B
   2. Point C

5.  The magnitude of the pressure gradient force, which is determined by the pressure difference (4 mb) divided by the distance (50 km) between consecutive isobars, would be:
   1. 4 mb/50 km
   2. 8 mb/25 km
   3. 4 mb/100 km
   4. 4 mb/25 km

 Using a calculator, divide the difference by the distance to obtain a decimal number for the Pressure Gradient Force, expressed in mb/km.

6. Suppose we changed Figure \(\PageIndex{3}\) so that the distance between each isobar was doubled to 100 km instead of 50 km. Assuming that the pressure difference (4 mb) stays the same, the pressure gradient force will:
   1. double
   2. decrease by 50%
   3. stay the same

7. On the other hand, suppose we cut the distance between each isobar in half, so that each isobar is only 25 km away from the other. Assuming that the difference (4 mb) stays the same, the pressure gradient force will:
   1. double
   2. decrease by 50%
   3. stay the same

8. The pressure gradient force directly affects the motion of air. As the pressure gradient force increases, wind speeds increase. Based on questions 5-7, winds would be strongest when:
   1. The isobars are far apart from each other
   2. The isobars are closer to each other

Let’s investigate this more using a real weather map. Figure \(\PageIndex{4}\) is a map of weather conditions, fronts, data, and isobars for the continental United States at 1200 UTC on October 23, 2016.

Figure \(\PageIndex{4}\): Surface Weather conditions for the United States at 1200 UTC on October 23, 2016. (Public Domain; American Meteorological Society via NCEP/NWS) Alternative description of image.

9. Based on the conclusions we made in question 8, the strongest winds should be present over:
   1. California
   2. Alabama and Mississippi
   3. Montana and North Dakota
   4. Maine, Vermont, and New Hampshire   

10. On the other hand, winds are much calmer elsewhere, such as in the ______ where many locations have a “circle” outlining their station model instead of a wind pole, indicating calm winds.
    1. Northeast
    2. Southeast
    3. Northern Great Plains

Figure \(\PageIndex{5}\): A zoomed in version of Figure \(\PageIndex{4}\), focused on the Northeast United States. Boston's wind barb has been enhanced to make it more visible. (Public Domain; American Meteorological Society via NCEP/NWS) Alternative description of image.

Let’s examine the influence of the Pressure Gradient Force on a location’s wind direction. Figure \(\PageIndex{5}\) is the same map as Figure \(\PageIndex{4}\), but zoomed in on the Northeast United States so that we can see the wind directions more clearly. Boston, in eastern Massachusetts, has an air pressure of 1001.2 mb (its upper-right number is 012), and winds coming from the West-Southwest at a speed of 20 kts.  

11. If the wind is coming from the West-Southwest, it is traveling TOWARDS the:
    1. West-Southwest
    2. East-Northeast
    3. North-Southeast
    4. East-Southeast 

Print out Figure \(\PageIndex{5}\) or open it in a computer paint program to answer the following few questions. Draw an arrow extending out of Boston in the direction that the wind is blowing TOWARDS. This is the direction of the NET force on the wind. Now, we will break this force down into its component forces, starting with the pressure gradient force. The pressure gradient force causes air to move from areas of high pressure to areas of low pressure. To determine the direction of the pressure gradient force, draw a red arrow, pointing out of Boston’s station circle that is perpendicular to the nearest isobar (which is just northeast of Boston), and pointing towards the direction of low pressure.

12. The pressure gradient force over Boston should be moving the wind towards the:
    1. Northwest
    2. Northeast
    3. Southwest
    4. Southeast

But the net wind direction is not the same. This suggests that there are other forces, in addition to the pressure gradient force, acting on the wind that influence its speed and direction.