6.3: Cyclonic and Anticyclonic Flow - The Hand Twist Model

Air pressure systems, as identified at the end of the last section, are essential components of larger-scale weather patterns. As we continue in this course, we will begin shifting from smaller-scale physical properties (such as humidity and temperature) to larger-scale weather systems, including mid-latitude cyclones, hurricanes, and thunderstorms. But first, we need to understand the general behavior of air around high- and low-pressure systems, and we can introduce a simple model to demonstrate it. 

Cyclonic Flow

Figure \(\PageIndex{1}\): Blank map of the United States with a low-pressure center in the middle. (Public Domain; John M Wolfson via Wikimedia Commons)

Cyclonic flow describes how air flows around a low-pressure system at Earth's surface. We’ll cover the “whys” of cyclonic flow more in depth in Investigation 7, but for now, we’ll focus on the general behavior of air as it flows around a low-pressure system. To demonstrate the flow, a simple model, the Hand-Twist model, can be employed. Here’s how it works: 

• Step 1: Print out Figure \(\PageIndex{1}\), which is a map of the United States with a low-pressure system in the middle indicated by a red “L”.
• Step 2: Place your non-writing hand, with your fingers fully spread out, over the map with your palm over the center of the red “L”.
• Step 3: Using a pencil, label your fingers 1, 2, 3, 4, and 5. It doesn’t matter in which order you label the fingers. However, once you label a finger, that finger should keep its number for the remainder of the exercise. 
• Step 4: Twist the hand, one quarter turn counter-clockwise (to the left) and bring your fingers into the center of the red “L” as you do so. 
• Step 5: Label the position of your fingers using the same numbers as you did in Step 3. 
• Step 6: Draw arrows connecting the labels for each corresponding finger. For example, draw an arrow connecting the first “1” from step 3 to the second “1” from step 5. The arrow should point in the direction of the second position. 

The following video provides a nice demonstration of the Hand Twist model.

Video Title

Duration of video: 2:15

Video introduction: The Hand Twist Model shows the direction of airflow around low- and high-pressure systems.

 

13. The five lines you drew following the Hand-Twist model steps create what appears to be a: 
    1. triangle
    2. spiral
    3. square
    4. octagon

14. From the direction of the arrows, we can say that air rotates __________ a low pressure:
    1. clockwise and away from
    2. clockwise and into
    3. counter-clockwise and away from
    4. counter-clockwise and into 

15. Additionally, the palm of your hand is a good indicator of rising/sinking motion in a pressure system. As you progressed through the steps of the hand-twist model, the palm of your hand _________, indicating that air __________ in a low-pressure region. 
    1. rose, rises
    2. rose, sinks
    3. sank, rises
    4. sank, sinks

Anticyclonic Flow

While cyclones are areas of low pressure, anticyclones are areas of high pressure. Therefore, the behavior of the air around a high-pressure system is called Anti-Cyclonic flow. We can use the Hand-Twist model again to demonstrate Anti-Cyclonic flow, as we did for Cyclonic Flow. Here’s how to do it:

• Step 1: Print out Figure \(\PageIndex{2}\), which is similar to Figure \(\PageIndex{1}\), but has high pressure at the center. 
• Step 2: Start by bunching the fingers of your non-writing hand together, and place them over the center of the blue “H.”
• Step 3: Label the position of each finger 1-5, just like you did with the low pressure. 
• Step 4: Turn your hand one-quarter twist clockwise (to the right) and extend out your fingers fully as you do so. 
• Step 5: Label the new position of each finger, using the same numbers for each finger as you did in Step 3. 
• Step 6: Draw arrows connecting each position, with the arrow pointing in the direction of the second position. 

Figure \(\PageIndex{2}\): Blank map of the United States with a high-pressure center in the middle. (Public Domain; John M Wolfson via Wikimedia Commons)

The five arrows should create a pattern very similar to that of the hand-twist model for low pressure, but there are a few key differences. 

16. From the arrows you drew on Figure \(\PageIndex{2}\), we can say that air flows _________ the high-pressure region.
    1. clockwise and away from
    2. clockwise and into
    3. counter-clockwise and away from
    4. counter-clockwise and into 

17. Moreover, the palm of your hand _______ while doing the hand-twist model for high pressure, indicating that air _________ in a center of high pressure.
    1. rose, rises
    2. rose, sinks
    3. sank, rises
    4. sank, sinks