6.4: Cyclonic and Anticyclonic Flow - The Hand Twist Model

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 to progress in this course, we will begin shifting from smaller scale physical properties (such as humidity and temperature) to larger scale weather systems, such as mid-latitude cyclones, hurricanes, and thunderstorms. In this investigation, we are going to uncover the general behavior of air around high and low pressure systems, and introduce a simple model that can help demonstrate this behavior. At the end of the investigation, we’ll apply the model to specific weather situations. 

Cyclonic Flow

Cyclonic Flow represents how air flows around a low pressure system at the surface of the earth. 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 called the hand twist model can be employed. Here’s how it works: 

• Step 1: Print out the image below, Figure 6.4.1, 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 the position of 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 label for the remainder of the exercise). 
• Step 4: Twist the hand, one quarter turn counter-clockwise (aka “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 fingers (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. 

Figure \(\PageIndex{1}\): Blank Map of the United States with a Low Pressure center in the middle. (Public Domain; Derivative of Blank Map of States via Wikimedia Commons)

A demonstration video can be found here

13. The five lines you drew from the steps above create what appears to be a: 
    1. Triangle
    2. Spiral
    3. Square
    4. Octagon
14. From the direction of the arrows you drew on figure 4 above, we can say that air rotates ____________________ around 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 indication 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 low pressure. 
    1. Rose, Rises
    2. Rose, Sinks
    3. Sank, Rises
    4. Sank, Sinks

Anticyclonic Flow

While Cyclones are areas of low pressure, Anti-cyclones are areas of high pressure. Therefore, the behavior of air around a high pressure system is called Anti-Cyclonic Flow. We can use the hand-twist model to demonstrate Anti-Cyclonic Flow in a manner similar to how we demonstrated Cyclonic Flow. Here’s how to do it:

• Step 1: Print out Figure 6.4.2, below, which is similar to Figure 6.4.1, but has a High Pressure at the center. 
• Step 2: Bunch 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 (aka “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; Derivative of Blank Map of States via Wikimedia Commons)

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

16. From the arrows you drew on Figure 5, we can say that anticyclonic motion, how air flows around high pressure, is ___________________________ the high pressure.
    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