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Investigation 6: Air Pressure, Isobars, and the Hand-Twist Model

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    Introduction

    Have you ever experienced your ears popping while on an airplane or going through a tunnel? The reason this happens is due to changes in air pressure, which is simply defined as the weight of the air above/around you, pressing down on you. In fact, from the surface of the earth to the top of the atmosphere, there are approximately 14.7 pounds of air weighing down on every square inch of your body, assuming you are at sea level. As you rise in elevation, you have less air above you, hence lower air pressure. The molecules of your body (along with other objects such as the air in a water bottle) match the outside air pressure by pushing outward with their molecules. When air pressure is low, this causes the molecules of your body to push outward less, which is why your ears pop. The same principle applies to objects containing air, such as water bottles (Figure 6.1). At higher elevations, there is less air pressure pushing down on the bottle, causing it to "inflate." As elevation decreases, air pressure increases, causing the bottle to compress slightly. You can recreate this phenomenon yourself: buy a sealed bag of chips (don't open it, as that will fill it with outside air), and drive up a tall hill or mountain. After ascending a few thousand feet, it will appear as if the bag has puffed up. Now drive back down, and you'll see the bag deflate.  

    The effects of air pressure and elevation on a water bottle. At lower elevations, air pressure is higher, and the bottle is compressed.
    Figure 6.1: The effects of air pressure on a water bottle. Lower elevations lead to higher pressures, thus crushing the bottle (Public Domain; via Wikimedia Commons)

    Air pressure is an essential component of weather for two main reasons: First, differences in air pressure exist at the Earth's surface, causing wind to blow. Second, air spirals into areas of low pressure and is forced to rise, producing clouds and stormy weather. Conversely, air spirals out of high pressure, causing it to sink, resulting in clear skies and sunny weather. In this investigation, we will explore how air pressure varies at the Earth's surface (even at sea level), identify areas of low and high pressure, and then examine how air behaves near these pressure zones. 

    Learning Objectives
    • Draw isobars on a weather map.
    • Using an isobar map, identify areas of low and high pressure.
    • Using the hand-twist model, distinguish how air behaves near low pressure from high pressure, and explore how that affects weather

    • 6.1: Surface Air Pressure Patterns
      We explain that air pressure, which is affected by gravity and varies with elevation, plays a crucial role in weather forecasting. Meteorologists use air pressure patterns to predict storms and other weather events. Measurements from different altitudes are standardized to sea level, known as Sea Level Pressure, enabling better insight into pressure variations across locations for more accurate weather predictions.
    • 6.2: Air Pressure on a Weather Map
      Section covers sea-level pressure and its significance in weather mapping, noting the decrease of air pressure with elevation. It details how weather stations calibrate measurements to sea level for better data interpretation, with an average sea-level pressure of 1013.25 mb. It introduces isobars, lines connecting equal pressure points, as tools for visualizing pressure distributions and recognizing weather patterns.
    • 6.3: High and Low Pressure Systems
      The role of isobars in identifying high and low pressure systems on weather maps is covered here, highlighting that a "bulls eye" pattern signifies pressure variations. It associates low pressure with stormy weather and high pressure with fair conditions. Additionally, it underscores the need to consider other weather factors, such as moisture and frontal systems, to gain a comprehensive understanding of weather conditions, as pressure systems alone are insufficient for complete analysis.
    • 6.4: Cyclonic and Anticyclonic Flow - The Hand Twist Model
      This section introduces Hand Twist Model for understanding air flow in weather patterns, detailing Cyclonic Flow (counter-clockwise into low pressure) and Anti-Cyclonic Flow (clockwise away from high pressure). It offers a practical guide for visualizing these movements using hand gestures on maps of the U.S. Mastery of these concepts is essential for analyzing broader weather systems in the course ahead.
    • 6.5: Cyclonic and Anticyclonic flow in real life.
      This portion of the activity focuses on analyzing a "mystery pressure" system in Eastern Louisiana from August 30, 2017, using surface observation maps and wind directions. It explains key concepts of convergence and divergence, depicting the behaviors of low and high pressure systems. Readers are encouraged to observe wind patterns and infer the pressure system's type by examining the weather conditions presented.
    • 6.6: Alternative Text Descriptions for Investigation 6
      A set of detailed alternative text descriptions for Investigation 6

    Thumbnail: A barometer. Dosen Barometer is licensed under CC-BY-SA 3.0.

    Investigation 6: Air Pressure, Isobars, and the Hand-Twist Model is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by LibreTexts.