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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 the weight of the air above/around you pressing down on you. The air molecules within your ears, along with other objects such as the air in a water bottle, match the external air pressure by exerting an equal outward force. When external air pressure decreases rapidly, the air in your ear needs to equalize, exerting a net outward pressure on your ear, which is why your ears pop. The same principle applies to other objects containing air, such as water bottles, as shown in 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, compressing the bottle slightly. You can recreate this phenomenon yourself: buy a sealed bag of chips (don't open it, as that will let outside air in), and drive up a tall hill or mountain. After ascending a few thousand feet, the bag will appear to have puffed up. Now drive back down, and you'll see the bag deflate.  

    The effects of air pressure and elevation on a water bottle.
    Figure 6.1: The effects of air pressure on a water bottle. At higher elevations, there is less atmospheric air pressure squeezing the bottle, thus, causing it to inflate. As elevation decreases, external air pressure increases, compressing the bottle. (Public Domain; M9matr0902 via Wikimedia Commons)

    Air pressure is an essential component of weather for two main reasons: first, differences in air pressure at Earth's surface cause wind. 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 areas, sinking and resulting in clear skies and sunny weather. Thus, surface air pressure patterns significantly influence the weather conditions. For example, hurricanes are centers of very low surface pressure. Fire weather conditions, such as heat and high winds, are created by the movement of air from high pressure to low pressure. Even the movement of air masses is strongly governed by air pressure systems. Because of this, meteorologists want to detect air pressure patterns so they can more accurately forecast storm position and movement, wind patterns, the advection of warm/cold/moist/dry air, and the potential for hazardous weather. 

    In this investigation, we will explore how air pressure varies at the Earth's surface, 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: 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.
    • 6.2: 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.
    • 6.3: 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.
    • 6.4: 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.
    • 6.5: Alternative Text Descriptions for Investigation 6
      A set of detailed alternative text descriptions for Investigation 6

    Thumbnail: A Dosen Barometer measuring air pressure. Dosen Barometer by Langspeed is licensed under CC-BY-SA 3.0.

    This page titled 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 Neel Desai & Alicia Mullens .