Meteorology

☁️Meteorology Unit 7 – Atmospheric Pressure and Wind

Atmospheric pressure and wind are fundamental concepts in meteorology, shaping weather patterns and climate. These forces drive air movement, creating high and low-pressure systems that influence our daily weather experiences. Understanding pressure gradients, wind formation, and measurement techniques is crucial for predicting weather patterns. From sea breezes to jet streams, various wind types play significant roles in atmospheric circulation and local weather phenomena.

What's Atmospheric Pressure?

  • Atmospheric pressure refers to the force exerted by the weight of the atmosphere on a unit area of surface
  • At sea level, standard atmospheric pressure is 1013.25 millibars (mb) or 29.92 inches of mercury (inHg)
  • Pressure decreases with increasing altitude because there is less air above to exert force
  • High-pressure systems have air sinking and diverging at the surface, resulting in generally clear, stable weather conditions
  • Low-pressure systems have air rising and converging at the surface, often associated with cloudy, unstable weather and precipitation
  • Pressure differences between high and low-pressure systems drive atmospheric circulation and wind patterns
  • Isobars are lines on a weather map connecting points of equal pressure, helping to visualize pressure gradients and wind flow

How Pressure Changes with Height

  • Pressure decreases exponentially with increasing altitude in the atmosphere
  • The rate of pressure decrease is approximately 1 mb per 10 meters in the lower atmosphere
  • At higher altitudes, the rate of pressure decrease slows down due to the thinning of the atmosphere
  • The pressure at the top of Mount Everest (8,848 m) is about one-third of the pressure at sea level
  • The relationship between pressure and height is described by the hypsometric equation, which takes into account temperature and humidity
  • In the International Standard Atmosphere (ISA) model, pressure decreases by half for every 5.5 km increase in altitude
  • Pressure changes with height affect the density of air, which has implications for aircraft performance and weather patterns
    • Aircraft require longer runways for takeoff at high-altitude airports due to the reduced air density

Measuring Atmospheric Pressure

  • Atmospheric pressure is measured using a barometer, which can be either a mercury barometer or an aneroid barometer
  • Mercury barometers measure pressure by balancing the weight of the atmosphere against a column of mercury
    • The height of the mercury column is proportional to the atmospheric pressure
  • Aneroid barometers use a sealed, flexible metal chamber that expands or contracts with changes in pressure
    • The movement of the chamber is translated into a pressure reading on a dial or digital display
  • Barometers are calibrated to display pressure in various units, such as millibars (mb), inches of mercury (inHg), or hectopascals (hPa)
  • Pressure readings are often adjusted to sea level pressure (SLP) to allow for comparison between locations at different elevations
  • Barographs are instruments that continuously record atmospheric pressure over time, creating a graph called a barogram
  • Altimeters in aircraft use pressure measurements to determine altitude above sea level, as pressure decreases with height

Pressure Systems and Weather Maps

  • High-pressure systems (anticyclones) are characterized by sinking air, clear skies, and generally stable weather conditions
    • Winds flow clockwise and outward around high-pressure systems in the Northern Hemisphere
  • Low-pressure systems (cyclones) are characterized by rising air, cloudy skies, and often unstable weather with precipitation
    • Winds flow counterclockwise and inward around low-pressure systems in the Northern Hemisphere
  • Weather maps use isobars to represent areas of equal pressure, allowing meteorologists to identify pressure systems and gradients
  • Closely spaced isobars indicate a strong pressure gradient and higher wind speeds, while widely spaced isobars suggest weaker winds
  • Troughs are elongated areas of low pressure, often associated with frontal systems and unsettled weather
  • Ridges are elongated areas of high pressure, typically bringing clear skies and stable conditions
  • The movement and interaction of pressure systems play a crucial role in determining weather patterns and the development of various weather phenomena (fronts, storms)

What Causes Wind?

  • Wind is the horizontal movement of air from areas of high pressure to areas of low pressure
  • Pressure gradients, which are the differences in pressure over a distance, are the primary driving force behind wind
  • The strength of the pressure gradient determines the wind speed, with stronger gradients resulting in faster winds
  • The Coriolis effect, caused by Earth's rotation, deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
    • This deflection creates the characteristic circular flow around pressure systems
  • Friction with the Earth's surface slows down wind speed and affects wind direction, especially in the lower atmosphere (planetary boundary layer)
  • Thermal differences, such as those between land and water or between different latitudes, can also generate wind patterns (sea breezes, jet streams)
  • Topography, including mountains and valleys, can channel or block wind flow, creating local wind patterns (mountain breezes, gap winds)
  • Convection, the vertical movement of air due to heating and cooling, can lead to the development of small-scale wind patterns (thermals, dust devils)

Types of Winds

  • Geostrophic wind is a theoretical wind that results from the balance between the pressure gradient force and the Coriolis force
    • It flows parallel to isobars at a constant speed and does not consider friction
  • Gradient wind is a more realistic approximation of wind flow that accounts for the pressure gradient force, Coriolis force, and centripetal acceleration
    • It flows parallel to curved isobars and varies in speed
  • Surface wind is the actual wind experienced near the Earth's surface, influenced by friction, topography, and local factors
  • Sea and land breezes are coastal wind patterns driven by temperature differences between land and water
    • Sea breezes blow from the cooler ocean to the warmer land during the day, while land breezes blow from the cooler land to the warmer ocean at night
  • Mountain and valley breezes are local wind patterns in mountainous areas, caused by temperature differences between the slopes and the valley floor
  • Jet streams are narrow bands of strong winds in the upper atmosphere, typically located at the boundaries between air masses of different temperatures
    • The polar jet stream and the subtropical jet stream are the most prominent examples
  • Monsoon winds are seasonal wind patterns that reverse direction between summer and winter, primarily affecting tropical and subtropical regions (South Asia, West Africa)

Wind Measurement and the Beaufort Scale

  • Wind speed is typically measured using an anemometer, which can be a cup anemometer, a propeller anemometer, or a sonic anemometer
    • Cup anemometers measure wind speed based on the rotation rate of three or four cups mounted on a vertical shaft
    • Propeller anemometers measure wind speed based on the rotation rate of a propeller mounted on a horizontal shaft
  • Wind direction is measured using a wind vane, which points in the direction from which the wind is blowing
  • Wind speed is usually expressed in meters per second (m/s), kilometers per hour (km/h), or knots (nautical miles per hour)
  • The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea or on land
    • It ranges from 0 (calm) to 12 (hurricane force) and describes the effects of wind on waves, trees, and structures
  • Wind gusts are sudden, brief increases in wind speed that can significantly exceed the average wind speed
    • Gusts are important for aviation, construction, and other activities sensitive to wind conditions
  • Wind shear refers to changes in wind speed or direction over a short distance, either horizontally or vertically
    • Wind shear can be hazardous to aircraft, especially during takeoff and landing

Pressure Gradients and Wind Patterns

  • Pressure gradients are the driving force behind wind, with wind flowing from high pressure to low pressure
  • The strength of the pressure gradient determines the wind speed, with stronger gradients resulting in faster winds
    • Closely spaced isobars on a weather map indicate a strong pressure gradient and high wind speeds
  • The direction of the pressure gradient force is perpendicular to the isobars, from high to low pressure
  • The Coriolis force, caused by Earth's rotation, deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
    • This deflection results in wind flowing roughly parallel to isobars, rather than directly across them
  • Geostrophic balance occurs when the pressure gradient force is balanced by the Coriolis force, resulting in geostrophic wind
  • Gradient wind balance occurs when the pressure gradient force, Coriolis force, and centripetal acceleration are in balance, resulting in gradient wind
  • Friction with the Earth's surface disrupts the geostrophic and gradient wind balance, causing wind to cross isobars at an angle (Ekman spiral)
    • The angle of deflection is greater over rough surfaces (land) than over smooth surfaces (water)
  • Convergence and divergence of wind can lead to vertical motion in the atmosphere, affecting weather patterns and the formation of pressure systems


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.