☁️Meteorology Unit 7 – Atmospheric Pressure and Wind

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