🌦️Atmospheric Science Unit 7 – Atmospheric Pressure and Wind Systems

Key Concepts and Definitions

• Atmospheric pressure the force exerted by the weight of the atmosphere on a unit area
• Isobars lines on a weather map connecting points of equal atmospheric pressure
• Wind horizontal movement of air from areas of high pressure to areas of low pressure
• Coriolis effect apparent deflection of moving objects, including wind, due to Earth's rotation
• Pressure gradient force (PGF) drives wind from high to low pressure areas
• Geostrophic wind theoretical wind resulting from the balance between PGF and Coriolis force
• Barometer instrument used to measure atmospheric pressure (mercury barometer, aneroid barometer)
• Anemometer device used to measure wind speed and direction (cup anemometer, wind vane)

Atmospheric Pressure Basics

• Atmospheric pressure decreases with increasing altitude due to less air above
• At sea level, average atmospheric pressure is approximately 1013.25 millibars (mb) or 29.92 inches of mercury (inHg)
• Pressure changes with weather systems high pressure associated with fair weather, low pressure with storms
• Pressure differences drive atmospheric circulation and wind
• Rapid pressure changes can indicate severe weather (rapidly falling pressure may signal an approaching hurricane)
• Air pressure affects various processes, including breathing, cooking, and weather balloon expansion
• Atmospheric pressure is a key factor in weather forecasting and understanding global circulation patterns

Factors Influencing Pressure

• Altitude atmospheric pressure decreases with increasing height above sea level due to less air above
  • Pressure decreases by about 1 mb for every 10 meters of altitude gain
• Temperature warm air is less dense and rises, leading to lower pressure; cold air is denser and sinks, causing higher pressure
• Moisture content moist air is less dense than dry air, resulting in lower pressure
• Gravity Earth's gravitational pull on the atmosphere contributes to atmospheric pressure
• Latitude pressure belts form due to uneven heating of Earth's surface and atmospheric circulation patterns
  • Equatorial low, subtropical highs, subpolar lows, and polar highs
• Topography mountains can block or channel air flow, affecting local pressure patterns
• Seasonal changes variations in solar radiation throughout the year influence global pressure patterns and wind systems

Global Pressure Systems

• Equatorial low (Intertropical Convergence Zone, ITCZ) low pressure belt near the equator due to intense solar heating and rising air
• Subtropical highs (Hadley cells) high pressure belts around 30°N and 30°S latitude, characterized by descending air and fair weather
  • Examples include the Bermuda High and the Pacific High
• Subpolar lows (Ferrel cells) low pressure belts around 60°N and 60°S latitude, formed by the convergence of polar and subtropical air masses
• Polar highs high pressure systems over the North and South Poles due to cold, dense air
• Monsoons seasonal wind patterns caused by pressure differences between land and ocean (Indian Monsoon, East Asian Monsoon)
• Semi-permanent pressure systems influence global wind patterns and climate (Aleutian Low, Siberian High)

Wind Formation and Patterns

• Wind is driven by pressure differences (pressure gradient force, PGF) from high to low pressure
• Coriolis effect deflects wind to the right in the Northern Hemisphere and left in the Southern Hemisphere
• Geostrophic wind results from the balance between PGF and Coriolis force, flowing parallel to isobars
• Surface friction slows wind and causes it to cross isobars at an angle (10-30°)
• Wind speed is determined by the pressure gradient steeper gradient results in stronger winds
• Local wind patterns influenced by topography (mountain-valley breezes, land-sea breezes)
  • Katabatic winds cold, dense air flowing downslope (Mistral in France, Santa Ana in California)
• Global wind patterns (Hadley cell, Ferrel cell, Polar cell) drive atmospheric circulation and heat transport

Measuring Pressure and Wind

• Mercury barometer measures atmospheric pressure using a column of mercury in a glass tube
  • Pressure is measured in inches of mercury (inHg) or millimeters of mercury (mmHg)
• Aneroid barometer measures pressure using a sealed, flexible metal cell that expands or contracts with changes in pressure
  • Often used in portable weather stations and altimeters
• Cup anemometer measures wind speed by counting the rotations of cups mounted on a vertical axis
  • Wind speed is typically reported in knots, miles per hour (mph), or meters per second (m/s)
• Wind vane indicates wind direction by pointing into the wind
  • Direction is reported using cardinal directions (N, NE, E, SE, S, SW, W, NW)
• Doppler radar can measure wind speed and direction by detecting the motion of precipitation or dust particles in the atmosphere
• Weather balloons (radiosondes) measure pressure, temperature, and humidity at various altitudes in the atmosphere

Real-World Applications

• Weather forecasting atmospheric pressure patterns and changes are used to predict weather conditions
  • High pressure generally associated with fair weather, low pressure with storms and precipitation
• Aviation atmospheric pressure affects aircraft performance (lift, engine efficiency) and altimeter settings
• Wind energy wind speed and consistency are crucial factors in selecting locations for wind turbines
• Agriculture wind can influence crop growth, pollination, and soil erosion
  • Windbreaks (trees, shrubs) can be used to reduce wind speed and protect crops
• Architecture and construction wind load must be considered when designing buildings and structures
• Sports and recreation wind affects activities such as sailing, kiteboarding, and golf
• Air pollution and dispersion wind patterns can transport pollutants and impact air quality in downwind areas

Common Misconceptions

• "Atmospheric pressure only affects the weather" pressure influences many aspects of daily life, including breathing, cooking, and tire inflation
• "Low pressure always means bad weather" while low pressure systems often bring storms, not all low pressure areas are associated with severe weather
• "Wind blows from east to west due to Earth's rotation" wind direction is primarily determined by pressure gradients and the Coriolis effect, not Earth's west-to-east rotation
• "Wind speed and wind velocity are the same" wind velocity is a vector quantity that includes both speed and direction, while wind speed is a scalar quantity
• "Atmospheric pressure is constant across Earth's surface" pressure varies with altitude, temperature, moisture content, and weather systems
• "The Coriolis effect is caused by centrifugal force" the Coriolis effect is an apparent force resulting from Earth's rotation, not centrifugal force
• "Atmospheric pressure only decreases with altitude" pressure can increase with altitude in specific weather conditions, such as temperature inversions