🧭Physical Geography Unit 8 – Atmospheric Pressure and Wind Patterns

What's the Deal with Atmospheric Pressure?

• Atmospheric pressure refers to the force exerted by the weight of the atmosphere on a unit area of Earth's surface
• At sea level, standard atmospheric pressure is defined as 1013.25 millibars (mb) or 29.92 inches of mercury (inHg)
• Pressure decreases with increasing altitude because there is less air above a given point
• High-pressure systems are associated with sinking air, clear skies, and generally fair weather
  • Air flows outward from the center of a high-pressure system
• Low-pressure systems are associated with rising air, cloudy skies, and stormy weather
  • Air flows inward towards the center of a low-pressure system
• Pressure differences between high and low-pressure systems drive the movement of air, creating wind

How Pressure Changes with Altitude

• Atmospheric pressure decreases exponentially with increasing altitude
• The relationship between pressure and altitude is described by the barometric formula: $P = P_0 \cdot e^{-\frac{Mgh}{RT}}$
  • $P$ is the pressure at a given altitude
  • $P_0$ is the pressure at a reference level (usually sea level)
  • $M$ is the molar mass of air
  • $g$ is the acceleration due to gravity
  • $h$ is the height above the reference level
  • $R$ is the universal gas constant
  • $T$ is the temperature in Kelvin
• The rate of pressure decrease with altitude is approximately 1 mb per 10 meters near sea level
• At higher altitudes, the rate of pressure decrease slows down due to the exponential nature of the relationship
• The troposphere, the lowest layer of the atmosphere, contains about 75% of the atmosphere's mass and nearly all of its water vapor and aerosols
• The tropopause, the boundary between the troposphere and the stratosphere, varies in altitude from about 8 km at the poles to 18 km at the equator

Global Pressure Belts: The Big Picture

• The global distribution of atmospheric pressure is characterized by alternating belts of high and low pressure
• Near the equator, the Intertropical Convergence Zone (ITCZ) is a low-pressure belt where the trade winds converge
  • The ITCZ shifts seasonally, following the sun's zenith point
• Subtropical high-pressure belts are located around 30°N and 30°S latitudes
  • These belts are associated with descending air, clear skies, and desert regions (Sahara, Kalahari, Australian Outback)
• Subpolar low-pressure belts are found around 60°N and 60°S latitudes
  • These belts are associated with the polar front, where cold polar air meets warmer mid-latitude air
• Polar high-pressure regions are located over the North and South Poles
  • These regions are characterized by cold, dense air that tends to sink and flow outward
• The pressure belts are not continuous and can be interrupted by seasonal changes and land-sea temperature contrasts

Wind Basics: Why Air Moves Around

• Wind is the horizontal movement of air from areas of high pressure to areas of low pressure
• Pressure gradient force (PGF) is the primary force that drives wind
  • PGF acts perpendicular to isobars (lines of constant pressure) from high to low pressure
• Coriolis force, caused by Earth's rotation, deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
  • The magnitude of the Coriolis force depends on wind speed and latitude, with the greatest effect at the poles and no effect at the equator
• Friction with Earth's surface slows down wind and causes it to cross isobars at an angle
  • The effect of friction is most significant within the planetary boundary layer (lowest 1-2 km of the atmosphere)
• Geostrophic wind is a theoretical wind that results from the balance between the PGF and the Coriolis force
  • Geostrophic wind flows parallel to isobars at a constant speed
• Gradient wind is a more realistic approximation that includes the effects of centripetal acceleration in curved flow around pressure systems
• Thermal wind describes the change in geostrophic wind with height due to horizontal temperature gradients

Major Wind Patterns Explained

• The global circulation of the atmosphere is driven by the unequal heating of Earth's surface and the Coriolis force
• Hadley cells are large-scale atmospheric circulation patterns that transport heat and moisture from the equator to the subtropics
  • Rising air near the equator (ITCZ) flows poleward at upper levels, descends in the subtropics, and returns to the equator as the trade winds
• Ferrel cells are mid-latitude circulation patterns that transport heat and moisture from the subtropics to the subpolar regions
  • These cells are driven by the convergence of the polar front and are characterized by rising motion and low pressure
• Polar cells are small-scale circulation patterns that occur over the poles
  • Cold, dense air descends over the poles and flows outward as the polar easterlies
• Jet streams are narrow bands of strong winds that flow near the tropopause
  • The polar jet stream is associated with the polar front and serves as a guide for mid-latitude storm systems
  • The subtropical jet stream is associated with the poleward edge of the Hadley cell and can contribute to the formation of subtropical high-pressure systems
• Monsoons are seasonal wind patterns that result from the differential heating of land and water
  • In summer, land heats up faster than water, leading to rising motion over land and a flow of moist air from the ocean (Southwest Monsoon in India)
  • In winter, land cools down faster than water, leading to sinking motion over land and a flow of dry air from the land to the ocean (Northeast Monsoon in India)

Local Wind Systems: Breezes and More

• Local wind systems are driven by small-scale temperature and pressure differences, often resulting from the differential heating of land and water surfaces
• Sea and land breezes are diurnal wind patterns that occur along coastlines
  • During the day, land heats up faster than water, leading to rising motion over land and a flow of cooler air from the sea (sea breeze)
  • At night, land cools down faster than water, leading to sinking motion over land and a flow of warmer air from the land to the sea (land breeze)
• Mountain and valley breezes are diurnal wind patterns that occur in mountainous regions
  • During the day, mountain slopes heat up, causing air to rise along the slopes (valley breeze)
  • At night, mountain slopes cool down, causing air to sink and flow downslope (mountain breeze)
• Katabatic winds are cold, dense winds that flow downslope under the influence of gravity
  • These winds can be particularly strong in Antarctica and Greenland, where cold air pools over the ice sheets and flows downslope to the coast
• Anabatic winds are warm, buoyant winds that flow upslope due to daytime heating of the mountain slopes
• Foehn winds are warm, dry winds that descend on the leeward side of mountain ranges
  • As air flows over a mountain range, it cools and condenses, releasing latent heat; on the leeward side, the air descends and warms adiabatically, resulting in a warm, dry wind (Chinook winds in the Rocky Mountains)

Measuring Pressure and Wind

• Atmospheric pressure is measured using a barometer
  • Mercury barometers measure the height of a mercury column that is supported by atmospheric pressure
  • Aneroid barometers use a sealed, flexible metal cell that expands or contracts with changes in pressure
• Pressure is typically reported in millibars (mb) or hectopascals (hPa), with 1 mb = 1 hPa
• Wind speed and direction are measured using an anemometer and a wind vane, respectively
  • Cup anemometers measure wind speed based on the rotation rate of three or four cups mounted on a vertical axis
  • Wind vanes indicate wind direction by aligning themselves with the wind flow
• Wind speed is typically reported in meters per second (m/s), kilometers per hour (km/h), or knots (1 knot = 1.852 km/h)
• Wind direction is reported as the direction from which the wind is blowing, using cardinal directions (N, NE, E, SE, S, SW, W, NW) or degrees (0° = N, 90° = E, 180° = S, 270° = W)
• Upper-air wind measurements are obtained using radiosondes, which are instrument packages attached to weather balloons
  • Radiosondes measure wind speed and direction at various levels in the atmosphere using GPS or radio navigation techniques
• Doppler radar can also be used to measure wind speed and direction by detecting the motion of precipitation particles or other airborne objects

Real-World Impacts on Weather and Climate

• Atmospheric pressure and wind patterns have significant impacts on weather and climate at various scales
• High-pressure systems are generally associated with fair weather, clear skies, and light winds
  • Subsidence in high-pressure systems can lead to the formation of temperature inversions, trapping pollutants near the surface (smog in Los Angeles)
• Low-pressure systems are associated with stormy weather, cloudy skies, and strong winds
  • Cyclones, such as mid-latitude low-pressure systems and tropical cyclones, can bring heavy precipitation, strong winds, and storm surges
• The position and strength of the jet streams can influence the development and trajectory of mid-latitude storm systems
  • A strong, zonal (west-to-east) jet stream tends to keep storm systems moving quickly, while a weak, meridional (north-south) jet stream can lead to more persistent weather patterns
• Monsoon winds play a crucial role in the distribution of precipitation in many parts of the world
  • The Asian Monsoon brings vital rainfall to agricultural regions in India, Southeast Asia, and China
  • Variations in monsoon strength and timing can lead to droughts or floods, with significant impacts on food production and the economy
• Local wind systems can influence the microclimate of a region
  • Sea breezes can moderate temperatures and bring moisture to coastal areas, while land breezes can lead to the formation of offshore fog
  • Mountain and valley breezes can affect the distribution of temperature, moisture, and air pollutants in mountainous regions
• Long-term changes in atmospheric pressure and wind patterns, such as those associated with climate change, can have far-reaching effects on regional and global climate
  • Shifts in the position of the ITCZ or the strength of the monsoons could alter precipitation patterns and impact water resources in many parts of the world
  • Changes in the frequency or intensity of mid-latitude storm systems could affect the distribution of temperature and precipitation in the mid-latitudes