Meteorology Unit 3: Air Pressure and Wind Systems

• Unit 3 of Meteorology focuses on winds, air pressure impact, small and large scale wind systems plus wind determination/measurements

Atmospheric Pressure

• Pressure exerted by a mass of air above a given location
• Air pressure decreases as height increases
• Warm air aloft usually means high pressure
• Cold air aloft usually means low pressure
• Pressure gradient force is established between two adjacent areas

Daily Pressure Variations

• Changing temperature causes predictable air pressure changes
• High temperature, low pressure happens during summer in the SW United States
• Winter comes with low temperature and high pressure
• Pressures vary daily based on the sun's energy warming Earth
• Tropical regions have biggest daily changes due to direct radiant energy

Pressure Measurements

• Barometers measure air pressure
• Typically recorded in millibars of mercury (mb Hg)
• Mercurial barometers are not commonly used
• Aneroid (no liquid) barometers are used in most applications

Surface & Upper-Level Charts

• Isobars connect equal pressure points
• Isobars are drawn every 4 mb of pressure
• Isobars usually flow to points "close to" 1000, 1004, 1008 mb et
• Lines will not cross and often form loops
• Air pressure changes with elevation, pressures are adjusted to sea-level
• Isobaric charts (aka constant pressure chart) are used by meteorologists
• High pressure areas are shown by ridges
• Low pressure areas are shown by troughs
• Winds blow clockwise around high pressure (anti-cyclone)
• Winds blow counterclockwise around low pressure (cyclonic storms or depressions)
• Charts are to determine wind flows and weather system movements

Newton's Laws of Motion

• First Law: Objects stay at rest or in motion unless acted upon by external force
• Second Law: Force = mass x acceleration (F=ma)
• To determine which direction the wind will blow, Pressure-gradient, Coriolis, and friction forces must be observed

Forces That Influence Winds

• Pressure Gradient Force is defined at Difference in pressure/distance, abbreviated as PG = Δρ/d
• Large Pressure gradient is formed when the change is rapid over small distance
• Small Pressure gradient is formed when the change is over large distance
• Closely spaced isobars are steep Pressure gradients, with winds
• Far apart isobars are gentle Pressure gradients, with low winds
• Winds blow from high to low pressure

Coriolis Force

• The result of Earth’s rotation that deflects winds and currents
• Winds deflect clockwise in the N Hemisphere and counterclockwise in the S Hemisphere
• Deflection increases with wind speed
• Greatest deflection is at poles, none at equator
• Winds blow clockwise around high pressure, counterclockwise around low pressure, opposite in S. Hemisphere

Friction Force

• Ground friction slows the wind
• Extends up to ~1000m (~3300ft) from the surface
• Aloft has less friction, so winds are faster
• Friction reduces wind speed and the Coriolis force

Winds + Vertical Air Motions

• Winds blow inward at low pressure, so air rises
• Winds blow outward at high pressure, so air sinks
• Vertical motion is slower than horizontal wind motion
• Air flows upward from high pressure at the ground to lower pressure aloft
• The downward force of gravity balances the vertical movement, which is called hydrostatic equilibrium

Scales of Atmospheric Motion

• Wind refers to air in motion and it occurs on many levels
• Microscale is the smallest scale
  • With diameters a few meters or smaller
  • Are short-lived
  • Sway branches and swirls dust
• Mesoscale is the "city-scale"
  • Includes local breezes (land/sea)
  • Spans few - 100 km
  • Lasts longer than microscale
  • Includes thunderstorms/tornadoes
• Macroscale: the largest scale, includes both synoptic and global scales
  • Synoptic scale is aka weather map scale
    • Shows patterns over several days over regions that span across US
    • Includes big storms that move across the US such as typhoon/hurricane
  • Global scale is planetary: wind patterns range over the entire Earth

Small Scale Winds Interacting with the Environment

• Turbulence is disturbed flow/air that produces wind gust and eddies
• Friction decreases as one moves away from surface
• Wind speeds increase with height
• Surface heating affects turbulence, creating thermals and convection cells
• Vertical motion is known as thermal turbulence
• Simultaneous occurrences of surface heating and friction interaction create very strong & gusty winds
• Eddies are small volumes of air in larger flows, which can happen suddenly
• Wind shear results when wind suddenly changes speed/direction
• Wind shear is dangerous for aircraft, creating "clear air turbulence"
• Winds erode soil
• Shelterbelts (wind breaks) of trees are to change erosion of farming soil
• Winds create waves, affected by duration of time plus how far wind blows over water

Local Wind Systems

• Thermal circulations are brought about by changes in air temperature
  • Thermal highs (cold):
    • Regions of surface highs
    • Created as the atmosphere cools
  • Thermal lows (warm):
    • Regions of surface lows
    • Created as the atmosphere warms
• These systems are shallow and weaken with height

Sea and Land Breezes

• Results from more faster heating/cooling of air over land versus over water
• DAYTIME brings in a sea breeze
  • Land heats up fast, and air above it warms rises, creating a thermal low
  • Water remains cooler, and is a thermal high
  • Surfaces breezes move from water to land
• NIGHTTIME brings in a land breeze
  • Land cools more quickly, making thermal high
  • Water remains warmer, becoming thermal low
  • Surface breezes move from land toward water
• Breezes are named where they start

Mountain and Valley Breezes

• Due to unequal heating at top of slopes and valley below
• DAYTIME in the Mt. area = thermal low while Valley air is cooler = thermal high
  • Surface breeze comes FROM the valley, called valley breeze
• NIGHTTIME in the Valley air warmer = thermal low while Mt cools more quickly = thermal high
  • Surface breeze comes FROM the mountain = mountain breeze

"Specialty Winds"

• Winds limited to certain regions/conditions
• Santa Anas
  • Warm, dry winds blow downhill into S. California; dry vegetation and destructive of wildfire
• Chinook
  • Warm/dry winds blow down eastern side of Rocky Mountains = destructive to roofs/cars
• Desert Winds
  • Haboob: Dust clouds (100+ km high) leading thunderstorms in Sudan and U.S. Southwest
  • Dust Devils: Hot, clear days, a few meters wide and under 100 meters in height

Seasonal Winds

• Monsoons blow from one direction during the summer and the opposite direction during winter
  • Are well-developed Asia wind patterns
  • Land breeze occurs in winter: Cold land creates thermal high.
  • Sea breeze occurs in summer: Warm land air temperature creates thermal low.
  • Essential to agriculture/drinking water to Asia

Determining Wind Direction & Speed

• wind direction is the direction from which wind blows
  • called "onshore" if blows from water onto land
  • called "offshore" if blows from land toward water
  • termed "upslope" if moves uphill
  • termed "downslope" if moves downhill
• Wind may be given in degrees: 0°/360° = N; 90° = E; 180° = S; 270° = W

Influence of Prevailing Winds

• prevailing wind is the wind direction that happens most
• Influences the climate of a region
• Can affect City planning
• Applied in predicting invasive plant spreading
• Can predict dust & ash from volcanic activity
• A wind rose represents these winds in planning/building wind farms

Wind Measurements

• Wind vane: measures wind direction
• Anemometer: measures wind speed
• Aerovane: measures wind speed and direction
• Measurements above the surface require weather balloon
  • Balloons have scientific instruments for measuring humidity, pressure and temperatures
  • Balloons typically pop in the stratosphere around 30 km
• Rockets/radar/satellites determine wind information above 30 km

Winds: Global Systems

General Circulation of the Atmosphere

• Global patterns emerge when winds are averaged, unequal heating caused by solar radiation
• The tropics experience a heat gain, and the polar regions suffer a net loss

3 Cell Model of Air Circulation

• Hadley cell: Thermal circulation of air rises at equator & sinks at 30° latitude:
  • Reaches tropopause and moves horizontally
  • Dry air descends near 30° latitude, forming clear skies, warm temperatures, and rare precipitation (deserts).

Polar Cell

• The Polar cell is similar to the Hadley cell, yet located at 60° latitude
  • Weaker than Hadley cells due to decreased energy
  • high pressure, clear skies, and lower precipitation

Ferrel Cell

• The Ferrel cell is in between Hadley and Polar cells (30°-60° latitude)
  • Boundary (Polar front) creates mid-latitude weather systems
  • Weaker than Hadley cells

Additional Aspects of These Cells

• Doldrums are warm air at the equator that create "no winds"
• The InterTropical Convergence Zone (ITCZ) is pronounced "itch"
  • Where air rises and huge thunderstorms and clouds develop
• Horse Latitudes are between Hadley and Ferrel cells and have "no winds"
  • Many deserts are located here
  • Sailors would toss horses overboard at this location to lighten ship/load on ship
• Trade Winds go ~30° and move equator-ward
  • located in Hadley cells
• Westerlies go ~30° and move to poles
  • located in Ferrel cells
• Polar Easterlies are cold air coming from the poles to 60°
• Polar Front is in between the westerlies and the polar easterlies - Low pressure creates clouds and storms

General Circulation & Precipitation Patterns

• Abundant precipitation is expected where air rises, such as topics with ITCZs -Between 40°-55° latitude
• Little precipitation is expected where air sinks
  • Located near 30° latitude and near poles
• Summers have high precipitation and winters have clear dry weather
  • B/twn doldrums and horse latitude zones
• Other factors can disrupt the latitude pattern
  • Some cities have consistent precipitation due to inflow of moist tropical air
  • Other regions have periods of low precipitation due to inflow of dry cooler air

Jet Streams

• Jet streams are swiftly flowing currents in a narrow atmosphere zone
• typically found at the tropopause
• caused by a sudden and intense pressure change along a front
• Jet Stream Locations:
  • Subtropical - near 30° latitude
  • Polar - near polar front
    • Wavy west-east pattern and strongest in winter

Atmospheric-Ocean INteractions

• Surface ocean currents are influenced heavily by wind
  • Wind currents move faster
  • Water circulation and wind follow the Coriolis force
    • Clockwise in N. hemisphere and counterclockwise in S. hemisphere
• Gulf Stream is a significant warm current off the east coast of US
  • Transports warmth into higher latitudes, providing warmth and moisture for cyclones for energy to Atlantic Drift, warming the area and giving milder climate
• Eastern edge of ocean is warm and currents move toward the poles
  • Western edge of ocean is cool and currents flow from equator

Upwelling

• Is what occurs on Pacific coast
• Strong air blows top surface, which moves the water, then Cold water moves in from the bottom, becoming nutrient dense, but with low clouds/ fog
• Makes for good condition for fishing

El Nino Southern Oscillation (ENSO)

• Is a periodic ocean that is a phenomenon of the atmosphere.
  • Strong countercurrent at equator makes surface warmer, preventing upwelling
  • Alters air flow over continents, for example making some area more rainy
    • Can lessen hurricane formation
      • Because increased winds disrupts organization.
      • Is about ever ~7 years or so

La Nina

• Occurs after events of Nino’s - Makes the water cooler