Atmospheric Circulation & Wind Systems Notes

Atmospheric Circulation & Wind Systems

Atmospheric Pressure

  • The atmosphere is an ocean of air with weight, constantly exerting pressure on the Earth’s surface due to gravity.
  • Air pressure decreases with increasing elevation.
  • A barometer is used to measure air pressure.

Factors Affecting Air Pressure

  • Altitude/elevation is not the only factor.
  • Other factors include:
    • Intensity of heating from insolation
    • Global or regional air circulation

Horizontal Pressure Variations

  • Two types:
    • Thermal
    • Dynamic

Thermal Pressure Variations

  • Warm Air:
    • Air at the surface warms, its volume expands, and its density decreases.
    • This causes air to rise, resulting in low pressure at the surface.
  • Cold Air:
    • Air at the surface gets cold, its volume decreases, and its density increases.
    • This causes air to sink, resulting in high pressure at the surface.

Dynamic Pressure Variations

  • Related to broad patterns of atmospheric circulation.
    • Subtropical highs: result from dynamic air motion related to the sinking of convectional cells initiated at the equatorial low.
    • Subpolar lows: result from the collision of opposing winds, which cause air to rise.

Wind Speed & Direction

  • Controlled by three forces:
    • Pressure-gradient force: the change in air pressure across Earth’s surface.
    • The Coriolis effect: caused by the Earth’s rotation.
    • Friction

Isobaric Map

  • A map that shows surface pressure.
    • “H” indicates areas of high pressure.
    • “L” indicates areas of low pressure.

Basic Pressure Systems

  • Two types:
    • Cyclone (low) “L”
      • An area where air is rising.
      • Convergent wind circulation.
    • Anticyclone (high) “H”
      • An area where air is descending.
      • Divergent wind circulation.

The Coriolis Effect

  • The apparent deflection of winds and water as a result of the Earth’s rotation.
  • Decreases at lower latitudes and has no effect at the equator.

Wind

  • Wind is the horizontal movement of air in response to differences in atmospheric pressure.
  • "Winds always blow from high to low."

Global Pressure Belts

  • Equatorial low
  • Subtropical high
  • Subpolar low
  • Polar high

Wind Belts

  • Trade winds: 5°-25° N/S (strong, persistent winds)
  • Horse Latitudes: 25°-35° N/S (weak winds, not persistent)
  • Westerlies: 35°-60° N/S (storms; persistent)
  • Polar easterlies: 75°-90° N/S (storms; persistent)

Seasonal Pressure Differences

  • Global belts shift:
    • Northward in July
    • Southward in January
  • Wind systems shift with the seasons as well.

Winter (in the Northern Hemisphere)

  • High pressure cells develop over land areas because cold air sinks.
  • Subpolar lows develop over the ocean because they are comparatively warmer.

Winds Aloft, Geostrophic winds, and the Jet Stream

Geostrophic Wind

  • Upper level winds subject to pressure gradients and the Coriolis effect only.
  • No surface friction (therefore, they are very fast).

Jet Stream

  • River of wind.
  • Guides weather patterns.
  • Moves west to east.
  • Two per hemisphere.
  • Occur where pressure gradients are strongest.
  • Planes can use them to get places faster.

Asian Monsoon

  • 180° reversal of wind flow.
  • Winter = dry
  • Summer = wet
  • Caused by differences in heating of land vs. sea and the orographic effect of the Himalayas.

Santa Ana Winds

  • Start in Fall, as seasons change, and some regions begin to cool.
  • High pressure builds in Great Basin (Nevada, Utah, Idaho).
  • Clockwise circulation channels wind toward CA.

Santa Anas (cont.)

  • Elevation difference: Cajon Pass, 4,000 ft; Santa Monica, 0 (sea level).
  • Air heats up at Dry Adiabatic Rate (5.5°F/1,000ft) as it is compressed by the change in elevation.
  • Local mountains act like a dam (wind pushes forcefully through passes).
  • Winds are very dry.
  • Pose extreme wildfire danger.

El Niño

  • Occurs at 3 to 5 to 8 year intervals.
  • Changing pressure patterns cause strong trade winds to become weak westerlies.
  • Warming of the waters of the eastern Pacific (2-8°F).
  • Major change in sea-surface temperatures can shift weather patterns across the globe.
    • Dry areas become wet
    • Wet areas become dry
  • Named after the baby Jesus because the warming of the waters is first noticed in December in South America.

La Niña

  • Sometimes follows an El Niño event.
  • Cooling of ocean water (2-4°F).
  • Normal conditions become acute.
    • Dry areas become drier
    • Wet areas become wetter (more rainfall)

Ocean Currents

  • Exchange heat between low and high latitudes.
    • Move warm waters poleward and cold waters toward the Equator.
    • Warm currents tend to be on the eastern side of continents (ex. the Gulf Stream).
    • Cold currents tend to be on the western side of continents (the California Current).

Ocean Currents (cont.)

  • Surface currents are driven by prevailing winds.
    • Move in broad circulatory patterns called gyres.
  • Deep currents are powered by changes in temperature and density in surface waters that cause them to sink.

Thermohaline Circulation

  • Cold, salty surface water sinks in the N. Atlantic Ocean.
  • Deep, cold currents develop.
  • Slow upwelling occurs in the Indian Ocean, W. Pacific, and coast of Antarctica.