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Winds and Atmospheric Kinetics

Sep 25, 2024, 8 min read

What Causes Winds?

  • Atmospheric pressure exists due to the continuous collision of molecules in the atmosphere.
  • Temperature gradients are the main driver for atmospheric motion.
    • Insolation causes an increase in temperature, which causes an increase in volume, and a decrease in air pressure.
    • The above also results in pressure gradients. Wind is primarily driven by pressure gradients

Rules for Wind Flow

Pressure Gradients

  • Winds flow from areas of high pressure (low temperature) to low pressure (high temperature).
  • Warmer winds flow up due to the expansion of the gas. Conversely, colder winds flow down.
  • The strength of wind flow is directly proportional to the magnitude of the pressure gradient
  • High pressure zones are formed when wind is flowing downwards. Conversely, low pressure zones are formed when wind is flowing upwards.
    • The amount of energy retained by the ocean compared to the continent is important here as this determines where high / low pressure zones tend to form over.

Friction

  • Friction causes atmospheric winds near the surface to be slower due to more obstacles, though this is restricted in the friction layer of the atmosphere.
  • Wind flows around terrain and becomes more turbulent as a result.
  • Wind tends to be stronger in regions with little continent.

Coriolis Effect

  • The flow of wind above the friction layer is more influenced by the rotation of the planet via the Coriolis Effect
    • Winds travel faster near the center of the Earth since points here cover more distance per unit time.
    • Winds travel slower at the poles for the same reason.
    • Winds deflected towards the poles will appear to move ahead of other polar winds.
    • Winds deflected towards the equator will appear to move behind other equatorial winds.
    • The winds themselves favor moving in the direction of Earth’s rotation.
    • The apparent deflection is directly proportional to the latitude (because of the circumference at that point getting smaller).
    • The apparent deflection is directly proportional to the amount of force (i.e., either mass or acceleration) that the wind already exerts.
    • The apparent deflection is directly proportional to the altitude the wind is blowing as friction dominates less at higher altitude.
    • At the geostrophic level the flow of wind is fully deflected. It is no longer influenced by pressure gradients.

Low and High Pressure Zones

  • In the Northern Hemisphere at geostrophic levels, wind flows counterclockwise around a low pressure zone, and clockwise around a high pressure zone.

  • In the Southern Hemisphere at geostrophic levels, wind flows clockwise around a low pressure zone, and counterclockwise around a high pressure zone.

  • Applying friction to the rules above results in winds that converge rotating towards the center. This also explains the formation of cyclones and anticyclones.

Major Wind Belts

  • Circulation Cells - systems which involve the circulation of wind across the depth of the troposphere
    • Hadley Cells - these span the Equator
    • Polar Cells - these span the Poles
      • These winds rotate around the poles due to the strong Coriolis effect.
      • These winds also generally cause low pressure zones around the poles which gives stormy weather.
    • Ferrel Cells - these span the mid-latitude regions and are generally weaker.
      • They are created due to the shearing force of the Hadley cells and Polar cells.
      • However, they do not have a stable temperature gradient that can sustain their existence.
  • Jet Streams - very fast flows of air along the boundaries of some circulation cells that form due to shearing of their circulation cells and the Coriolis effect.
    • They do not track perfectly circular routes. The contour has ridges (towards the poles) and troughs (towards the equator).
    • The ridges and troughs are not constant and can change over time
    • Rossby Waves - bends in the polar jet stream influence the weather. When a trough moves farther away from the pole, the cold Arctic air extends farther away from the pole.
  • Westerlies and Easterlies - are winds that generally come from the West and East respectively.
  • Trade Winds- winds that generally blow from the northeast or the southeast and are found in the tropics. They blow eastward due to the Coriolis effect.

Kinds of Winds

Sea and Land Breezes

  • Both phenomena are observed in coastal area and are governed by insolation (see here).
  • The sea breeze is caused by land heating up much faster during the day. The land breeze is caused by land cooling down much faster at night.
    • In both cases, the side that is warmer will have the air rising up due to increased temperatures, and falling down otherwise. This creates vertical motion.
  • As a result of the specific heat of land and water, during the night, wind flow is reversed.
  • The strength of both is proportional to the changes in temperature.

Valley and Mountain Breezes

  • Both phenomena are observed in rocky terrain and are governed by changes in elevation and topography. They are due to the fact that the atmosphere heats up much slower than the surface (see here).
  • The valley breeze is caused by land at lower elevations heating up much faster. The mountain breeze is caused by land at higher elevations being cooled down much faster.
    • This means that during the day, valleys are hotter, and during the night ,valleys are colder.

Urban Heat Islands

  • Urban areas are usually warmer than rural areas due to:
    • A lack of trees and standing water.
    • Machinery that produce more heat or light.
    • Road and building materials (i.e., concrete) that absorb a lot of heat.
  • These urban areas are called urban heat islands.

Other Kinds

  • Chinook Winds
    • It can cause sudden rises in temperature even during winter.
  • Katabatic Winds
    • They are especially strong if they are channeled down a valley to the point they can desiccate the area
  • Santa Ana Winds
    • They can contribute to the production of destructive wildfires due to their extreme dryness.
  • Sirocco
    • Causes “blood rain” due to picking up red sand that mixes with rainfall.
    • Perceived as causing general unease and an irritable mood, in addition to bringing heat and dust.
  • Monsoon
    • A seasonal wind that reverses depending on the season. Monsoons bring dry conditions and wet conditions depending on the direction.
    • The change in direction is brought about by changes in insolation which causes a migration of the ITCZ.

Effects of Winds

  • In general, winds carry dust, clouds, fog, and pollution in the direction of the wind.
  • Winds can also contribute to environmental destruction (i.e., desiccation or by causing and spreading wildfires).
  • In some people, the wind can have a physiological effect (general unease), in addition to bringing irritants towards them.
    • People with weak hearts are more likely to have a heart attack when exposed to strong winds.
  • Winds also bring about storms by moving areas of high or low pressure. This is most apparent in midlatitude regions.

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