Atmospheric Circulation

 Chapter 6

“You can hear the wind but can’t tell where it comes from or where it is going …”

John 3:8 (NLT)

 “A fierce storm arose. High waves began to break into the boat until it was nearly full of water.  Jesus was sleeping at the back of the boat with his head on a cushion.  Frantically they woke him up, shouting, ‘Teacher, don’t you even care that we are going to drown?’ 

When he woke up, he rebuked the wind and said to the water, ‘Quiet down!’   Suddenly the wind stopped, and there was a great calm.  And he asked them, ‘Why are you so afraid? Do you still not have faith in me?’ 

And they were filled with awe and said among themselves, ‘Who is this man, that even the wind and waves obey him?'”

 Mark 4:37-41 (NLT)

There is an introductory video here:

I.  Atmospheric Circulation – Wind!

The atmosphere moves – we experience this as wind!

This fact is important because:

  • energy is redistributed (e.g. from the tropics toward the poles)
  • pollution (both natural and human-induced) is dispersed and moved around the globe
  • global weather patterns are produced
  • ocean currents are generated

Atmospheric circulation is categorized as (know these for your exam!):

  • primary circulation – general, worldwide patterns (e.g. westerly winds that dominate mid latitude regions, like North America)
  • secondary circulation – regional circulation patterns (e.g. monsoons that are limited to South and South-east Asia)
  • tertiary circulation – local winds. (e.g. anabatic/katabatic winds that affect a local area, like your neighbourbood)

II. What is “wind”?

Wind is simply defined as the horizontal movement of air across the earth’s surface.

Wind is produced by differences in air pressure between one location and another.  Air flows from high pressure to low pressure.  Air pressure is measured using a barometer.

Wind speed is measured by an anemometer (in km (kilometres)/h or m(miles)ph or k(knots)ph).  The Beaufort Wind Scale  is a more “experiential” way of categorizing wind speed based on what you might actually observe.  The Beaufort Scale goes from 0 (calm) to 12 (hurricane force).  The Wikipedia page has a great description/pictures for the scale.

Wind direction is measured by a wind vane.  Winds are named for the direction they blow from.  Winds are NOT named by where they are blowing to!  This is important!  And for some people counter-intuitive.  Know this (for the exam…?)

  • e.g. a westerly blows from the west toward the east
  • e.g. a nor’easter blows from the north-east toward the south-west

In the Bible, the most deadly wind in prophetic literature, for instance, is the east wind.  Why?  Think about Israel.  An east wind blows hot and dry, from the east, off the deserts of the Middle East (see Job 15:2, Hosea 13:15).  West winds tend to be cooler and wetter, blowing from the west, off the Mediterranean Sea, bringing moisture and moderate temperatures.

  • *See Figure 6.5, “A wind compass” 4CE; p. 148(3CE, p. 138) for the “names” of the major winds.
A. Primary Circulation (Global wind patterns)

On a global scale, dominant wind patterns are the results of:

1. Pressure Gradient Force

6 pgf(Figure 6.8a, “Three physical forces…” 4CE; p. 152 (6.7a;3CE, p. 141)

The density of air (or air pressure) varies both vertically and horizontally.  Vertically, air density (air pressure) decreases as altitude increases, because the earth’s gravitational pull upon air is weaker.  The higher you go, the less dense – or the thinner – the air becomes (remember Chapter 3).

Horizontally, temperature affects density (air pressure).  Normal air pressure at sea level is 1013.2 mb (millibars).  This can also be expressed as 101.3 kPa (kilopascals) or 29.92 inches of mercury (USA).

Air pressure “normally” ranges between 980 mb to 1050 mb.

Horizontal variations in air density within the atmosphere cause air to be pushed from areas of high pressure to areas of low pressure. (see the map to the right)   Air always moves from HIGH to LOW pressure!  Think about a balloon:  you blow into it to create a higher pressure than the air around it; but if you let go of the stem, the high pressure air in the balloon will rush out!6 isobar

  • In a high-pressure area, air descends (it is denser/heavier than the air around it).  As it sinks, it spreads out, moving along the surface toward lower pressure areas.  High pressure areas are clear (more later).
  • In a low pressure area, air rises (it is less dense/lighter than the air around it).  As it rises, it pulls air toward itself, air converges.  Low pressure areas are cloudy, often stormy (more later)

If the isobars are far apart (i.e. air pressure changes only a little bit over a large distance – a gradual pressure gradient), winds will be light. (see the map to the right)

If the isobars are very close (i.e. air pressure changes a lot in a very short distance – a steep pressure gradient) winds will be very strong. (see the map to the right)

2. Coriolis Force

(Figure 6.8b 4CE p. 152(6.7b; 3CE, p. 141))

On a non-rotating earth, winds would flow straight from high to low pressure.  But, the earth is rotating quickly!  (believe it or not at 1700 km/h at the equator; 650 km/h at our latitude – it has to make one complete revolution every 24 hours!).  Thus, although the air may move straight in the atmosphere, from the earth it will appear to move in a curved path – the air really is going straight, but the earth is rotating underneath it!  (See Figure 6.7, “The Coriolis Force” 4CE; p. 150(6.8; 3CE, p. 142)).

What you need to know:

  • Air appears to deflect to the right in the northern hemisphere and to the left in the southern hemisphere.
  • The Coriolis force (this deflection) is greatest at the poles and negligible at the equator.

The Coriolis Force is a difficult concept to understand.  Read the section in your text to help you!  Don’t worry if you don’t totally understand why or how it works!  Do know its effects:

  • right deflection in N;
  • left deflection is S;
  • and its relative strengths at different locations:
    • greatest at poles;
    • negligible at equator

If the Coriolis force and pressure gradient force are exactly equal, the wind will blow parallel to the isobars; this called a geostrophic wind (only occurs in the upper atmosphere).

Note that the Coriolis force is NOT responsible for little twists in water draining from your sink or bathtub!  Such water does not travel far enough to be affected the earth’s rotation.  Sink/tub swirls are the result of little eddies and currents in your sink (caused by the motion of your hands or something else) – not the Coriolis force!  Some people who live on the equator make money from gullible tourists by demonstrating water swirling one way out of a basin just one side of the equator … then they hop to the other side of the road to show water swirling the other way out of a basin on the other side.  In reality they cleverly twist the basins to give the desired effect!  It is slight of hand, not Coriolis!  Save your money! 

Click here for a full discussion of this “urban legend.”  If I ask you on an exam question about whether the Coriolis force causes water in your drain to drain to the right of left, make sure you answer that the Coriolis force has nothing to do with it!  SO, if I ask you about whether or not the Coriolis Force affects the drain in your sink, you will know the correct answer, right???

3. Friction

(Figure 6.8c ; 4CE; p.152(6.7c; 3CE, p. 141))

Land surfaces are relatively rough and resist airflow because of friction; it tends to slow wind speed up to heights of 500 m into the atmosphere.

You know this!  If you want to get out of the wind, what do you do?  Get behind something!

B.  Global Circulation Patterns

The basic cause of atmospheric motion is the pressure gradient caused by unequal heating by insolation (solar radiation).  The Coriolis force and friction then modify this motion.

*** Study Figures 6.1a and 6.1b, “General atmospheric circulation model” (4CE, pp. 156-157; (Figure 6.11; 3CE, p. 146)).  Know it inside-out!  It will be on the exam in some form or other!  Be prepared!  ***

1. The Inter-tropical Convergence Zone (ITCZ) or Equatorial Low-Pressure Trough

The region of the Earth that uniformly receives the most insolation, and thus is the warmest region of the Earth, is the equator.  Around the equator air expands and rises upwards, flowing toward the poles at high altitudes.  The equatorial region, then, is a low-pressure area.

During a typical day, air rises, water evaporates from warm, tropical oceans, and mid-late afternoon thunderstorms are common. Over the oceans, surface winds tend to be light because the air rises upwards.

This region is known as the doldrums — not a good place to sail because you can often be becalmed for long periods of time!

I spent some time in Singapore (1°N) … it’s true.  Each day there was virtually no wind.  And each began sunny, clouded up by noon — and poured cats and dogs from about 2 – 6 p.m.  Then, as the atmosphere cooled, it cleared off by dusk.

Over continents this region extends large distances in summer; the intense radiation, evaporation and rising air gives rise to the seasonal monsoons (Arabic for season — discussed below).

2. N.E./S.E. Trade Winds

6 tradewindsBecause there is low pressure at the surface in the ITCZ, air is drawn from north and south of the equator back, along the surface toward the equator.  This creates reliable winds that blow from about 20-30°N and 20-30°S toward the equator.  So, from about 20-30°N and 20-30°S toward the equator, relatively consistent strong winds blow from a high-pressure belt (subtropical highs [20°-30°N/S] – discussed next) toward the equatorial low-pressure belt (from the N.E. [in N] or S.E. [in S]).

In the days of sail, these were vital for trade — hence the name!  They are most important over the oceans.  Over continents, patterns are complicated by monsoon conditions.

3. Subtropical high pressure belts

The warm equatorial air cools as it rises and sinks back to the surface at about 20° to 30° N and S latitude, creating subtropical highs pressure belts and weak winds.

These regions were called the “horse latitudes,” reportedly because Spanish conquistadors travelling to Central and South America in sailing ships were often becalmed for weeks and were forced to eat horses they were transporting across the Atlantic!   Besides the doldrums (in the ITCZ), this is the location you may be becalmed as a sailor!  This is important to know … great exam question!

These areas are dominantly high pressure (dry air), with very few clouds and thus little precipitation.  Note that most major deserts (Gobi, Sahara, and eastern Australia) are in these latitudes.

Some of this descending air flows back to the equator; some flows toward the poles.

The typical rising-air-at-the-equator, sinking-air-in the subtropics, and returning-air-as-trade-winds forms what are known as Hadley cells (after the English scientist who first observed the pattern).

  • See Figures 6.1a and b(6.11 in 3CE).

Be able to locate and identify (and draw if needed) this circulation pattern – a Hadley cell.

4. Mid-latitude Westerlies

6 westerlyBeyond the subtropics, other forces are involved.  Friction is greatest at the surface and decreases with altitude; thus wind speed increases with altitude, as does the effect of the Coriolis force in an upward spiral deflecting wind direction and air movement toward the right in the Northern Hemisphere.  This creates a dominantly westerly (i.e. from the west) flow in the Northern Hemisphere.

Simple patterns are complicated by the friction caused by large air masses (northern hemisphere circulation patterns are much more irregular than southern hemisphere ones, because the north has more land).

In general mid-latitude winds are westerly.  In these region storms (also called lows, cyclones, or depressions) tend to develop over oceans and move towards the northeast as vast spirals of cloud.

These storm tracks tend to move north in the summer (giving California nice sunny summers) and south in the winter (causing California heavy rains in winter).

These winds are very inconsistent!  However there is a strong north-south transfer of air, bringing thermal energy north and moving cooler air back to the tropics.

5. Polar Easterlies

6 polar windsAround the poles weak easterly flows appear to exist.  In winter polar high-pressure cells often dominate, resulting in cold north-easterly flows (in the north) or south-easterly flows (in the south).

These appear to be related to the mid-latitude westerlies:  warm, moist winds carry air north where it is cooled, descends over the poles, and flows south as polar easterlies.

6. High Altitude Winds

Winds in the upper atmosphere often move in different directions than at the surface!  Because of the decreasing influence of friction, at high altitudes winds tend to be stronger and move more along the pressure gradient than across it (the Coriolis force).  If the wind speed is high enough a geostrophic wind will result.

a. Upper Westerlies

In the upper atmosphere the dominant winds are westerly (deflected by the Coriolis force) outside of the ITCZ.  They are strongest in the upper troposphere, 12-15 km above the earth with an average speed of 125 km/h; speeds of several hundred km/h are not uncommon.

Generally they are stronger in winter when temperature differences between tropics/mid-latitudes are greatest.  (summer speed may be as low as 50 km/h).

The practical implication of this is that it is normally quicker to fly from Vancouver to Halifax than vice versa!  (especially in winter!)

b. Rossby Waves and the Jet Stream

Around the poles, air appears to flow in a series of waves (Rossby waves, named after a meteorologist named Rossby!).  On part of its track it carries warm air northward (ridges).  Elsewhere it carries cold polar air southward (troughs).  These often seems to become “fixed” in winter, perhaps because of the influence of the Rockies and Himalayas (causing very cold, dry winters to Alberta!)

  • See Figure 6.13, “Rossby upper-atmosphere waves,” 4CE, p. 160; (Figure 6.15; 3CE p. 150).

Along these waves, bands of especially strong winds (up to 490 km/h) – the jet stream – occur.  These relatively narrow bands of rapidly flowing air form in areas with very high pressure gradients, where cold, polar air and warm, mid-latitude air come in contact (see the zonal index, below).

They are very important for air travel, both for speed reasons, and for turbulence reasons (they are very “rough”).

  • Today’s location of the jet stream over North America is available here.
  • For more global information and jet stream forecasts, click here.

A weaker, subtropical jet stream can occur where tropical and mid-latitude air converge.

These streams of extremely fast-moving upper level air were identified initially during World War II, but more extensively during the Korean War when jets began flying at high altitudes.  Pilots heading west, across the Pacific toward Korea, discovered that though their air speed might be 400-500 km/h, their ground speed could be 0 km/h!  They were, in fact, going nowhere!  They were flying into a very strong westerly (from the west) jet stream.

It is, obviously, wisest not to fly in a jet stream (unless you’re traveling east, when it can really zip you along … very turbulently!)

Note that jet streams change with the seasons.

You may have noticed on weather forecasts they often talk about the jet stream … a big line on the weather map shaped like a wave – a Rossby wave, in fact.  To the north of the jet stream is usually cold air.  To the south warm air.  (More about air masses in Chapter 8).

Along the jet stream itself, there can be nasty storm events as the cold and warm air collide – and move quickly.  For instance, an Alberta Clipper is a fast moving low pressure storm system that often brings nasty weather to the eastern Prairies and central Canada.

One of the issues associated with climate change appears to be changes in the jet stream.  This is leading to weather patterns tending to becoming stuck over areas for weeks on end. It also drives cold weather further south and warm weather further north. This change has been implicated in long seasons of cold/stormy weather in eastern North America, storms/floods in the UK, and extended hot/drought seasons in Australia and South Western USA.  BBC News – Wavier jet stream ‘may drive weather shift’

c. The Zonal Index

Simply put, the atmosphere usually has two distinct air masses:  a dome of cold air over each pole and a band of warm air around the middle.  Where these two meet is referred to as the Arctic (or Polar) Front (this is where one jet stream almost always occurs); this retreats to high north in the summer and extends further south in the winter.  Along this front pressure gradient differences are highest, causing high winds – the jet stream.  This typically happens in a wavy line (Rossby waves).

The strength of these gradient differences is referred to as the zonal index; it tends to go from a strong to weak condition on a 4-6 week cycle which tends to cause 4-6 week cycles of cooler/warmer temperatures in mid-latitudes (us!).  As it weakens it tends to create mid-latitude cyclones (discussed later).

C. Secondary circulation (regional circulation patterns)

Some regional wind systems change direction seasonally.  These are often called “monsoons” — after the Arabic word for season (“mausim”).  They occur in a number of countries around the world.

The strongest monsoons develop over southern and eastern Asia (although weaker monsoons exist over Africa, and even Australia).

The two key ingredients for the Asian Monsoon are a large land mass and a large ocean – namely southern Asia (Bangladesh, India, Nepal, Pakistan and Sri Lanka) and the surrounding Arabian Sea and the Indian Ocean.

In winter, the Asia-European landmass is dominated by an intense, cold, high-pressure area (little insolation and heating) – cold and dry!  Consequently, winds tend to flow offshore toward the low-pressure ITCZ.  Thus, in winter, cold, dry winds blow off central Asia.

In spring, dry winds continue although the land begins to heat.

From April on, the pre-monsoon heat builds over the land.  As the land heats, the air heaats.  And hot air … rises!  With time and more intense heating, this will result in continuous rising of less dense air (as the land warms faster) over the land.  Over Asia-Europe low pressure forms over the land, especially over North India and the Himalayas.

6 monsoon1Meanwhile, over the oceans the air is relatively cooler and denser (remember water heats much more slowly than land does).  So the ocean is low relatively high pressure.

To maintain the energy balance air starts flowing from the oceans (now relatively high pressure) to the land (now relatively low pressure thanks tot the land heating so rapidly).

This brings moisture rich southwest winds across southern Asia – the ‘wet’ phase.  From June-September, the Asian continent begins to super-heat and become a huge  low-pressure area as the heated air rises.  Meanwhile the Indian Ocean is under a subtropical high-pressure cycle.  The temperature difference between the land and sea can be as much as 20°C – land temperatures in India can even exceed 45°C, while the surrounding water in the Bay of Bengal and the Arabian Sea remains in the low 20s.

As a result, warm, wet winds blow onshore.  When these warm, wet winds reach the warm landmass and the Himalaya Mountains, the air rises, water vapour condenses, and precipitation results.

The devastating floods in Thailand in 2011 resulted from violent monsoon rains:  2011 Thailand floods – Wikipedia, the free encyclopedia

As the land and ocean begin to cool in late summer and into autumn, the land loses heat quicker than the ocean. The wind reverses during this ‘dry’ phase, becoming a north-easterly, blowing from the land, out to sea.  These are dry winds, blowing from the land.

See Figure 6.15, “The Asian monsoons,” 4CE, p. 161; (Figure 6.19; 3CE p. 154).

D. Tertiary circulation (local winds)
1. Sea and Land Breezes
  • See Figure 6.16 “Land-sea breezes …” 4CE, p. 163; (Figure 6.17; 3CE p. 152).

In coastal areas, during the day, land areas heat quickly.  This causes the air to rise, creating areas of low pressure.  Adjacent sea areas heat more slowly, causing less atmospheric heating and high pressure remains.  During the day, winds tend to blow on-shore, from the ocean onto the land (a sea breeze).  Winds tend to grow in strength throughout the day as the land gets warmer and warmer; the water heats much more slowly.  As evening approaches and the heating of the land decreases, the sea breezes tend to drop as well.  Sea breezes are strongest mid-afternoon, and decrease toward evening.  Sea breezes are also strongest in summer when the land warms most during daylight.

At night the opposite happens.  The land cools quickly, becoming an area of high pressure.  The sea cools slower, become – relatively – low pressure.  Consequently at night winds often blow off-shore, as land breezes.

So, if you want to take sailing lessons – do so mid-afternoon (best winds!).  Don’t take morning or evening lessons (like me!), when winds are lightest or non-existent.

You may also notice that coastal cities are cloudy during the day (moist air coming onshore) but clear out at night (dry air blowing from inland).

2. Valley and Mountain Breezes
  • See Figure 6.17, “Pattern of mountain-valley breezes …” 4CE, p. 164; (Figure 6.18; 3CE p. 153).

During the day, ground in mountain valley areas is heated.  As the ground heats quickly, air rises and breezes blow upslope.  These are called anabatic winds.  As air from the valley continues to be heated and rise throughout the day, warm, light breezes from the valley, up the slope develop.

At night, the ground cools quickly, cooling the air above it.  As the air cools, it becomes denser and sinks.  On a slope this means it flows downhill.  This is called a katabatic wind.  Thus at night, bitter cold winds may flow downhill in mountainous regions.  If this cold air collects in a depression a very cold “frost hollow” can develop.  Local examples include Chinook winds (Alberta),  Santa Ana winds (California), and the mistral (France).

Note also that in the Northern Hemisphere, a slope with a southerly aspect (that is, the slope faces south), receives more solar radiation than a north-facing slope.  Therefore, it’s warmer!  In the north this great!  Further south it may mean that south slopes are barren (too much insolation!) and only north slopes support vegetation.

Often there are warm, upslope winds on south-facing slopes, but cold, downslope winds on north-facing slopes.

  • READ Focus Study 6.1, 4CE pp.166-167; 3CE, pp. 156-157.

III.  Ocean Currents

Ocean currents are created by frictional drag of global winds.  Wind pushes water into moving, creating currents.  The currents, then, mirror the dominant wind patterns in each region.  They are also affected by the Coriolis Force.

Overview this section so you understand their influence on global temperatures and climate.  We do these in more depth in the other course.

IV. Wind Power

Use of wind actually goes back centuries as windmills were used to drive pumps, grind flour, and perform other tasks.

In much of Canada, wind has tremendous potential as a renewable power source (although it also has its issues and its critics).

As of June 2015, wind power generating capacity was over 10,000 megawatts (MW), providing about 4% of Canada’s electricity demand (mostly in Ontario, Qeubec, and Alberta).   The Canadian Wind Energy Association has  a strategy to grow to a capacity of 55,000 MW by 2025, meeting 20% of our energy needs.

Worth reflecting on …
  1. Theologian Alister McGrath discusses – science is very successful at finding out certain things, but it has limits. When it comes to questions of meaning, does science have anything to say?  What is the role of science? – YouTube.  What do you think?
  1. “The basic theme of this book is simple,” writes Alister McGrath in a book that looks at the natural world from a Christian perspective. “It suggests that we reclaim the idea of nature as God’s creation and act accordingly, bringing attitudes and actions into line with beliefs.  We have been entrusted, corporately and individually, with the jewel of God’s creation and given the responsibility of tending and nurturing it, before passing it on to others.  We are like curators of the great art collection, who are accountable to posterity as well as to the present for our tending of its treasures.  We must learn to appreciate and prize this entrustment, as perhaps the greatest privilege this earth can offer.  And more than this: we must see nature as a continual reminder and symbol of a future renewed creation, a world that we do not yet know but believe to lie over the horizons of our human existence.”

(Alister McGrath, The Re-enchantment of Nature, page 184.)

How do you feel about that?  Do you agree?  Feel free to discuss these on the course discussion site …

To review …

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Scripture quotations marked (NLT) are taken from the Holy Bible, New Living Translation, copyright © 1996. Used by permission of Tyndale House Publishers, Inc., Wheaton, Illinois 60189. All rights reserved