Lecture 2: Large Scale Circulation Patterns

Question 1

Where is wind speed greatest and why?

Answer 1

Over oceans, as friction is greatest over land.

Question 2

What direction does the wind flow in the mid lats?

Answer 2

West to East

Question 3

What are spiral structures in the tropics known as? Describe them.

Answer 3

Trade Winds - occupy almost half the globe in the belt between the ITCZ and subtropical highs.
At the surface, winds tend to the equator, becoming easterly above friction layer.
Accompanied by aggregation of small, uniform clouds - the result of latent heat transfer from sea surface.

Question 4

What are the consequences of the trade winds?

Answer 4

Provide steady climate. Convergence of moisture in the winds feed equatorial trough, whose position therefore determines the extent of easterlies / westerlies.

Question 5

What is a consequence of the equatorial trough being further north, and when are the trade winds most southerly?

Answer 5

If equatorial trough is further north, trade winds will be restricted in the Northern hemisphere.
Trades are most southerly in January, extending to the equator.

Question 6

The doldrums are found equatorward of the main trade belt in the East Pacific and Atlantic, but what are they characterised by?

Answer 6

Light, variable winds.

Question 7

Describe the ITCZ (aka equatorial trough)

Answer 7

Shallow trough of low pressure near the equator.
Almost static over oceans are seasonal temperature changes small.
Winds are predominantly westerly and carry rain, whilst at higher latitudes it may cause monsoons and reverse direction seasonally.

Question 8

What happens to the ICTZ during summer?

Answer 8

Sweeps polewards, reaching 30-40 degrees north over China.

Question 9

What altitude and latitude are the westerlies strongest at?

Answer 9

35 degrees north and south - 10km above land.

Question 10

Where are the mid-lat westerlies initiated, and what does this form?

Answer 10

In areas of strong temperature gradient, forming circular patterns of low pressure and rotational strong winds (lows/cyclones/depressions)
Rotating stroms formed transport energy polewards of sub-tropical anticyclones.

Question 11

Where are the westerlies at their max intensity?

Answer 11

Aleutian and icelandic lows.

Question 12

Why are the westerlies more consistent in the Southern Hemisphere?

Answer 12

The ocean expanse rules out the chance of stationary pressure systems developing.

Question 13

Where do the polar easterlies occur?

Answer 13

Between the polar high pressure and subpolar low pressure.

Question 14

What controls circulation around the south pole, and what does this cause?

Answer 14

The antarctic ice cap - so anticyclones frequently occur over East Antarctica and strong SE winds develop around margin of ice plateau. (However Westerlies predominate over seas off west antarctica).

Question 15

The pressure gradient force is the basic force of atmospheric motion. What is it proportional to, and what movement then occurs?

Answer 15

Proportional to gradient in pressure, so air moves from ares of high to low pressure.

Question 16

Where is lateral pressure greatest, and why do variation in lateral pressure occur?

Answer 16

In areas with most dense atmosphere. Therefore variation occurs as the result of different intensities of solar heating.

Question 17

What are isobars, and theoretically how should air move in relation to them?

Answer 17

Lines of equal pressure - so air should move at 90 degrees to them, down the pressure gradient.

Question 18

The magnitude of the pressure gradient force determines movement, what does this mean for the relationship between wind speed and distance between isobars?

Answer 18

Inversely proportional.

Question 19

Earths rotation should cause wind to blow parallel to isobars. Why does this not occur, and what geographic variation is therefore observed?

Answer 19

Friction alters this, so angle between wind flow and isobars greater over land than ocean.

Question 20

What is the earths eastward velocity at the equator?

Answer 20

1660km/h

Question 21

How do eastward velocities change as you go north, and why?

Answer 21

Decrease, as radius/circumference of circle at that latitude declines.

Question 22

How does observed velocity differ towards the poles?

Answer 22

It will increase, as it is the velocity at equator - velocity at circle (which we know decreases with latitude)

Question 23

Describe the apparent velocities of an object moving from the northern to southern hemisphere.

Answer 23

Whilst moving southwards in the northern hemisphere, its apparent eastward velocity will decline, and once crossed equator, its apparent westward velocity will grow.

Question 24

Why do objects on the earths surface maintain direction they face, but launched objects turn right?

Answer 24

Objects on the earths surface rotate about a reference frame within them, consistent to earths rotation rate. Launched objects maintain the same rate of rotation about local axis, but this is now too slow for increased eastward velocity through atmosphere so turns right.

Question 25

How is deflection affected by latitude and why?

Answer 25

Decreases with latitude, as there is no deflection at equator, due to no local rotation.

Question 26

What is geostrophic flow, and where can it be found?

Answer 26

Where pressure gradient force and coriolis effect balance and air travels along contours of constant pressure. Found in tropical high pressure systems.

Question 27

What is the effect of friction on geostrophic flow?

Answer 27

Where friction slows down the surface winds, the coriolis effect is unable to balance the pressure gradient force.

Question 28

Annual mean wind speed is highest 35 degrees north and 32 degrees south, at 10km altitude, what does this coincide with?

Answer 28

The sub-tropical jet stream.

Question 29

What determines jet stream formation?

Answer 29

The westerlies and tropical easterlies, as well as regions of rapid temperature gradient.

Question 30

Describe the formation of Hadley cells.

Answer 30

Air warmed by the oceans rises to 20km, spreads polewards and releases temp energy to atmosphere therefore starting to sink again.

Question 31

Name the types of vertical cell circulations that exist from the equator, polewards.

Answer 31

Hadley, Ferrel, Polar

Question 32

What determines the max N/S extent of Hadley cells?

Answer 32

The maximum strength of the West to East flow.

Question 33

Momentum =

Answer 33

mass x velocity

Question 34

Angular momentum =

Answer 34

Angular velocity x distance from axis of rotation x mass

Question 35

Therefore, how can angular momentum be conserved?

Answer 35

A small radius

Question 36

Planetary angular momentum =

Answer 36

Earths rotation + motion relative to earth

Question 37

How does a declining radius help to explain wind speed increasing as you get nearer the poles?

Answer 37

If radius decreases, whilst angular momentum of earth due to rotation decreases, the only way for momentum to be conserved is an increase in wind speed.

Question 38

How does a declining radius affect the point at which hadley cells break down?

Answer 38

Declining radius means that wind speeds must increase as you move polewards in the Hadley cell. At the point at which hadley cells breakdown, the wind speeds must therefore be at their max.

Question 39

Name the 4 monsoonal regions.

Answer 39

• SE Asia / Australia
• North America (Originates in mexico)
• South America (esp.Brazil)
• Africa (Western Sahel)

Question 40

How do monsoons form?

Answer 40

• Surface heating over land in summer much greater than over ocean.
• Air at surface becomes less dense and rises
• Cools and water condenses out as rainfall
• In winter the land cools down and circulation reverses.

Question 41

Tibetan mountains are a raised plateau, how do they affect the indian monsoon?

Answer 41

They accentuate the effects of heating, causing strong reversals between air flowing off the continent, Dec-Feb, and air flow on in June-August.