Chapter 11 -- Notes

Question 1

What are the Time and Space Scales for Atmospheric Motions?

Answer 1

• Macroscale
  
  • ​Planetary
    
    • Weeks or longer
    • 1000-40,000 km
    • Westerlies and trade winds
  • Synoptic
    
    • Days to Weeks
    • 100-5000 km
    • Mid-latitude cyclones, anticyclones, and hurricanes
• Mesoscale
  
  • ​Minutes to hours
  • 1-100 km
  • Thunderstorms, tornadoes, and land-sea breeze
• Microscale
  
  • ​Seconds to minutes
  • <1 km
  • Turbulance, dust devils, and gusts

Question 2

When are **Mountain breezes **and **Valley breezes **most frequent?

Answer 2

Mountain breezes during the cold season

Valley breezes during the warm season

Question 3

What are the **Chinooks, **and the **Foehns? **

Answer 3

Warm, dry winds sometimes move down the east slopes of the Rockies, where they’re called **chinooks, **and the Alps, where they are called **foehns. **Another chinooklike wind that occurs in the United States is the **Santa Ana. **Found in southern California, these hot, desiccating winds greatly increase the threat of fire in this already dry area.

Question 4

What is a Katabatic or fall wind?

What are the most famous of these winds?

Answer 4

These winds originate when cold air, situated over a highland area such as the ice sheets of Greenland or Antartica, is set in motion. Under the influence of gravity, the cold air cascades over the rim of a highland like a waterfall.

Although the air is heated adiabatically the initial temperatures are so low that the wind arrives in the lowlands still colder and more dense than the air it displaces. In fact, this air *must *be colder than the air it invades, for it is the air’s greater density that causes it to descend.

As this frigid air descends, it occastionally is channeled into narrow valleys, where it acquires velocities capable of great destruction.

The **Mistral, **which blows from the French Alps toward the Mediterranean Sea. Another is the **Bora, **which originates in the mountains of Yugoslavia and blows to the Adriatic Sea.

Question 5

When is a **Country breeze **most likely to occur?

What is one of the unfortunate consequences of the country breeze?

Answer 5

During a relatively clear, calm night.

Pollutiants emitted near the urban perimeter tend to drift in and concentrate near the city’s center.

Question 6

What is Hadley’s Model or A Single-Cell Circulation Model?

Answer 6

Where equatorial air rises until it reaches the tropopause, where it begins to spread toward the poles. Eventually, this upper-level flow would reach the poles, where cooling would cause it to sink and spread out at the surface as equatorward-moving winds. As this cold polar air approached the equator, it would be reheated and rise again.

Although correct in principle. Hadley’s model does not take into account the fact that Earth rotates on its axis. Hadley’s model wound better approximate the circulation of a *nonrotating *planet.

Question 7

What is a Hadley Cell?

Why does air in the Hadley cell subside in a zone between 20° and 35° latitude?

What is that zone of subsidence popularily called?

Answer 7

In the Zones between the equator and roughly 30° latitude north and southis called the **Hadley cell. **Near the equator, the warm rising air that releases latent heat during the formation of cumulus towers is believed to provide the energy to drive the Hadley cells. The clouds also provide the rainfall that maintains the lush vegatation of the rain forests of southeast Asia, equatorial Africa, and South America’s Amazon Basin.

As the flow aloft in Hadley cells move poleward, it begins to subside in a zone between 20° and 35° latitude. Two factors contribute to this general subsidence:

1. As upper-level flow moves away from the stormy equatorial region, where the release of latent heat of condensation keeps the air warm and bouyant, radiation cooling increases the density of the air.
2. The Coriolis force causes a general pileup of air (convergence) aloft. As a result, general subsidence occurs in the zones located between 20° and 35° latitude.

This subsiding air is relatively dry, because it has released its moisture near the equator. In addition, the effect of adiabatic heating during descent further reduces the relative humidity of the air. Consequently, this zone of subsidence is the site of the world’s subtropical deserts.

Because winds are generally weak and variable near the center of this zone of descending air, this region is popularly called the horse latitudes.

Question 8

What are the **Trade Winds? **

Answer 8

From the center of the horse latitudes, the surface flow splits into a poleward branch and an equatorward branch. The equatorward flow is deflected by the Coriolis force to for the reliable **trade winds, **In the Northern Hemisphere, the trade winds blow from the northeast. In the Southern Hemisphere, the trades are from the southeast.

Question 9

What is are the Doldrums?

Answer 9

The trade winds from both hemispheres meet near the equator in a region that has a weak pressure gradient. This region is called the d**oldrums. **Here light winds and humid conditions provide the monotonous weather that is the basis for the expression “down in the doldrums.”

Question 10

What winds blow at 30° to 60° latitude?

Answer 10

The Westerlies

Unlike in the Hadley cell, the net surface flow is poleward, and because of the Coriolis force, the winds have a strong westerly component. These **Prevailing westerlies, **are much more sporadic and less reliable than the trade winds. The migration of cyclones and anticyclones across the midlatitudes disrupts the general westerly flow at the surface.

Question 11

What do we know about the Polar easterlies?

Answer 11

It is generally understood that subsidence near the poles produces a surface flow that moves equatorward and is deflected into the **polar easterlies **of both hemispheres. As these cold polar winds move equatorward, they eventually encounter the warmer westerly flow of the midlatitudes. The region where the flow of warm air clashes with cold air has been named the **polar front. **

Question 12

Which pressure zone is associated with the warm rising branch of the Hadleys cells, near the equator?

Answer 12

The Equatorial Low.

This region of ascending moist, hot air is marked by abundant precipitation. Because it is the region where the trade winds converge, it is also reffered to as the Intertropical convergence zone (ITCZ).

Question 13

What are the pressure zones known as the Subtropical highs?

Answer 13

In the belts about 20° to 35° on either side of the equator, where the westerlies and trade winds originate and go their separate ways, are the **subtropical highs. **

These zones of pressure are caused mainly by the Coriolis deflection, which restricts the poleward movement of the upper-level branch of the Hadley cells. As a result, a high-level pileup of air occurs aroun 20° to 35° latitude. Here a subsiding air column and diverging winds ar the surface result in warm and clear weather.

Generally, the rate at which air accumulates in the upper troposphere exceeds the rate at which the air descends and spreads out at the surface. Thus, the subtropical highs exist throughout most of the year and are regarded as *semipermanent *features of the general circulation.

Question 14

What are the **subpolar lows? **

What are the polar highs?

Answer 14

Situated at about 50° to 60° latitude, in a position corresponding to the polar front, the polar easterlies and westerlies clash to form a convergent zone known as the **subpolar low. **This zone is responsible for much of the stormy weather in the middle latitudes, particularly in the winter.

Near the Earth’s poles are the **Polar highs, **from which the polar easterlies originate. The high-pressure centers that develop over the cold polar areas are generated by entirely different processes than those that create the subtropical highs. The polar highs exhibit high surface pressure mainly because of surface cooling. Because air near the poles is cold and dense, it exerts ahigher than average pressure. These are prominent features of the winter circulation ove the northern continents. Subsidence within these air columns results in clear skies and divergent surface flow. The resulting winds are called *polar easterlies. *

Question 15

What happens in winter to the pressure zones?

Answer 15

Polar highs develop over the northern continents, weakening is observed in the subtropical anticyclones situated over the oceans. Further, the average position of the subtropical highs tend to be closer to the eastern margin of the oceans. The **Azores high **is positioned close to the northwest coast of Afica. Two intense sumipermanent low-pressure centers. Named the **Aleutian low **and the **Icelandic low, **these cyclonic cells are situated over the North Pacific and North Atlantic, respectively. So many cyclones are present that these regions of the globe are almost always experiencing low pressure.

Question 16

What happens in Summer to the pressure zones?

Answer 16

High surface temperatures over the continents generate lows that replace the winter highs. These thermal lows consist of warm ascending air that induces inward directed surface flow. The strongest of these low-pressure centers develops over southern Asia. A weaker thermal low is also found in the southwestern United States.

The subtropical highs in the Northern hemisphere migrate west-ward and become more intense than in the winter months. These strong high-pressure centers dominate the summer circulation over the oceans and pump warm moist air onto the continents that lie to the west of these highs. This results in an increase in precipitation over parts of easter North America and Southeast Asia.

The subtropical high found in the North Atlantic is positioned near the island of Bermuda, hence the name **Bermuda high **this high is located near Africa and goes by the name *Azores high *during winter.

Question 17

What is upwelling?

Answer 17

Upwelling, the rising of cold water from deeper layers to replace warmer surface water, is a common wind-induced vertical movement. It is most characteristic along the eastern shores of the oceans, most notably along the coasts of California, Peru, and West Africa.

Upwelling occurs in these areas when winds blow toward the equator parallel to the coast. Because of the Coriolis force, the surface-water movemet is directed away from the shore. As the surface layer moves away from the coast, it is replaced by water that “upwells” from below the surface. This slow upward flow from depths of 50 to 300 meters brings water that is cooler than the original surface water and creates a characteristic zone of lower temperatures near the shore.

Question 18

What are the **“backbones” **of the ocean current?

Answer 18

North and South of the equator are two westward moving currents, the North, and South equatorial currents, which derive their energy principally from the trade winds that blow from the northeast and southwest, respectively, toward the equator. Because of the Coriolis force, these currents are deflected poleward to form clockwise spirals in the Northern Hemisphere and counterclockwise spirals in the Southern Hemisphere.

Question 19

What is an Él Niño?

Answer 19

Near the end of each year a warm countercurrent, which flows eastward along the equator, develops. As a result , warm water begins to accumulate along the coasts of Ecuador and Peru.

Normally, these warm countercurrents, or

Question 20

How do Él Niños effect the economies of Ecuador and Peru?

Answer 20

These unusually strong countercurrents amass large quantities of warm water that block the upwelling of colder, nutrient-filled water. As a result, the anchovies, which support the population of game fish, starve which devastates the fishing industry. At the same time, some inland areas that are normally arid receive above average rainfall. In these areas, yields of cotton and other crops are far above average.

Question 21

What is a Monsoon?

Answer 21

A wind system that exhibits a pronounced seasonal reversal in direction. In general, winter is associated with winds that blow off the continents and summer with winds that blow from the sea to land. Thus the summer monsoon is usually associated with abundant precipitation.