Chapter 1, Review Questions - UK Ag Weather Center



Chapter 8, Review Questions

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1. Provide a definition of wind.

a. Wind is the local motion of air measured relative to the rotating Earth.

2. What causes horizontal air pressure gradients? How do air parcels respond to a horizontal air pressure gradient?

a. Horizontal pressure gradients are caused by air pressure changes along a surface of constant altitude. Horizontal pressure gradients cause air parcels to move toward the area of lowest pressure.

3. What is the relationship between the horizontal wind speed and the spacing of isobars on a surface weather map?

a. Horizontal wind speed is strong where the pressure gradient is steep, indicated by closely spaced isobars. Horizontal wind speed is light or calm where the horizontal pressure gradient is weak, indicated by widely spaced isobars.

4. Why does the Coriolis Effect reverse direction between the Northern and Southern Hemispheres?

a. The reversal in direction of Coriolis Effect between the Northern and Southern Hemispheres stems from the contrast in our perspective of Earth’s rotational direction in the two hemispheres. To an observer at the North Pole, Earth appears to rotate in a counterclockwise direction whereas to an observer at the South Pole, Earth appears to rotate in a clockwise direction.

5. Describe how the Coriolis Effect varies with wind speed and latitude.

a. The Coriolis Effect causes the horizontal wind to be deflected to the right of its initial direction in the Northern Hemisphere, and to the left of its initial direction in the Southern Hemisphere. The magnitude of the Coriolis Effect varies with wind speed; the faster the wind the greater the Coriolis Effect. The amount of deflection also varies with latitude, with the maximum deflection at the poles and no deflection at the equator.

6. How does the roughness of Earth’s surface affect horizontal wind speed and direction within the atmospheric boundary layer?

a. Earth’s surface roughness (frictional resistance) slows the wind and causes the wind to blow across isobars and toward low pressure.

7. Provide an example of how gravity influences air motion.

a. Gravity causes air parcels to be pulled towards the Earth. It influences air that is ascending or descending, such as updrafts or downdrafts in a thunderstorm or the downhill drainage of cold air in the mountains.

8. What forces are balanced in the geostropic wind?

a. The geostrophic wind results from a balance between the horizontal pressure gradient force and the Coriolis Effect.

9. Why does the circulation within an anticyclone favor generally fair weather?

a. Viewed from above, surface winds in a Northern Hemisphere anticyclone blow clockwise and outward. Surface winds diverge from the center of the anticyclone and air descends from aloft. Descending air undergoes compressional warming and the relative humidity decreases so that fair weather is usually associated with an anticyclone.

10. Why does the circulation within a cyclone usually bring cloudy, stormy weather?

a. Viewed from above, surface winds in a Northern Hemisphere cyclone blow counterclockwise and inward. Surface winds converge toward the center of the cyclone and air ascends. Ascending air undergoes expansional cooling and the relative humidity increases so that cloudy, stormy weather is usually associated with a cyclone.

Chapter 8, Critical Thinking Questions

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1. Why is radiation fog more likely near the center of an anticyclone than near the center of a cyclone?

a. At the center of an anticyclone, skies tend to be clear and the humidity low as a result of sinking air. Also, the horizontal air pressure gradient is typically very weak over a broad area around the center of the system bringing calm conditions or light winds. Calm or light winds coupled with clear skies allow for intense nocturnal radiational cooling. These conditions favor the development of radiational fog.

2. Distinguish between the geostrophic wind and the gradient wind.

a. The gradient wind curves whereas the geostrophic wind is straight. Both are horizontal and occur above the atmospheric boundary layer.

3. The pattern of horizontal winds blowing about the center of a high-pressure system implies the existence of a centripetal force. Explain why.

a. Viewed from above in the Northern Hemisphere, horizontal winds blow counterclockwise about the center of a low-pressure system. Curved motion implies a net centripetal force.

4. Why are horizontal winds associated with a sloping pressure surface (e.g., 700-mb surface)?

a. A pressure surface features uniform air pressure all along that surface (isobaric surface). Where an isobaric surface is sloping upward or downward, air pressure differences (gradients) occur along a horizontal plane. Where there is a horizontal pressure gradient, there is wind. Sloping of an isobaric surface is caused by horizontal temperature variations in the air column beneath the pressure surface.

5. Describe the relationship between a high pressure system and an air mass.

a. Air masses develop under large, slow moving high pressure systems because the air in a high pressure system is relatively uniform in temperature and humidity over a broad area.

6. Along a coastline, cumuliform clouds are more likely with an onshore wind (directed from water to land) than an offshore wind (directed from land to water). Explain why.

a. Onshore winds cause surface winds to slow and undergo horizontal convergence as a result of air blowing from a smooth to rough surface. This induces upward motion and promots the formation of cumuliform clouds. Offshore winds cause air to accelerate and undergo horizontal divergence as a result of air blowing form a rough to smooth surface. This induces downward motion of air, which inhibits vertical motion.

7. Upper-air support for a developing cyclone requires horizontal divergence. Explain why.

a. Horizontal divergence aloft allows converging air at the surface to rise.

8. Suppose that a cyclone is centered over St. Louis, MO. Describe the type of air mass advection to the southeast and to the northwest of the storm center.

a. For a cyclone over central St. Louis, warm air advection occurs to the southeast of the storm center and cold air advection to the northwest.

9. In view of Newton’s first law of motion, is the gradient wind a consequence of balanced forces? Explain your answer.

a. No. The gradient wind is not the consequence of balanced forces because the gradient wind is curved motion rather than straight-line motion. According to Newton’s first law of motion, a net force must operate to confine air motion to a curved path.

10. What is hydrostatic equilibrium? Is vertical motion of air possible with hydrostatic equilibrium? Explain your answer.

a. Hydrostatic equilibrium is a balance between the vertical pressure gradient force and gravity. Vertical motion is possible with hydrostatic equilibrium, however, particles will move at a constant velocity.

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