Chapter 7 Questions

Describe how the wind blows around highs and lows aloft and near the surface in the Northern Hemisphere.

(a) In the Northern Hemisphere, clockwise around high and counterclockwise around low aloft. At the surface, friction induces a component directing toward low pressure. In the free troposphere, the balance is between pressure gradient and the Coriolis forces. Near ground, it is a three balance ‐including friction. (b)In the Southern Hemisphere, the geostrophic wind switches the direction

What can cause the air pressure to change at the bottom of a column of air?

A colder column of air loses pressure more rapidly with height, thus at higher altitudes, warm air has higher pressure. A warm air column near a cold air column will transfer air molecules to the lower-pressure air at the top of the cool column, increasing the air pressure at the bottom of the column. At the same time, the cool column will now have a higher pressure at the bottom, and thus transfer air back to the warmer column.

If clouds above you are moving from southwest to northeast, in which general direction would you expect to find an upper- level low pressure area? If the surface wind on this same day is blowing from the southeast, in which general direction would you expect to find a surface mid-latitude cyclonic storm?

Air will start moving towards low pressure ,then the coriolis force will cause the wind to start spin.In the northern hemisphere winds spun in a counter clockwise direction around surface low pressure center's.The winds also spiral inward toward the centre of low, this is called convergence.When the converging air reaches the centre of low , it starts to rise.Rising air expands because it is moving into Lower pressure surroundings at higher altitude, the expansion causes it to cool.If the air is moist and it is cooled enough clouds will form and may then begin to rain or snow.You often see cloudy skies and stormy weather associated with surface low pressure.Surface high pressure centres are pretty much just the opposite situation.Winds spin clockwise and spiral outward.The outward motion is called divergence. Air sinks in the centre of surface high pressure to replace the diverging air.The sinking air is compressed and warms.This keeps clouds from forming ,so clear skies are normally found with high pressure. Once the winds start to blow , they can affect and change the temperature pattern.Temperature changes from south to north but not from west to East.

Explain why, in the Northern Hemisphere, the average height of contour lines on an upper level isobaric chart tend to decrease northward.

As you go north in the Northern Hemisphere you generally hit colder temperatures; cold air aloft is associated with low pressure, thus the contour lines that measure constant pressure tend to decrease northward.

On an upper-level chart, is cold air aloft generally associated with low or high pressure? What about warm air aloft ?

Cold air aloft = low pressure, warm air aloft = high pressure

Why does air pressure decrease with height more rapidly in cold air than in warm air?

Cold air is smaller and denser, thus moving upward moves past a larger amount of molecules than in warm air, which is larger and less dense.

Why do upper-level winds in the middle latitudes of both hemispheres generally blow from the west?

High altitudes winds at mid-latitudes blow from the west because they are influenced by the earth rotation. But it depends on the scale and time you are talking about

What is the force that initially sets the air in motion?

Pressure Gradient Force

How does sea-level pressure differ from station pressure? Can the two ever be the same?

Sea-level pressures differ from station pressure in that station pressure is corrected for temperature, gravity, and instrument error and is taken at the altitude of the station. The two can be the same if a station is located at sea level

What is a geostrophic wind? Why would you not expect to observe a geostrophic wind at the equator?

the geostrophic wind is a theoretical wind that results from a balance between the Coriolis force and the pressure gradient force (PGF). It is a good approximation for the observed wind in middle latitudes more than ~1 km above the surface. At the equator there is no Coriolis force, thus there cannot be a geostrophic wind.