Quiz #6 Study Questions
Chapter 6, pp. 141-151
Chapter 7, pp. 169-185
Turn in Monday, April 24 by start of class. Answers should be on a separate sheet of paper and legible. This is worth up to 5 points extra credit towards Quiz #6
- Why do we have an atmosphere? What two forces keep our atmosphere in place? What do we call the balance between these two forces?
- The force of gravity keeps air molecules close to the surface of the earth. If gravity were the only force involved, the atmosphere might collapse under its own weight. This does not happen because the downward force of gravity is balanced by the upward pressure gradient force (high pressure near surface, lower presure aloft). This is called hydrostatic balance.
- Describe the geostrophic wind.
- This arises from a balance between the pressure gradient force (which sets air in motion) and the Coriolis force (which deflects moving air to the right in Northern Hemisphere). When the pressure gradient is uniform (i.e. isobars are straight and parallel on weather map) the geostrophic wind is directed along isobars with low pressure lying to the left (in Northern Hemisphere).
- Why do upper level winds in the middle latitudes of both hemispheres blow from west to east?
- (See discussion on page 149 and Figure 4). In general, warmer air is located along the equator and cooler air lies to the north and south of the equator. Remember that differences in air temperature can result in differences in the thickness of a column of air extending up to the 500 millibar level (see figure 6.2 and Figure 2 on p. 141). Thus differences in air temperature set up pressure gradients between the equator (higher pressure, warmer air) and the middle latitudes (lower pressure, cooler air). The pressure gradient force intially moves air poleward. The Coriolis force deflects this air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The resulting flow will then be from west to east in both hemispheres. Because we are taking about winds in the upper troposphere (say 500 millibars) we can ignore friction.
- Describe the gradient wind flow around a low pressure center. Which direction is the blowing? What forces are at work and in what directions are they acting? Are the wind speeds faster than or slower than the geostrophic wind?
- When air flows in a circular fashion around a center of high or low pressure, there must be a net force directed inwards. In this case, the pressure gradient force and the Coriolis force are not in balace, so the wind flow is not geostrophic. In the case of a lower pressure center (and let's assume we're in the Northern Hemisphere, the gradient wind around a low is in the counter-clockwise direction. The pressure gradient force is directed inward at all points around the low. The coriolis force is directed to the right of the direction of motion. For this case of counterclockwise flow, the Coriolis force will always be directed outward, away from the low pressure center. Wind speeds around a mid-latitude low are less than geostrophic (sub-geostropic).
- Explain how the friction force works. What effect does it have on surface winds? Why is this effect important?
- Friction results from collisions between moving air molecules and a stationary surface, and also from collisions between individual molecules near the surface of the earth where the density of air moilecules is greatest. Friction slows surface winds. Slower moving air is not deflected as much by the Coriolis force. As a result, the pressure gradient force can become slightly larger than the Coriolis force, and air will flow across isobars from high pressure to low pressure. This cross-isobaric flow generates convergence and divergence of surface air which is related to vertical air motions.
- Explain how a dust devil may form on a hot, clear day in Tucson.
- On a hot day sirface heating will cause air to rise through convection. A column of rising hot air (or a "thermal") can form in a flat, open space such as a field or broad area of desert. Wind flow at the surface may aquire rotation as it is deflected by obstacles on the ground or by flowing over surfaces of variying roughness. When a rotating mass of air is lifted by a strong thermal, the column of fluid is stretched and the rotation increases. The resulting column of rapidly rotating air rising from the surface is a dust devil.
- What is the underlying cause of the general circulation of the atmosphere? Explain how this could lead to the formation of a Hadley cell.
- The general circulatin is driven by differential heating of the earth's surface by the sun. At the equator, where the heating is greatest, the warm air rises and clouds form (the inter-tropical convergence zone or ITCZ forms here; surface pressure is low). As it spreads laterally to higher latitudes, the air cools and sinks. As the air sinks, in is compresed and heats up adiabatically. The average location for the descending branch of the Hadley Cell is 30 degrees latitude. At this point, the sirface pressure is high (the sub-tropical high). Hot, clear, calm conditions are common here. To replace the rising air, air flows in along the sirface from higher latitudes into the ITCZ. This inflow is referred to as the trade winds, and it is directed from NE to SW in the Northern Hemisphere, and from SE to NW in the Southern Hemisphere.
- What feature of the earth causes the observed three-cell structure of the general circulation?
- The 3-cell model (se p 171 and Figure 20 on p. 178) arises from the earth's rotation.
- Tucson's latitude is 32 degrees north. What wind and surface pressure features of the general circulation are likely to affect our weather here?
- Our locatin puts us between the sub-tropical high pressure near 30 degrees N and the mid-latitude westerlies which are present at middle latitudes. Looking at figure 7.15a we see that in January, our climate is influenced by the Pacific high and surface wind flow is generally from the NW. From Figure 7.15b we see that in July the Pacific High moves northward, while a thermal low forms over the desert southwest due to surface heating by the sun. The Bermuda high is to out east, and in summer the counter-clockwise flow around the thermal low, combined with clockwise flow around the Bermuda high, directs moist air from the Gulf of Mexico toward SE Arizona, giving us out summer rainy season.
- Describe the changes in Pacific sea surface temperature and winds along the equator that take place during a major El Nino/Southern Oscillation event? How do these changes affect the weather over the southwestern U.S.?
- As Figure 7.15 shows, surface winds along the equator in the Pacific Ocean are usually from east to west. Under "normal" conditions, this causes water to flow from east to west along the equator. Cold water from deep in the ocean off the S. American coast is drawn up to replace the water flowing westward ("upwelling"). During El Nino, the wind pattern reverses, so air flows from west to east. This shuts off the cold upwelling water off the S. American coast, and sea surface temperatures off the coast of Ecuador and Peru increase dramatically (see Figure 7.25). As Figure 7.26 shows, the southwestern United States doesn't experience much change in weather due to El Nino when you look at many ENSO events. But the last two events (winter 1993 and 1998) brought lots of rain to Tucson. Go figure.
Figures to study: Chapter 6: 14, 17, 18, 20; Chapter 7: 13, 14, 15, 16, 25, 26