Download Quiz # 3

EART 80C – Quiz 3
August 15, 2008
Name:_________________________________
1. Describe the process of droplet break-up (include a diagram). What is the size limit of a rain
drop and why? What role does this play in precipitation formation?
Droplet break up occurs when the air resistance against a droplet it so great that it beings to change the shape of the droplet
(into a pancake with a dent and then almost like a donut) until the droplet is divided into several smaller drops. This limits
the size of the rain drops to about ¼ of an inch or around 6mm. Air resistance increases as droplet size increases, so as the
droplets grow they can undergo breakup.
2. Describe what the pressure gradient force is. Also, define isobars. Explain how these two
concepts are connected.
The pressure gradient force is the change in pressure over some horizontal distance. This change is caused by
differences in heating (temperature). Isobars are contours of constant P. They are related in two specific ways:
The pressure gradient force is at right angles to isobars (if that’s all we knew about) and increases as isobars get
closer together.
3. Describe the sequence of events that leads to El Nino (include a diagram).
The onset of ElNino is marked by abnormal weather patterns that drastically affect the economies of Ecuador and Peru. It
is an ocean phenomenon, that occurs most specifically in the Equatorial Pacific. As the SE trade winds decrease in strength
(weaken) the warm water can make it farther across the Pacific towards South America (Peru). Normally, the trade winds
and strong equatorial currents flow toward the west. At the same time, the strong Peruvian current causes upwelling of cold
water along the west coast of South America. When the Southern Oscillation occurs the pressure over the western and
western Pacific flip-flops. This causes the trade winds to diminish, leading to an eastward movement of warm water along
the equator. As a result, the surface waters of the central and eastern Pacific warm. Without the dramatic temperature
differences between the western and eastern Pacific you do not see the same patter of precipitation forming over the
Indonesian Islands. Thus you get fires in Indonesia, and a decrease in upwelling (and decreased nutrient loading of the
coastal waters) off the South American Coast.
4. What is the mid-latitude jet-stream? Why does it form? Why is it an important part of
forecasting weather in the United States?
Mid-latitude jet stream is a relatively narrow band of air that flows particularly fast. It is usually found at an altitude
between 7 to 12 km, and is usually between 100 to 500 km wide, and a few km in thickness. It can be thought of as the
fastest moving part of a river, in the sense that there is always some wind at this altitude, but this is the part that flows
fastest. Typical wind speeds are around 100 to 200 km/h.
It forms along the polar front, where air at the surface is converging. In the NH, air coming to the polar low from the N is
very cold, and air from the S is very warm. When these air masses meet, there’s a strong contrast in T in a very short
distance. This T diff in short distance leads to a large pressure gradient force (which points to the N at these altitudes). This,
in turn, causes wind speeds to be very high in this region. At these altitudes, coriolis is the only other force that acts on
moving air parcels, resulting in geostrophic flow and therefore generally westerly flow. [In reality, it also exhibits a wavy
pattern as it flows around the Earth, the result of the Earth’s rotation].
It is an important part of forecasting weather in the United States because it both supplies energy to the circulation of
surface storms and directs their paths of movement.