Physics A: Problem Set 13: Doppler effect

sample problem

1. There are hundreds if not thousands of television stations across North America that claim to use "doppler radar" when reporting weather related news. In keeping with the general level of hype that is 21st century TV weather, most if not all of these stations never show actual doppler radar images in their broadcasts. Instead, what they show are reflectivity images. A radar pulse is sent out, the strength of the echo is measured, and the results are displayed as a certain color that indicates the intensity of precipitation. A typical color code would be green for light rain, yellow for moderate, red for intense, and so on.

   A real doppler radar image like the one below shows something different. In this mode, a radar pulse is sent out, the frequency of the echo is measured, and the results are displayed as a certain color that indicates the velocity of the precipitation. The colors are often assigned in a manner that imitates the color shifts seen when the doppler effect is applied to visible light — red, orange, and yellow for raindrops blowing away from the radar antenna and green, blue, and violet for raindrops blowing toward the radar antenna. (Color assignments vary from one radar system to another, however, so these are not absolute rules.)

   

   The image above was taken from the NOAA National Weather Service radar station in Dodge City, Kansas on 7 May 2007 that was operating in doppler mode.

   1. The radar site that produced this image is located in Dodge City (the upper left corner). The radar image in this region is red on the west side side and blue on the east side.
      1. What is the wind direction in the red region?
      2. What is the wind direction in the blue region?
      3. What is the overall airflow pattern in this region?
   2. There is an unusual two-color region located in the center of this image just to the southwest of a small town called Greensburg.
      1. What is the wind direction in the red region?
      2. What is the wind direction in the blue region?
      3. What is the overall airflow pattern in this unusual two-color region?
      4. What is probably going on here?
   • Note: Wind direction indicates where the wind is coming from, not where the wind is going to.

homework

2. Bats use echolocation to visualize their world. They send out ultrasonic squeaks from their little bat mouths and then listen for the echos with their giant bat ears. The characteristics of the echo will vary depending on the characteristics of the object reflecting the squeak.
   1. How will the echo from a large object (like a tree trunk) compare to a small one (like a tasty moth)?

      

   2. How will the echo from an object moving away from the bat (like an escaping moth) compare to one from an object moving toward the bat (like a moth about to become lunch)?

      
3. When viewed from the northern hemisphere of the Earth, light coming from the left side of the Sun is seen to have a slightly shorter wavelength than the light from the right side. (The situation is reversed when the Sun is viewed from the southern hemisphere.) What do these observations tell you about the Sun?