Companion to the reading passage How Doppler Radar Decodes the Sky. Answer each question, then click Check. Multiple-choice and number questions grade themselves; for written answers, compare yours with the model answer and checklist, then score yourself honestly.
Doppler weather radar stations use a rotating antenna to send short pulses of microwaves into the atmosphere. The radar transmits microwaves with a frequency of 2.80 × 109 Hz, and the microwaves travel through the air at 3.00 × 108 m/s. When a pulse strikes raindrops, hail, or snow, a small part of its energy is reflected back to the antenna as an echo. The station measures the time between sending a pulse and receiving its echo in order to locate the storm. Table 1 shows echo times for three storm cells detected during one sweep of the antenna.
Data are representative values for practice.
Table 1: Echo Times for Three Storm Cells
| Storm Cell | Time from Pulse Sent to Echo Received (s) |
|---|---|
| A | 2.0 × 10−4 |
| B | 4.0 × 10−4 |
| C | 8.0 × 10−4 |
1. Which mathematical representation can be used to determine the distance from the radar station to storm cell B?
2. Substitute numerical values into a mathematical representation that can be used to determine the wavelength of the microwaves transmitted by the radar. Then calculate the wavelength. (Type powers of ten as, for example, 3.00e8.)
Doppler radar can also measure how fast precipitation is moving toward or away from the station. When precipitation moves toward the radar, the echo returns with a slightly higher frequency than the transmitted pulse. When precipitation moves away from the radar, the echo returns with a slightly lower frequency. Meteorologists compared the frequency shift of the echo from three areas of a storm with the speed of the precipitation along the radar beam, which they found by tracking the precipitation over several minutes. Their results are shown in Table 2.
Data are representative values for practice.
Table 2: Echo Frequency Shift and Precipitation Speed
| Area | Direction of Motion | Speed Along Radar Beam (m/s) | Echo Frequency Shift (Hz) |
|---|---|---|---|
| A | toward radar | 10 | +187 |
| B | away from radar | 15 | −280 |
| C | toward radar | 24 | +448 |
3. A meteorologist makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response with evidence from Table 2.
4. Which row correctly describes the echo from precipitation in Area B, compared with the pulse transmitted by the radar? (Frequency of echo / Wavelength of echo / Speed of echo)
Inside a rotating thunderstorm, a radar velocity map can show strong motion toward the radar right beside strong motion away from the radar. This pattern means the air is circulating, a warning sign that a tornado may form. Newer dual-polarization radars, which send pulses with both horizontal and vertical orientations, can also detect tumbling debris lofted by a tornado.
Diagram 1: A view from above shows a parcel of air moving at constant speed in a horizontal circle of radius 500 m around a center point, O. The circulation is counterclockwise as seen from above. The radar station is far to the south. At the moment shown, the parcel is directly east of point O and is moving due north.
Diagram 1: Circulating Air Parcel (view from above)
5. In Diagram 1 (north is up and east is to the right), choose the direction of the net force acting on the air parcel at the moment shown. Then, assuming a 1.0-kg sample of air in the parcel moves at 20 m/s, determine the magnitude of the net force on it.
6. Storm spotters report a steady wind of 20 m/s blowing from west to east in a region that is due north of the radar station. The echo from precipitation in this region shows a frequency shift of 0 Hz. Which statement best explains this result?