Learning Lesson: AM in the PM

Overview

Demonstrate that AM radio signals can travel many 100's of miles at night. The student will listen to as many radio stations as possible obtaining the call signs and places of origin during the evening (after sunset) hours.

TOTAL TIME 30 minutes to two hours during the evening (or early in the morning before sunrise).
SUPPLIES Radio with an AM band; Pen/pencil
PRINTED/AV MATERIAL Radio Station Reception Form (pdf)
TEACHER
PREPARATION
Supply the students with a list of the local AM radio stations.
SAFETY FOCUS NOAA Weather Radio

Procedure

  1. Most homes should have some sort of portable AM radio. If not, automobile radios will often work well. (Have the student to ask a parent or guardian to help supervise them while listening to AM radio stations in an automobile. Students without a valid driver's license are not to be left alone with keys to the vehicle.)
  2. Search for a radio signal that is not from a local station. (Most will be faint but the reception is usually clear enough to understand.) Stations broadcasting sporting events are easy to identify.
  3. Listen for the station identifier "call sign". The call sign is a three or four letter identifier beginning with the letter "W" or "K". In the U.S. stations are required to broadcast their call sign within 5 minutes of the top of the hour.
  4. Log the call sign and location (city) of the transmission. Also note the quality of the signal. Was it loud? Soft? Fade in and out? etc, and the what was broadcast; news, sports, etc., Was there a lot of static?
  5. Search for another signal and repeat.
  6. You can compare the students results with the list of clear channel stations. It is entirely possible that students hear broadcasts that are not local and not one of the powerful nighttime radio stations.

Discussion

During the daytime, the distance the AM radio signal travels is the distance the ground wave travels based upon the power of the transmitter. The signal also reaches the ionosphere.

The D-Layer of the ionosphere plays an interesting role. While there are no radio signals reflected off this layer it does absorb AM radio signals. Because of the absorption of the signal, there are also more radio stations transmitting during the daytime and these stations can often transmit at higher power.

Comparison showing how the D-layer affects daytime and night time radio transmissions.

At night, the D-Layer disappears and the transmitted signal can then bounce off the ionosphere and return back to the earth. As a result, some low power stations must cease transmission at sunset while others reduce their transmitted power to reduce interference.

However, there are high power clear channel stations that can broadcast all night. It is these stations the students will most likely find.

Live weatherwise

NOAA Weather Radio (NWR) is a nationwide network of radio stations broadcasting continuous weather information direct from a nearby National Weather Service office. NWR broadcasts National Weather Service warnings, watches, forecasts and other hazard information 24 hours a day.

Working with the Federal Communication Commission's (FCC) Emergency Alert System, NWR is an "all hazards" radio network, making it your single source for comprehensive weather and emergency information. NWR also broadcasts warning and post-event information for all types of hazards--both natural (such as earthquakes and volcano activity) and environmental (such as chemical releases or oil spills).

Known as the "Voice of NOAA's National Weather Service," NWR is provided as a public service by the National Oceanic & Atmospheric Administration (NOAA), part of the Department of Commerce. NWR includes more than 900 transmitters, covering all 50 states, adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands, and the U.S. Pacific Territories. NWR requires a special radio receiver or scanner capable of picking up the signal.

For more information, go to the NOAA weather Radio website.

Fast Facts

The scientific unit of pressure is the Pascal (Pa) named after after Blaise Pascal (1623-1662). One pascal equals 0.01 millibar or 0.00001 bar. Meteorology has used the millibar for air pressure since 1929.

When the change to scientific unit occurred in the 1960's many meteorologists preferred to keep using the magnitude they are used to and use a prefix "hecto" (h), meaning 100.

Therefore, 1 hectopascal (hPa) equals 100 Pa which equals 1 millibar. 100,000 Pa equals 1000 hPa which equals 1000 millibars. The end result is although the units we refer to in meteorology may be different, their numerical value remains the same. For example the standard pressure at sea-level is 1013.25 millibars and 1013.25 hPa.