Fire department VHF radio signals that are monitored by CARDINAL NEWS were remarkably stronger from communities outside Chicagoland on Wednesday, September 29, 2021. Normally signals that reach Arlington Heights from Woodstock or Wauconda, for example, are fair quality with some static. Today these radio signals were clearer than usual. On most days, Cary, Fox River Grove, McHenry, Wauconda, and Woodstock, for example, are about at the edge of the tolerable range for understanding radio messages reaching Arlington Heights because of static (a low signal to noise ratio).
Also today, radio signals from communities well outside Chicagoland were reaching Arlington Heights with signal strength greater than the average strength received from Fox River Grove and McHenry (as examples).
The reason could be caused by clear skies, low humidity, and a strong temperature gradient that is characteristic of warm Fall days when air at the surface is heated by the sun, but air at higher altitudes gets cooler compared to Summer air at these high altitudes.
Today, radio signals that aren’t usually detected at all in Arlington Heights reached our radios from Wisconsin and Michigan. For example, on Wednesday, September 29, 2021 fire department radio signals from Menomonee Falls, northwest of Milwaukee, Wisconsin; and Galesburg, west of Kalamazoo, Michigan were not only detectable, but were stronger than the usual average, everyday signals from Cary, Fox River Grove, and McHenry in Illinois.
Menomonee Falls in Wisconsin is about 70 miles north of Arlington Heights.
Galesburg, Michigan is about 130 miles east of Arlington Heights.
Cary, Illinois is about 16 miles northwest of Arlington Heights.
Fox River Grove, Illinois is about 14 miles from Arlington Heights.
McHenry is about 22 miles northwest of Arlington Heights.
Wauconda is about 14 miles northwest of Arlington Heights.
According to a study by Tan Fan Ting Valerie from the School Science and Technology in Singapore, warmer air reduces the transmission of strong, better-quality radio signals, but when cool air flows above warm air at higher altitudes, the radio signals can be boosted.
Consider the situation on a warm Fall day such as today. Warmer air is found near the surface of the earth, but cooler air exists at higher altitudes. In other words, in Fall on a warm day, the temperature difference from high altitude to low altitude is greater than in Summer and Winter. In Summer, the high altitude air is not as cool as during Fall; therefore, the DIFFERENCE in warmth of air at the surface compared to the warmth available at high altitude is not as great in the Summer. In any season, higher altitude air is almost always cooler than surface air. The high altitude air in the Fall is even cooler compared to Summer. In Winter, surface air and high altitude air temperatures are also more similar, as both altitudes contain cool or cold air compared with a weaker temperature gradient compared to Fall.
Normal Atmosphere in Any Season
Usually, within the lower atmosphere (the troposphere) the air near the surface of the Earth is warmer than the air at higher altitudes, largely because the atmosphere is heated from below as solar radiation warms the Earth’s surface. Air directly above the surface warms the layer of the atmosphere directly above the surface. Air temperature also decreases with an increase in altitude because higher air is at lower pressure, and lower pressure causes lower temperature. Lower temperatures at high altitudes also result from the heat being radiated at greater distances (more distance to cool off) from the heat of the sun reflected and absorbed on the earth’s surface. Note: Certain weather conditions and topographical characteristics can cause an inversion, which means warmer temperatures exist at higher altitudes compared to the surface temperatures.
Now, let’s reconsider the fire department radio. If a transmitting antenna is extended into a duct of cool air at a high altitude, the radio waves enters at a very low angle of incidence and the VHF and UHF transmissions used by fire department radios are propagated far beyond normal distances, therefore leading to better signal reception and transmission.
Due to the different densities and refractive index of the warm and cool air, a temperature gradient causes radio waves to be refracted towards earth and then reflected back upwards. This process repeats and therefore causes the range of propagation of a good radio signal to be greater.
When a season, such as Fall, creates a greater difference between the cool air above and warm air at the earth’s surface, this could be a primary reason for the stronger radio signals at greater distances.
Other factors, such as humidity and cloud cover, and air pressure could also affect radio signals. Humidity is directly related to air pressure. High humidity increases air pressure with more water molecules in the air to degrade a radio signal. Today’s relative humidity was low for Chicago during the day, but was high during the night from Tuesday to Wednesday. Leaves on trees can also interfere with radio signals.
In summary …
1) increased air temperature weakens radio signals,
2) increased humidity weakens radio signals; but a
3) stronger temperature gradient with colder air above and warmer air at the surface bounces strong radio signals to greater distances.
These factors could also affect TV “cable cutters” who try to detect as many TV channels as possible on their televisions while getting signal reception with an attic or roof antenna.
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