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Macaque monkey,  Crab-eating macaque (Macaca fascicularis) in Lopburi, Thailand.  Photo courtesy of 'Chris huh' and Wikipedia.

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Interacting with nature by K:

How to Offer Wild Birds Shelter in the Winter

Not all birds migrate south for the winter.  Winter is a hard season for birds, and many risk freezing to death at night. It doesn't take much effort or money to provide shelter for them, and it can make a huge difference to the little feathered guys!

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Radio signals bend in the ionosphere soup

Q: I have always wondered why AM radio signals coming from a distance are much stronger at night, especially cloudy ones and why FM signals seem stronger early mornings. --Dennis and Treen

A: Earth's atmosphere changes how radio waves travel. Out in space they zip along at light speed and travel vast distances: from Earth to Pluto, for example.

Right: [U.S. Navy] Wave travel: line of sight and bouncing off the ionosphere

Here on Earth, when a signal travels in a straight line from transmitter to receiver, Earth blocks the signal at the horizon. Viewed from eye level, the horizon is about 2.5 miles away but, from the top of a 500-foot transmission tower, it's about 100 miles distant. Line-of-sight signals, thus, go about 100 miles.

When we pick up an AM signal from farther than that, a cloud-like layer of ionized particles in the air (called the ionosphere F-layer) bent the radio wave down to reach us. See figure. When the radio wave enters the F-layer air, it slows down and therefore bends to a new direction. A soda straw in a glass of water appears bent due to the same phenomenon.

The ionosphere bends signals best at night because the Sun is no longer ionizing the atmosphere then. That's why you pick up distant AM signals at night. An AM signal can hop all the way around the world at night, bending down from the ionosphere and reflecting back up from Earth: hopping in that fashion and ultimately going vast distances.

On the other hand, FM radio waves go right through the ionosphere. Consequently, most FM signals are line-of-sight.

Water in the atmosphere weakens all signals but the higher frequencies more than the lower ones. For this reason, water reduces FM strength much more than AM since FM frequencies are about 100 times greater than AM. In fact, AM is not much affected even by rain. This argues against AM signals being stronger on cloudy nights. However, it may explain why you pick up FM signals better in the morning.

"FM repeater stations, which rely on long distance pick up of an FM signal, often experience reduced signal level during the hotter summer months..." says Lewis Downey, engineer at KUER FM in Salt Lake City, Utah. This may be due to more moisture in the air during summer, which weakens FM signals. The morning, however, has cooler temperatures and lower humidity. Consequently, the air contains less water to disrupt the FM signal. Thus, the FM signal is somewhat stronger in the cool of the morning.

(Answered by April Holladay, science correspondent, January 23, 2002)

Further Surfing:

USATODAY.com: Humidity

KUER FM90: Tuning tips

BBC: Better reception

US Navy: Propagation of waves

New Zealand Amateur Radio: Proagation of radio signals

Clouds in a glass of beer by Craig F. Bohren, Wiley Science, 1987, pp. 10, 11.

 

 

 

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