Editing Radio propagation (section)

==Practical effects==
The average person can notice the effects of changes in radio propagation in several ways.

In [[AM broadcasting]], the dramatic ionospheric changes that occur overnight in the mediumwave band drive a unique [[broadcast license]] scheme in the United States, with entirely different [[transmitter power output]] levels and [[directional antenna]] patterns to cope with skywave propagation at night. Very few stations are allowed to run without modifications during dark hours, typically only those on [[clear-channel station|clear channels]] in [[NARBA|North America]].<ref>
{{cite report
 |title=Why AM stations must reduce power, change operations, or cease broadcasting at night
 |date=2015-12-11
 |language=en
 |publisher=U.S. Federal Communications Commission
 |url=https://www.fcc.gov/media/radio/am-stations-at-night
 |access-date=2017-02-11
}}</ref> Many stations have no authorization to run at all outside of daylight hours.

For [[FM broadcasting]] (and the few remaining low-band [[TV station]]s), weather is the primary cause for changes in VHF propagation, along with some diurnal changes when the sky is mostly without [[cloud cover]].<ref>
{{cite web
 |title=VHF/UHF Propagation 
 |publisher=Radio Society of Great Britain
 |website=rsgb.org
 |language=en-GB
 |url=http://rsgb.org/main/get-started-in-amateur-radio/operating-your-new-station/vhfuhf-propagation/
 |access-date=2017-02-11
}}</ref> These changes are most obvious during temperature inversions, such as in the late-night and early-morning hours when it is clear, allowing the ground and the air near it to cool more rapidly. This not only causes [[dew]], [[frost]], or [[fog]], but also causes a slight "drag" on the bottom of the radio waves, bending the signals down such that they can follow the Earth's curvature over the normal radio horizon. The result is typically several stations being heard from another [[media market]] – usually a neighboring one, but sometimes ones from a few hundred kilometers (miles) away.  [[Ice storm]]s are also the result of inversions, but these normally cause more scattered omnidirection propagation, resulting mainly in interference, often among [[weather radio]] stations. In late spring and early summer, a combination of other atmospheric factors can occasionally cause skips that duct high-power signals to places well over 1000&nbsp;km (600 miles) away.

Non-broadcast signals are also affected. [[Mobile phone signal]]s are in the UHF band, ranging from 700 to over 2600&nbsp;MHz, a range which makes them even more prone to weather-induced propagation changes. In [[urban area|urban]] (and to some extent [[suburb]]an) areas with a high [[population density]], this is partly offset by the use of smaller cells, which use lower [[effective radiated power]] and [[beam tilt]] to reduce interference, and therefore increase [[frequency reuse]] and user capacity. However, since this would not be very cost-effective in more [[rural]] areas, these cells are larger and so more likely to cause interference over longer distances when propagation conditions allow.

While this is generally transparent to the user thanks to the way that [[cellular network]]s handle cell-to-cell [[handoff]]s, when [[international boundary|cross-border]] signals are involved, unexpected charges for international [[roaming]] may occur despite not having left the country at all. This often occurs between southern [[San Diego]] and northern [[Tijuana]] at the western end of the [[Mexico–United States border|U.S./Mexico border]], and between eastern [[Detroit]] and western  [[Windsor, Ontario|Windsor]] along the [[Canada–United States border|U.S./Canada border]]. Since signals can travel unobstructed over a [[body of water]] far larger than the [[Detroit River]], and cool water temperatures also cause inversions in surface air, this "fringe roaming" sometimes occurs across the [[Great Lakes]], and between islands in the [[Caribbean]]. Signals can skip from the [[Dominican Republic]] to a mountainside in [[Puerto Rico]] and vice versa, or between the U.S. and British [[Virgin Islands]], among others. While unintended cross-border roaming is often automatically removed by [[mobile phone company]] billing systems, inter-island roaming is typically not.