Why Can You Hear Distant AM Radio Stations at Night?
― When the Sun Sets, the Layer of Sky That Absorbed Radio Waves Disappears
A faraway station that was completely silent by day suddenly comes in after dark. People who work or play with radio know this from experience: radio waves travel farther at night. The waves haven't changed. What has changed is the sky about 80 km over your head.
You're driving at night with the AM radio on. Faintly, over the programme you were listening to, a broadcast in a language you don't know starts to creep in.
Turn the dial a little and you pick up stations from regions hundreds of kilometres away, even broadcasts from across the sea. Try the same thing in daytime and you hear only noise.
Amateur radio operators, and people who have worked with radio on ships, know the rule by heart: at night aim low frequencies at distant stations, by day aim high ones.
Only two reasons
About 100 km up there is a thin layer of air that the Sun's light has given an electric charge. Radio waves bounce off this layer like light off a mirror and come back down beyond the horizon.
Only during the daytime, a little below the ceiling (60 to 90 km up), a layer forms that absorbs radio waves. When the Sun sets, this layer vanishes and the waves can reach the ceiling.
So distant stations come in at night because the "bouncing ceiling" is always there, and only at night is the "absorbing floor" taken away. Let's go through it step by step.
Radio waves bounce off a "ceiling" in the sky and travel far
Radio waves are a kind of light, so they basically travel in straight lines. The Earth is round, so a straight-travelling wave normally can't get past the horizon.
High in the sky, however, the Sun's ultraviolet light and X-rays strip electrons off air molecules. This layer, full of free-roaming electrons, is called the ionosphere.
When a radio wave below a certain frequency enters a layer rich in electrons at a slant, it is bent gradually and turned back downward. AM radio waves (roughly 0.5 to 1.6 megahertz) are the kind that are easily turned back this way.
Look at the right side of Figure 1. At night, a wave sent obliquely upward from the transmitter bounces off the layer about 100 km up and comes down hundreds of kilometres away. This is called a "sky wave." In good conditions it bounces repeatedly between the ground and the ceiling and travels even farther.
A "absorbing layer" that appears only by day was erasing the distant stations
The ionosphere is not a single sheet. It is divided into several tiers by height. The lowest tier (60 to 90 km up) sits where the air is still fairly dense.
In dense air, electrons shaken by a radio wave quickly collide with the molecules around them. Each time, some of the wave's energy is lost as heat. So this tier swallows radio waves before they can bounce back.
This low tier can exist only while sunlight keeps stripping off electrons. After sunset, the electrons quickly rejoin nearby molecules and the tier almost disappears. The upper, reflecting tier has thinner air, so its electrons survive into the night.
So only at night can AM waves slip through the absorbing tier and bounce off the one above. By day, distant stations are silent not because their waves fail to arrive, but because the waves are absorbed on the way.
Absorption is stronger for lower frequencies. High-frequency shortwaves pass through this tier even in daytime and bounce off a higher tier. The radio enthusiasts' rule of "high frequencies by day, low frequencies by night" fits this mechanism well.
Operators chasing long-distance contacts know that links along the boundary between day and night on Earth (the dawn or dusk line) tend to work well. Near that boundary, the absorbing tier is fading away while the reflecting tier is still dense, which is the usual explanation.
How far do night waves reach?
The distance a wave covers in a single bounce is set by the height of the bounce and the roundness of the Earth. A wave leaving at a grazing angle to the horizon and turning back at about 100 km can come down at most about 2,200 km away (the calculation is in the fold-out at the end).
Use the slider in Figure 2 to change the bounce height. The higher the tier it bounces off, the farther it reaches. In practice, waves leaving at a grazing angle are weak, so AM radio at night usually reaches from several hundred kilometres to around 1,000 km. That is still an enormous difference from daytime.
Summary
High in the sky there are layers, made by sunlight, that bounce radio waves back. During the day, an absorbing layer forms just below them, so AM waves can't travel far. When the Sun sets, the absorbing layer disappears, and the waves bounce off the upper layer and come down hundreds of kilometres away.
Night-time radio carries far not because the waves get stronger.
It is because the layer of sky that absorbed them by day vanishes at sunset.
This "electric layer of the sky" can also be disturbed by huge explosions on the Sun. See also Can a huge solar explosion cause a blackout on Earth? The reason distant sounds carry well at night is a completely different mechanism from radio. Reading Why can you hear distant sounds better at night? alongside this one makes the difference clear.
- With an AM radio (a real radio that receives radio waves, not a smartphone app), find several frequencies during the day where you hear nothing, and note them down.
- One or two hours after sunset, tune to the same frequencies. See whether distant regional stations or foreign broadcasts come in.
- If you have the energy, listen again around dawn. As the sky brightens, you can hear the distant stations quietly fade away.
Radio waves weaken inside steel-framed buildings, so try it by a window. What you hear also changes with the season and with solar activity.
For those who want to know more ― terms, formulas, and links to textbooksEach part is labelled with its level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics or earth science
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity specialist courses (radio engineering, upper-atmosphere physics) not taught in high school
- ResearchNot yet taught as settled fact even at university; researchers are still studying it
Junior highTerms: this phenomenon has names
- Ionosphere: the region from about 60 km to over 1,000 km up where sunlight splits some of the air into electrons and ions. From the lowest up, it is divided into the D, E and F layers.
- Sky wave (ionospheric reflected wave): a radio wave that bounces off the ionosphere and returns to the ground. This is the wave that lets AM radio travel far at night.
- Ground wave (surface wave): a radio wave travelling along the ground. Daytime AM radio reaches you mainly this way, and it weakens with distance.
Junior highHigh schoolChecking with a formula: how far does one bounce reach?
For a wave leaving at a grazing angle to the horizon and turning back at a layer of height h, we find the maximum distance it covers along the ground. Let the Earth's radius be R.
| In symbols | d ＝ 2 × √( 2 × R × h ) |
| In words | Distance reached ＝ 2 × the square root of (2 × Earth's radius × bounce height) |
| Where it comes from | The distance from height h to the horizon (by the Pythagorean theorem, √(2Rh) when h is much smaller than R), added for the upward and downward legs |
| Symbol | Meaning and unit |
| d | Maximum distance along the ground reached in one bounce (km) |
| R | Earth's radius (km) |
| h | Height at which the wave bounces (km) |
| Earth's radius R | About 6370 km |
| Bounce height of night AM radio (E layer) | About 100 km |
| Example: bounce height of daytime shortwave (F layer) | About 300 km |
| 2 × R | 2 × 6370 ＝ 12740 |
| Then multiply by h (100 km) | 12740 × 100 ＝ 1274000 |
| Square root of 1274000 (distance to the horizon) | About 1129 km |
| Upward and downward legs | 1129 × 2 ＝ 2258 |
| For comparison: h = 300 km | 12740 × 300 ＝ 3822000 |
| Square root of 3822000 | About 1955 km |
| Upward and downward legs | 1955 × 2 ＝ 3910 |
Night AM radio could in theory reach at most about 2,300 km in one bounce, a little beyond Tokyo to Taipei (about 2,100 km). This is the upper limit for a wave leaving at a grazing angle. Stable reception in practice is from much closer.
High schoolHigh school+Why does a layer of electrons bend radio waves?
High schoolRadio waves are electromagnetic waves, in which electric and magnetic fields oscillate. Refraction happens because the speed of the wave (strictly, the speed of its phase) changes with the medium. In a layer rich in electrons, the refractive index for radio waves becomes less than 1, and a wave entering at a slant is bent downward.
High school+The highest frequency at which a wave sent straight up is turned back is called the critical frequency. It is proportional to the square root of the electron number density (four times the electron density doubles it). A wave entering at a slant can bounce at even higher frequencies. The practical version of this limit, used in radio work, is the maximum usable frequency.
UniversityD-layer absorption and radio waves in plasma
How radio waves propagate in the ionosphere is described as electromagnetic waves in plasma by the Appleton–Hartree equation (magneto-ionic theory). Once the collision frequency (how often electrons hit neutral molecules) is included, the refractive index gains an imaginary part, and this is the D-layer absorption. The absorption is roughly inversely proportional to the square of the frequency, which is why medium waves are strongly absorbed by day. The D layer is mainly produced by ionization of nitric oxide by the Sun's hydrogen Lyman-alpha radiation, and by ionization by X-rays.
📖 Derivations and further reading: Wikipedia, "Ionosphere" (Japanese edition) ／ NICT Space Weather Forecast, "Ionosphere" (Japanese)
ResearchWhat is still not well understood
- Suddenly appearing dense layers. In summer near Japan, a very dense layer of electrons (sporadic E) can suddenly appear at E-layer height, letting radio waves of high frequencies that normally wouldn't come back travel far. Differences in wind direction are thought to gather the electrons, but predicting when and where it will appear is still said to be difficult.
- Shaking from below. Air waves created by typhoons, big eruptions and large earthquakes have been observed to reach the ionosphere and make its electron density fluctuate. How large and how widespread the effect is remains under study.
- Changes at the moment of sunrise and sunset. How fast the D layer forms and disappears at the dawn and dusk boundaries varies with season and solar activity, and the details are still being observed.
So the content of this article, too, is "an explanation of what is understood so far." The ionosphere is a living layer, shaken from both the Sun above and the ground below.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "Structure of the atmosphere," "Reflection and refraction of light" | Radio waves bouncing off a layer of the sky |
| High school | Physics: "Electromagnetic waves"; Earth science: "Structure of the atmosphere" | The ionosphere's tiers, turning back by refraction |
| High school+ | Physics (advanced): "Refractive index and wave speed" | Critical frequency, maximum usable frequency |
| University | Radio engineering, upper-atmosphere physics | Appleton–Hartree equation, D-layer absorption |
| Research | Space weather, coupling of atmosphere and ionosphere | Sporadic E, shaking from below |
| ― | Links to daily life | Night-time radio interference, choosing radio frequencies, gathering information in disasters |
- National Institute of Information and Communications Technology (NICT), Space Weather Forecast, "Ionosphere"
- Wikipedia (Japanese edition), "Ionosphere"
- The Nobel Prize in Physics 1947: Edward V. Appleton (awarded for observations of the ionosphere)
- K. Davies, Ionospheric Radio, Peter Peregrinus (a textbook on radio propagation in the ionosphere)
*This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. How far radio waves reach varies greatly with season, time of day, solar activity and reception conditions.