question: Why are you able to see “waves” of heat when you look down a road on a hot day.
comment: Hot air has a lower density than cold air. Hot air rises and cold air takes its place, so you get a sort-of mini upward wind (it's called [convection]( URL_1 )) close to the surface of the road. Because they have different densities, hot air and cold air have slightly different optical properties, and [refraction]( URL_0 ) happens in the convection area.
passages: In 1966, Labrecque told "Take One" magazine, "I found I could go further in experimenting with the effects of various lenses on heat. For example, we found out how heat waves can be seen coming off the road on a hot day. We first noticed the effect approaching the camera way in the distance. With everything telescoped, it produced a unique effect. Notice how, on the right of the line of cyclists (they were on a straight road seven miles long), the telephone poles seem to be a narrow fence. In reality, there is a 180 feet between each pole.
   Temperature variations in the air can also cause refraction of light. This can be seen as a heat haze when hot and cold air is mixed e.g. over a fire, in engine exhaust, or when opening a window on a cold day. This makes objects viewed through the mixed air appear to shimmer or move around randomly as the hot and cold air moves. This effect is also visible from normal variations in air temperature during a sunny day when using high magnification telephoto lenses and is often limiting the image quality in these cases.
   BULLET::::- 1913 – In July, a heat wave struck California. During this heat wave, Death Valley recorded a record high temperature of at Furnace Creek, which still remains the highest ambient air temperature recorded on Earth.
  
  BULLET::::- 1923–1924 – During a period of 160 such days from 31 October 1923 to 7 April 1924, the Western Australian town of Marble Bar reached .
   Atmospheric internal waves can be visualized by wave clouds: at the wave crests air rises and cools in the relatively lower pressure, which can result in water vapor condensation if the relative humidity is close to 100%. Clouds that reveal internal waves launched by flow over hills are called lenticular clouds because of their lens-like appearance. Less dramatically, a train of internal waves can be visualized by rippled cloud patterns described as herringbone sky or mackerel sky. The outflow of cold air from a thunderstorm can launch large amplitude internal solitary waves at an atmospheric inversion. In northern Australia, these result in Morning Glory clouds, used by some daredevils to glide along like a surfer riding an ocean wave. Satellites over Australia and elsewhere reveal these waves can span many hundreds of kilometers.
   The optical phenomenon occurs because rays of light are bent when they pass through air layers of different temperatures in a steep thermal inversion where an atmospheric duct has formed. (A thermal inversion is an atmospheric condition where warmer air exists in a well-defined layer above a layer of significantly cooler air. This temperature inversion is the opposite of what is normally the case; air is usually warmer close to the surface, and cooler higher up.)
   Fata Morgana mirages tremendously distort the object or objects which they are based on, such that the object often appears to be very unusual, and may even be transformed in such a way that it is completely unrecognizable. A Fata Morgana can be seen on land or at sea, in polar regions or in deserts. This kind of mirage can involve almost any kind of distant object, including such things as boats, islands, and coastline.
   From the above observation it is clear that the author observed an inferior mirage green flash, when the much warmer surface of the desert with the help of the rising Sun was fighting the cool morning air, producing in the process a green flash – one of nature's great spectacles.
  
  A green flash might be also seen with a rising or setting Moon.br
   Air temperature variations close to the surface can give rise to other optical phenomena, such as mirages and Fata Morgana. Most commonly, air heated by a hot road on a sunny day deflects light approaching at a shallow angle towards a viewer. This makes the road appear reflecting, giving an illusion of water covering the road.
  
  Section::::Water waves.
   It is not yet known exactly how much wave energy is available to heat the corona. Results published in 2004 using data from the TRACE spacecraft seem to indicate that there are waves in the solar atmosphere at frequencies as high as (10 second period). Measurements of the temperature of different ions in the solar wind with the UVCS instrument aboard SOHO give strong indirect evidence that there are waves at frequencies as high as , well into the range of human hearing. These waves are very difficult to detect under normal circumstances, but evidence collected during solar eclipses by teams from Williams College suggest the presences of such waves in the 1– range.
   Heat wave
  
  A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary, a heat wave is usually measured relative to the usual weather in the area and relative to normal temperatures for the season. Temperatures that people from a hotter climate consider normal can be called a heat wave in a cooler area if they are outside the normal climate pattern for that area.
   "Rough Waves" impressed Ogata’s follower Sakai Hōitsu, who in 1805 created his work "Waves", in which he tried to convey a similar frightening atmosphere and a premonition of danger. At some time in the past, Hōitsu's family had commissioned work from Ogata, and so a collection of Ogata's pictures was available for Hōitsu to study in detail. The images in "Waves", created in ink on silver-plated paper, were more abstract than those in Ogata's work, but retained their demonic character.
   The atmospheric internal waves that form wave clouds are created as stable air flows over a raised land feature such as a mountain range, and can form either directly above or in the lee of the feature. As an air mass travels through the wave, it undergoes repeated uplift and descent. If there is enough moisture in the atmosphere, clouds will form at the cooled crests of these waves. In the descending part of the wave, those clouds will evaporate due to adiabatic heating, leading to the characteristic clouded and clear bands. The cloud base on the leeward side is higher than on the windward side, because precipitation on the windward side removes water from the air.
   BULLET::::- 2007: Tomczyk et al. reported the detection of Alfvénic waves in images of the solar corona with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. However, these observations turned out to be kink waves of coronal plasma structures.
   Section::::History.:The coronal heating problem.
  
  The study of Alfvén waves began from the coronal heating problem, a longstanding question in heliophysics. It was unclear as to why the temperature of the sun's corona is hot (about one million degrees Kelvin) compared to its surface (i.e. the photosphere), which is only a few thousand degrees. Intuitively, it would make sense to see a decrease in temperature when moving away from a heat source, but this does not seem to be the case even though the photosphere is denser and would generate more heat than the corona. 
   In scuba diving, a thermocline where water drops in temperature by a few degrees Celsius quite suddenly can sometimes be observed between two bodies of water, for example where colder upwelling water runs into a surface layer of warmer water. It gives the water an appearance of wrinkled glass that is often used to obscure bathroom windows and is caused by the altered refractive index of the cold or warm water column. These same schlieren can be observed when hot air rises off the tarmac at airports or desert roads and is the cause of mirages.
  
  Section::::Other water bodies.
   A Fata Morgana is most commonly seen in polar regions, especially over large sheets of ice that have a uniform low temperature. It may, however, be observed in almost any area. In polar regions the Fata Morgana phenomenon is observed on relatively cold days. In deserts, over oceans, and over lakes, however, a Fata Morgana may be observed on hot days.
   The mechanism for the forcing of the wave, for example the generation of the initial or prolonged disturbance in the atmospheric variables, can vary. Generally, waves are either excited by heating or dynamic effects, for example the obstruction of the flow by mountain ranges like the Rocky Mountains in the U.S. or the Alps in Europe. Heating effects can be small-scale (like the generation of gravity waves by convection) or large-scale (the formation of Rossby waves by the temperature contrasts between continents and oceans in the Northern hemisphere winter).
   Fata Morgana mirages are most common in polar regions, especially over large sheets of ice with a uniform low temperature, but they can be observed almost anywhere. In polar regions, a Fata Morgana may be observed on cold days; in desert areas and over oceans and lakes, a Fata Morgana may be observed on hot days. For a Fata Morgana, temperature inversion has to be strong enough that light rays' curvatures within the inversion are stronger than the curvature of the Earth.
   Heat haze, also called heat shimmer, refers to the inferior mirage experienced when viewing objects through a layer of heated air; for example, viewing objects across hot asphalt or through the exhaust gases produced by jet engines. When appearing on roads due to the hot asphalt, it is often referred to as a highway mirage.
   BULLET::::- In the course of Project MaxWave, researchers from the GKSS Research Centre, using data collected by ESA satellites, identified a large number of radar signatures that have been portrayed as evidence for rogue waves. Further research is under way to develop better methods of translating the radar echoes into sea surface elevation, but at present this technique is not proven.
   In recent years, infrared images have revealed dozens of examples of "infrared HH objects". Most look like bow waves (similar to the waves at the head of a ship), and so are usually referred to as molecular "bow shocks". The physics of infrared bow shocks can be understood in much the same way as that of HH objects, since these objects are essentially the same – supersonic shocks driven by collimated jets from the opposite poles of a protostar. It is only the conditions in the jet and surrounding cloud that are different, causing infrared emission from molecules rather than optical emission from atoms and ions.
   BULLET::::- 1900 – The historical heatwave of the center of Argentina between the first eight days of February 1900 known as "the week of fire" affected the city of Buenos Aires and Rosario with temperatures of up to but with a very high index of humidity that elevated the sensation of heat to severely affecting the health of people causing at least more than 478 fatalities.
  
  Section::::20th century.
  
  BULLET::::- 1901 – The 1901 eastern United States heat wave killed 9,500 in the Eastern United States.
   It is possible to observe the Novaya Zemlya effect in any place, where the temperature variations are great enough to produce a high refraction.
  
  Section::::Green flash.
  
  Green flash is a rare optical phenomenon that occurs during or shortly after set and during or before rise of a bright astronomical object, when a green spot is visible for a short period of time above a mirage of an astronomical object or its set/rise point.
   Other famous observations that were called "mirages" may actually be referring to looming. One of those was described in "Scientific American" on August 25, 1894 as "a remarkable mirage seen by the citizens of Buffalo, New York". Such looming—sometimes with apparent magnification of opposite shores—have been reported over the Great Lakes. Canadian shorelines have been observed from Rochester, New York, across Lake Ontario, and from Cleveland across Lake Erie. The landforms over distant, normally beyond the horizon, were sometimes perceived as away.
   A green flash from the moon and bright planets at the horizon, including Venus and Jupiter, can also be observed.
  
  Section::::Mirage.:Fata Morgana.
  