question: Why does heat appear to warp the air?
comment: Light bends when it passes between mediums with different refractive indexes. This is how lenses work, and why when you put you hand under water it seems to be in a different place than it actually is. Air slightly changes its refractive index when it changes temperature, so hot air will bend light and appear to make things seen through it ripple as the hot air mixes with cooler air.
passages: 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.
   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.)
   The refractive index of air is close to, but just greater than 1. Systematic variations in refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers onboard ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of Earth's surface.
  
  The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage.
  
  Section::::Circulation.
   As the temperature of air increases, the index of refraction of air decreases, a side effect of hotter air being less dense. Normally this results in distant objects being shortened vertically, an effect that is easy to see at sunset where the sun is visible as an oval. In an inversion, the normal pattern is reversed, and distant objects are instead stretched out or appear to be above the horizon, leading to the phenomenon known as a Fata Morgana or mirage.
   The use of the term "mired" dates back to Irwin G. Priest's observation in 1932 that the just noticeable difference between two illuminants is based on the difference of the reciprocals of their temperatures, rather than the difference in the temperatures themselves.
  
  Section::::Examples.
  
  A blue sky, which has a color temperature "T" of about 25,000 K, has a mired value of "M" = 40 mireds, while a standard electronic photography flash, having a color temperature "T" of 5000 K, has a mired value of "M" = 200 mireds.
   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.
   Section::::Discovery and naming.
  
  STEVE has been observed by auroral photographers for decades, with some evidence to suggest that observations may have been recorded as early as 1705. However, the first accurate determination of what STEVE is was not made until after the formal "discovery" in late 2016.
   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.
   BULLET::::- Sources, by urbanization level – whether the source is within a city or not is relevant in that urban areas constitute a so-called "heat island" and the heat rising from an urban area causes the atmosphere above an urban area to be more turbulent than the atmosphere above a rural area
  
  BULLET::::- Urban source – emission is in an urban area
  
  BULLET::::- Rural source – emission is in a rural area
  
  BULLET::::- Sources, by elevation
  
  BULLET::::- Surface or ground-level source
  
  BULLET::::- Near surface source
  
  BULLET::::- Elevated source
  
  BULLET::::- Sources, by duration
   Section::::Sounding rockets.
  
  After a few milliseconds, the energy of the shock front will no longer be great enough to heat the air into incandescence. At that point, the shock front becomes invisible, a process known as "breakaway". This makes the shock wave difficult to diagnose beyond this boundary.
   Air masses are large bodies of air with similar properties of temperature and humidity that form over source regions. The warm air mass behind a warm front is not only warmer, but often (but not always) also higher in humidity than the colder air preceding it. Because of a warm air mass’s higher temperature and thus lesser density, mixing between the two air masses is unlikely. Being light, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning. The boundary between the two air masses has a gradual slope of 1:200 and lifting is slow but persistent.
   The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. In the 1862, during the American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle, because they could not hear the sounds of battle only six miles downwind.
  
  Section::::Vertical component.:Planetary boundary layer.:Effects on architecture.
   Albedo and climate in some areas are affected by artificial clouds, such as those created by the contrails of heavy commercial airliner traffic. A study following the burning of the Kuwaiti oil fields during Iraqi occupation showed that temperatures under the burning oil fires were as much as 10 °C colder than temperatures several miles away under clear skies.
  
  Section::::Examples of terrestrial albedo effects.:Aerosol effects.
   As in the case of advection turning, when there is a cross-isothermal component of the geostrophic wind, a sharpening of the temperature gradient results. Thermal wind causes a deformation field and frontogenesis may occur.
  
  Section::::Examples.:Jet stream.
   In principle, we cannot directly see a difference in temperature, a different gas, or a shock wave in the transparent air. However, all these disturbances refract light rays, so they can cast shadows. The plume of hot air rising from a fire, for example, can be seen by way of its shadow cast upon a nearby surface by the uniform sunlight.
  
  Section::::Sunlight shadowgraph.
   An urban heat island occurs where the combination of heat-absorbing infrastructure such as dark asphalt parking lots and road pavement and expanses of black rooftops, coupled with sparse vegetation, raises air temperature by 1 to 3 °C higher than the temperature in the surrounding countryside.
   Section::::Historical examples.
   This optical phenomenon occurs because rays of light are strongly 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. In calm weather, a layer of significantly warmer air can rest over colder dense air, forming an atmospheric duct which acts like a refracting lens, producing a series of both inverted and erect images.
   Section::::Fictional character biography.:One Year Later.
   Thermals are caused by local differences in temperature, pressure, or impurity concentration in the vertical. Temperature differences can cause air currents because warmer air is less dense than cooler air, causing the warmer air to appear "lighter." Thus, if the warm air is under the cool air, air currents will form as they exchange places. Air currents are caused because of the uneven heating of Earth's surface.
   Atmospherics
  
  In marketing the term atmospherics is used to describe the discipline of designing commercial spaces. Atmospherics was coined by Philip Kotler in a 1973 article in the Journal of Retailing. 
   In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a "pinhole" and the escape of the gas is due to the pressure difference between the container and the exterior. Under these conditions, essentially all molecules which arrive at the hole continue and pass through the hole, since collisions between molecules in the region of the hole are negligible. Conversely, when the diameter is larger than the mean free path of the gas, flow obeys the Sampson flow law.
   At hypersonic speeds, this effect becomes much more pronounced, due to the exponential reliance on the Mach number. Since the boundary layer becomes so large, it interacts more viscously with the surrounding flow. The overall effect of this interaction is to create a much higher skin friction than normal, causing greater surface heat flow. Additionally, the surface pressure spikes, which results in a much larger aerodynamic drag coefficient. This effect is extreme at the leading edge and decreases as a function of length along the surface.
  
  Section::::Physics.:Entropy Layer.
   Atmospheric heat comes from long wave radiation from the soil and, mostly, from the water surface (oceans, lakes, rivers), since water is a not diathermanous body and covers three quarters of Earth's surface. Diathermancy cause subsidence on damp or water surfaces. That is because these areas tend to absorb heat radiation directly from the Sun but very slowly and also emits this radiation to the atmosphere very slowly. Therefore, cold ocean currents have very clear skies, without clouds, because subsidence from cold and heavy air avoids or limits convection since they are opposite processes.
   All the above-mentioned results are for situations with no heating effects. A numerical model study have shown that, when a surface in a street canyons heats up, it changes the characteristics of the vortex flow. And heating of different surfaces; upwind wall, downwind wall, canyon floor, changes the vortex flow in different manners.
  
  Section::::Effects.:Air quality.
  
  The modification of temperature and wind by the presence of a street canyon, consequently affect the air quality inside the street canyon.
  