question: Why is the air cold when you fan yourself on a very hot day? Doesn't motion generate heat?
comment: Moving air takes away heat faster from your skin. That's why it feels cool. The effect is even bigger when you're sweating, the water that evaporates from your skin cools it even faster and more efficient. Motion doesn't necessarily generate heat. I assume what you mean is friction that generates heat. But the air speed generated by a fan is way too slow to generate any friction heat.
passages: BULLET::::6. In his "Novum Organum" (1620), Francis Bacon concludes that heat is the motion of the particles composing matter. In Francis Bacon, "Novum Organum" (London, England: William Pickering, 1850), from page 164: " … Heat appears to be Motion." From p. 165: " … the very essence of Heat, or the "Substantial self" of Heat, is motion and nothing else, … " From p. 168: " … Heat is not a uniform Expansive Motion of the whole, but of the small particles of the body; … "
   Section::::Heat transfer.
  
  Section::::Heat transfer.:Heat transfer between two bodies.
  
  Referring to conduction, Partington writes: "If a hot body is brought in conducting contact with a cold body, the temperature of the hot body falls and that of the cold body rises, and it is said that a "quantity of heat" has passed from the hot body to the cold body."
  
  Referring to radiation, Maxwell writes: "In Radiation, the hotter body loses heat, and the colder body receives heat by means of a process occurring in some intervening medium which does not itself thereby become hot."
   Free, or natural, convection occurs when bulk fluid motions (streams and currents) are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid. "Forced" convection is a term used when the streams and currents in the fluid are induced by external means—such as fans, stirrers, and pumps—creating an artificially induced convection current.
  
  Section::::Mechanisms.:Convection.:Convection-cooling.
  
  Convective cooling is sometimes described as Newton's law of cooling: 
  
  Section::::Mechanisms.:Convection vs. conduction.
   Maxwell writes that convection as such "is not a purely thermal phenomenon". In thermodynamics, convection in general is regarded as transport of internal energy. If, however, the convection is enclosed and circulatory, then it may be regarded as an intermediary that transfers energy as heat between source and destination bodies, because it transfers only energy and not matter from the source to the destination body.
   Is not the Heat of the warm Room convey'd through the vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum? And is not this Medium the same with that Medium by which Light is refracted and reflected, and by whose Vibrations Light communicates Heat to Bodies, and is put into Fits of easy Reflexion and easy Transmission?
   According to the laws of thermodynamics, all particles of matter are in constant random motion as long as the temperature is above absolute zero. Thus the molecules and atoms which make up the human body are vibrating, colliding, and moving. This motion can be detected as temperature; higher temperatures, which represent greater kinetic energy in the particles, feel warm to humans who sense the thermal energy transferring from the object being touched to their nerves. Similarly, when lower temperature objects are touched, the senses perceive the transfer of heat away from the body as feeling cold.
   BULLET::::- The wind chill factor measures the effect of wind speed on cooling of the human body below 10 °C (50 °F). As airflow increases over the skin, more heat will be removed. Standard models and conditions are used.
   Now, if the only way the system exchanges energy with its surroundings is through heat transfer, the heat rate can be used to calculate the energy balance, since
  
  where we have integrated the heat flux formula_1 over the surface formula_10 of the system.
  
  In real-world applications one cannot know the exact heat flux at every point on the surface, but approximation schemes can be used to calculate the integral, for example Monte Carlo integration.
  
  Section::::See also.
  
  BULLET::::- Radiant flux
  
  BULLET::::- Latent heat flux
  
  BULLET::::- Rate of heat flow
  
  BULLET::::- Insolation
  
  BULLET::::- Heat flux sensor
  
  BULLET::::- Relativistic heat conduction
   Convection may be described as the combined effects of conduction and fluid flow. From the thermodynamic point of view, heat flows into a fluid by diffusion to increase its energy, the fluid then transfers (advects) this increased internal energy (not heat) from one location to another, and this is then followed by a second thermal interaction which transfers heat to a second body or system, again by diffusion. This entire process is often regarded as an additional mechanism of heat transfer, although technically, "heat transfer" and thus heating and cooling occurs only on either end of such a conductive flow, but not as a result of flow. Thus, conduction can be said to "transfer" heat only as a net result of the process, but may not do so at every time within the complicated convective process.
   BULLET::::- Heat Wave appears in the "" episode "Requiem for a Scarlet Speedster!", voiced by Steven Blum. He assists Captain Cold and Weather Wizard in a crime spree.
  
  Section::::In other media.:Television.:Arrowverse.
   Section::::Description.
  
  Section::::Description.:Physical explanation.
   Although Carathéodory himself did not state such a definition, following his work it is customary in theoretical studies to define heat, , to the body from its surroundings, in the combined process of change to state from the state , as the change in internal energy, , minus the amount of work, , done by the body on its surrounds by the adiabatic process, so that .
   Section::::History.
   It is likely that Robert Stirling's air engine of 1818, which incorporated his innovative "Economiser" (patented in 1816) was the first air engine put to practical work. The economiser, now known as the regenerator, stored heat from the hot portion of the engine as the air passed to the cold side, and released heat to the cooled air as it returned to the hot side. This innovation improved the efficiency of Stirling's engine and should be present in any air engine that is properly called a Stirling engine.
   Section::::History.
   In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. The mechanisms include conduction, through direct contact of immobile bodies, or through a wall or barrier that is impermeable to matter; or radiation between separated bodies; or isochoric mechanical work done by the surroundings on the system of interest; or Joule heating by an electric current driven through the system of interest by an external system; or a combination of these. When there is a suitable path between two systems with different temperatures, heat transfer occurs necessarily, immediately, and spontaneously from the hotter to the colder system. Thermal conduction occurs by the stochastic (random) motion of microscopic particles (such as atoms or molecules). In contrast, thermodynamic work is defined by mechanisms that act macroscopically and directly on the system's whole-body state variables; for example, change of the system's volume through a piston's motion with externally measurable force; or change of the system's internal electric polarization through an externally measurable change in electric field. The definition of heat transfer does not require that the process be in any sense smooth. For example, a bolt of lightning may transfer heat to a body.
   Convective heat transfer, or convection, is the transfer of heat from one place to another by the movement of fluids, a process that is essentially the transfer of heat via mass transfer. Bulk motion of fluid enhances heat transfer in many physical situations, such as (for example) between a solid surface and the fluid. Convection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid (diffusion) and heat transference by bulk fluid flow streaming. The process of transport by fluid streaming is known as advection, but pure advection is a term that is generally associated only with mass transport in fluids, such as advection of pebbles in a river. In the case of heat transfer in fluids, where transport by advection in a fluid is always also accompanied by transport via heat diffusion (also known as heat conduction) the process of heat convection is understood to refer to the sum of heat transport by advection and diffusion/conduction.
   Air movement can have a significant influence on human thermal comfort. Wind chill in cold conditions is considered detrimental, but air movement in neutral to warm environments is considered beneficial. This is because normally under conditions with air temperatures above about 74°F, the body needs to lose heat in order to maintain a constant internal temperature. 
   BULLET::::- Forced convection: when a fluid is forced to flow over the surface by an internal source such as fans, by stirring, and pumps, creating an artificially induced convection current.
  
  In many real-life applications (e.g. heat losses at solar central receivers or cooling of photovoltaic panels), natural and forced convection occur at the same time (mixed convection).
   As a common noun, English "heat" or "warmth" (just as French "chaleur", German "Wärme", Latin "calor", Greek θάλπος, etc.) refers to (the human perception of) either thermal energy or temperature. Speculation on thermal energy or "heat" as a separate form of matter has a long history, see caloric theory, phlogiston and fire (classical element).
  
  The modern understanding of thermal energy originates with Thompson's 1798 mechanical theory of heat ("An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction"), postulating a mechanical equivalent of heat.
   BULLET::::- "Kinetic heating". As the airspeed increases, more and more molecules of air per second hit the aircraft. This causes a temperature rise in the Direct Reading thermometer probe of the aircraft due to friction. Because the airflow is thought to be compressible and isentropic, which, by definition, is adiabatic and reversible, the equations used in this article do not take account of "friction heating". This is why the calculation of static air temperature requires the use of the recovery factor, formula_8. Kinetic heating for modern passenger jets is almost negligible.
   The term “theory of heat”, being associated with either vibratory motion or energy, was generally used in contrast to the caloric theory, which views heat as a fluid or a weightless gas able to move in and out of pores in solids and found between atoms.
  
  Section::::See also.
  
  BULLET::::- Cold
  
  BULLET::::- History of thermodynamics
  
  BULLET::::- Larmor formula
  
  BULLET::::- Phlogiston
  
  BULLET::::- Thermodynamics
  
  BULLET::::- Timeline of thermodynamics, statistical mechanics, and random processes
  
  Section::::External links.
  
  BULLET::::- Fourier and the Theory of Heat – Massachusetts Institute of Technology
  
  BULLET::::- Mechanical Theory of Heat - Rochester Institute of Technology
   In many writings in this context, the term "heat flux" is used when what is meant is therefore more accurately called diffusive flux of internal energy; such usage of the term "heat flux" is a residue of older and now obsolete language usage that allowed that a body may have a "heat content".
  
  Section::::Heat transfer.:Microscopic view.
   While fans are often used to cool people, they do not actually cool air (electric fans may warm it slightly due to the warming of their motors), but work by evaporative cooling of sweat and increased heat convection into the surrounding air due to the airflow from the fans. Thus, fans may become ineffective at cooling the body if the surrounding air is near body temperature and contains high humidity. 
  
  Section::::History.
   "heat, in the earth as well as in the air, can come regularly only from the rays of the sun.
  
  I dare say, however, that I was lucky enough to imitate it a little in regards to some small fruits: I succeeded in making some ripen five or six weeks early, for instance, strawberries at the end of March, early vegetables and peas in April, figs in June, asparagus and lettuces in December, January …"
  
presuppositions: Because motion generates heat, fanning yourself on a hot day should not cool a person down.
corrections: Motion doesn't generate heat, what's actually happening, is fanning causes the movement of air, and the movement of air takes heat away from your skin. 
raw_presuppositions: Because motion generates heat, fanning yourself on a hot day should not cool a person down. Motion generates heat.
raw_corrections: Motion doesn't generate heat, what's actually happening, is fanning causes the movement of air, and the movement of air takes heat away from your skin.  Friction generates heat.