question: Why does steam appear in sub-boiling temperatures?
comment: I think steam are actually invisible. It is the condensate that are visible. Steam happens when warm humid air meets with cold air. The humidity in the warm air precipitate into steam. This happens in the shower and when you are boiling water as well. This is also exactly what cloud is.
passages: Steam
  
  Steam is water in the gas phase, which is formed when water boils or evaporates. Steam is invisible; however, "steam" often refers to wet steam, the visible mist or aerosol of water droplets formed as this water vapour condenses. At lower pressures, such as in the upper atmosphere or at the top of high mountains, water boils at a lower temperature than the nominal at standard pressure. If heated further it becomes superheated steam. 
   For example, if the vapor's partial pressure is 2% of atmospheric pressure and the air is cooled from 25 °C, starting at about 22 °C water will start to condense, defining the dew point, and creating fog or dew. The reverse process accounts for the fog burning off in the morning. If the humidity is increased at room temperature, for example, by running a hot shower or a bath, and the temperature stays about the same, the vapor soon reaches the pressure for phase change, and then condenses out as minute water droplets, commonly referred to as steam.
   Section::::History.
   Section::::Occurrences.
  
  Steam devils are seen on the Great Lakes in early winter. They occur in the Atlantic off the coast of the Carolinas when cold air from the continent blows across the Gulf Stream. Steam devils can occur on small lakes and even over hot springs, but rather more rarely than on large bodies of water. It is also possible for steam devils to form over wet land if the air is cold and the sun is heating the ground.
   By contrast, water vapor that includes water droplets is described as wet steam. If wet steam is heated further, the droplets evaporate, and at a high enough temperature (which depends on the pressure) all of the water evaporates, the system is in vapor–liquid equilibrium, and it becomes "saturated steam".
   At atmospheric pressure the boiling point of water is - liquid water at atmospheric pressure does not exist at temperatures higher than . At that moment, the water would boil and turn to vapor explosively, and the liquid water turned to gas would take up significantly more volume (~1,600-fold) than it did as liquid, causing a vapor explosion. Such explosions can happen when the superheated water of a Boiler escapes through a crack in a boiler, causing a boiler explosion.
  
  Section::::Mechanism.:BLEVEs without chemical reactions.
   At very low pressures (below about 0.006 atm), water cannot exist in the liquid state and passes directly from solid to gas by sublimation—a phenomenon exploited in the freeze drying of food. At very high pressures (above 221 atm), the liquid and gas states are no longer distinguishable, a state called supercritical steam.
   Water is said to "boil" when bubbles of water vapor grow without bound, bursting at the surface. For a vapor bubble to expand, the temperature must be high enough that the vapor pressure exceeds the ambient pressure (the atmospheric pressure, primarily). Below that temperature, a water vapor bubble will shrink and vanish.
  
  Superheating is an exception to this simple rule; a liquid is sometimes observed not to boil even though its vapor pressure does exceed the ambient pressure. The cause is an additional force, the surface tension, which suppresses the growth of bubbles.
   Boiling is also a phase transition from the liquid phase to gas phase, but boiling is the formation of vapor as bubbles of vapor "below the surface" of the liquid. Boiling occurs when the equilibrium vapor pressure of the substance is greater than or equal to the environmental pressure. The temperature at which boiling occurs is the boiling temperature, or boiling point. The boiling point varies with the pressure of the environment.
   Clouds of condensation can sometimes be seen rising from manholes in Manhattan through orange and white "chimneys". This can be caused by external water being boiled by contact with the steam pipes or by leaks in the steam system itself.
   Section::::Saturated steam.
  
  Saturated steam is steam that is in equilibrium with heated water at the same pressure, i.e., it has not been heated above the boiling point for its pressure. This is in contrast to superheated steam, in which the steam (vapor) has been separated from the water droplets then additional heat has been added.
  
  These condensation droplets are a cause of damage to steam turbine blades, the reason why such turbines rely on a supply of dry, superheated steam.
   Section::::Practical uses.:Flash boiling in cooking.
   At standard atmospheric pressure and low temperatures, no boiling occurs and the heat transfer rate is controlled by the usual single-phase mechanisms. As the surface temperature is increased, local boiling occurs and vapor bubbles nucleate, grow into the surrounding cooler fluid, and collapse. This is "sub-cooled nucleate boiling", and is a very efficient heat transfer mechanism. At high bubble generation rates, the bubbles begin to interfere and the heat flux no longer increases rapidly with surface temperature (this is the departure from nucleate boiling, or DNB).
   Superheated steam
  
  Superheated steam is a steam at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured.
   In precipitation models this is generally a prelude to models of the growth process itself. Sometimes precipitation is rate-limited by the nucleation process. This happens for example before one takes a cup of superheated water from a microwave and, when jiggling it against dust particles on the wall of the cup, enables "heterogeneous" nucleation that then rapidly converts much of that water into steam.
   Section::::Types of steam and conversions.
  
  Steam is traditionally created by heating a boiler via burning coal and other fuels, but it is also possible to create steam with solar energy. Water vapor that includes water droplets is described as "wet steam". As wet steam is heated further, the droplets evaporate, and at a high enough temperature (which depends on the pressure) all of the water evaporates and the system is in vapor–liquid equilibrium.
   Like all liquids, water boils when its vapor pressure reaches its surrounding pressure. In nature, the atmospheric pressure is lower at higher elevations and water boils at a lower temperature. The boiling temperature of water for atmospheric pressures can be approximated by the Antoine equation:
  
  or transformed into this temperature-explicit form:
  
  where the temperature formula_32 is the boiling point in degrees Celsius and the pressure formula_33 is in Torr.
  
  Section::::Dühring's rule.
  
  Dühring's rule states that a linear relationship exists between the temperatures at which two solutions exert the same vapor pressure.
  
  Section::::Examples.
   Natural convection, or free convection, occurs due to temperature differences which affect the density, and thus relative buoyancy, of the fluid. Heavier (denser) components will fall, while lighter (less dense) components rise, leading to bulk fluid movement. Natural convection can only occur, therefore, in a gravitational field. A common example of natural convection is the rise of smoke from a fire. It can be seen in a pot of boiling water in which the hot and less-dense water on the bottom layer moves upwards in plumes, and the cool and more dense water near the top of the pot likewise sinks.
   Section::::Table of specific latent heats.
  
  The following table shows the specific latent heats and change of phase temperatures (at standard pressure) of some common fluids and gases.
  
  Section::::Specific latent heat for condensation of water in clouds.
  
  The specific latent heat of condensation of water in the temperature range from −25 °C to 40 °C is approximated by the following empirical cubic function:
  
  where the temperature formula_4 is taken to be the numerical value in °C.
   Two different regimes may be distinguished in the nucleate boiling range. When the temperature difference is between approximately to above T, isolated bubbles form at nucleation sites and separate from the surface. This separation induces considerable fluid mixing near the surface, substantially increasing the convective heat transfer coefficient and the heat flux. In this regime, most of the heat transfer is through direct transfer from the surface to the liquid in motion at the surface and not through the vapor bubbles rising from the surface.
   The phase of water (i.e., whether it assumes the form of a solid, liquid, or gas) is determined by its temperature and pressure. For example, in the case of liquid-gas transition, at each pressure there is a special "transition temperature" formula_1 such that if the sample temperature is even a little above formula_1, the sample will be a gas, but, if the sample temperature is even a little below formula_1, the sample will be a liquid; see phase transition. For example, at the standard atmospheric pressure, formula_1 is 100 °C = 212 °F. The transition temperature formula_1 decreases with decreasing pressure (which explains why water boils at lower temperatures at higher altitudes and at higher temperatures in a pressure cooker; see here for more information). In the case of water vapor in air, the formula_1 corresponding to the partial pressure of water vapor is called the dew point. (The solid–liquid transition also happens around a specific transition temperature called the melting point. For most substances, the melting point also decreases with decreasing pressure, although water ice in particular - in its I form, which is the most familiar one - is a prominent exception to this rule.)
   Section::::Cause.:States of water.
   If steam is released normally, say by opening a throttle valve, the bubbling action of the water remains moderate and relatively dry steam can be drawn from the highest point in the vessel.
  
  If steam is released more quickly, the more vigorous boiling action that results can throw a fine spray of droplets up as "wet steam" which can cause damage to piping, engines, turbines and other equipment downstream.
   Section::::Types.:Transition.
  
  "Transition boiling" may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.
  
  The formation of bubbles in a heated liquid is a complex physical process which often involves cavitation and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface.
  
  Section::::Types.:Film.
   Boiler
  
  A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.
  
  Section::::Heat sources.
  
presuppositions: You can see steam. Steam is visible.
corrections: Steam is invisible; you are seeing the condensate. Steam is invisible, it is condensate that is visible.
raw_presuppositions: You can see steam. Steam is visible.
raw_corrections: Steam is invisible; you are seeing the condensate. Steam is invisible, it is condensate that is visible.