Steam on the glass of the window Tonight I observed a phenomenon that I saw that before thousands of times.
I saw Steam on the glass of the window, and I wondered when this happens, what change happens to heat transfer? is it makes it possible to cold weather can more easily transfer from outside to the room?
I would appreciate in advance for your answers.
 A: A few potential changes come to mind:

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*Conduction: The additional layer of water changes the conductive heat path between the window and the air (in the manner of an insulator, perhaps). But the layer is very thin, and water conducts heat quite well (with a thermal conductivity of ~1 W/m-K, similar to that of glass, for an R-value of ~0.001, hundreds or thousands of times worse than a good insulator). The expected difference seems negligible.

*Convection: The "pebbled" nature of isolated water droplets changes the nature of natural convection at the vertical surface of the window. The expected difference seems negligible, though it would be interesting to check a heat transfer handbook to see how much the convection coefficient is predicted to change.

*Radiation: The water droplets act like a lens to focus or reflect sunlight or outgoing IR radiation. Since the total irradiation of the glass would remain the same, the expected difference from focusing seems negligible, though the pretty patterns on the opposite wall might take your mind off the chilly room. Reflection may be a different story if incoming sunlight undergoes total internal reflection.

*Latent heat: By condensing directly at the window, the water distributes energy formerly stored in the air in your room at just about the worst place possible: directly adjacent to the cold window. It would be interesting also to compare the air heat transfer with this additional effective heat transfer that arises through mass transfer.

A: First, a few concepts from an engineering point of view.  The "gas" phase of a liquid is called vapor when the vapor can be condensed into a liquid at normal temperatures and pressures.  For example, water vapor is distinguished from "non-condensable" oxygen gas.  Water vapor is called steam; for engineering applications, steam refers to high-temperature water vapor. The fluid is in a saturated state when both liquid and vapor can be present. There is a specific saturation temperature at a given pressure. The quality determines the amounts of liquid and vapor present in a saturated state.  For example, 100% quality is saturated vapor with no liquid. If the temperature is greater than the saturation temperature at a given pressure, the only vapor is present and the vapor is said to be superheated.  If the temperature is less than the saturation temperature at a given pressure, only liquid is present and the liquid is said to be is subcooled. Steam Tables, available online, provide the saturation, superheated, and subcooled conditions for water over a wide range of temperatures and pressures.
Condensation of a vapor occurs when the vapor comes into contact with a surface, the temperature of which is maintained at a value lower than the saturation temperature of the vapor for the pressure at which it exists. The removal of thermal energy from the vapor causes it to release its latent heat of vaporization and, hence, to condense (change phase to liquid) on the surface.
Regarding your question about the effect of the condensate on the room, I assume you are referring to condensation on the inside of the window during cold weather. Inside the room, we have water vapor mixed with non-condensable air.  The water vapor in the air can be saturated or superheated depending on its partial pressure and the gas/vapor temperature.  The temperature that marks the appearance of liquid water is called the dew point; it is the saturation temperature for the partial pressure of the vapor. When the window inside surface temperature reaches the dew point condensation occurs.
A good engineering textbook, such as Heat Transfer by Chapman, explains the condensation process in detail.  A good textbook on thermodynamics discusses water vapor gas mixtures (psychrometrics); for example. Introduction to Thermodynamics by Sonntag and Van Wylen.
Here is my opinion as to the effect of condensation on the room.  Condensation involves both mass transfer (liquid forming on the window) and heat transfer through the liquid layer.  Condensation on the window removes energy from the room due to the latent heat removed from the vapor to change it to a liquid.  The condensation process is hindered by the presence of non-condensable air. The temperature of the window tends to increase due to the energy removed from the warm room to change liquid to vapor, but that is a small effect given the cool heat sink on the outside of the window. The effective heat transfer coefficient from the room to the window with condensation present is greater than without condensation due to the change in phase of the vapor, and this results in more cooling of the room. You can find typical heat transfer coefficients on the web and in engineering heat transfer books.  Bottom line: the phase change associated with condensation of vapor is more effective at cooling the room than is pure convection/conduction heat removal with no condensation.  For a large room, this may not be a significant effect.
