# Steam from a cup of coffee

I observed that, in winter there is more visible steam from a cup of coffee than in summer. Is there any phenomenon taking place here.

• I would be very careful with steam and vapour here, as they are ambiguous. What you can observe without a spectrometer is mist, which is tiny water droplets diffracting the light Nov 25, 2023 at 8:41

The amount of water that air can take up before the water creates fog or visible steam depends on temperature. The colder the air, the less water it needs to create fog/steam. It is the same principle when hot air rises, for example when pushed up a mountain and then it starts to cool down drastically --> It will rain. For more have a look at: Relative humidity in https://en.wikipedia.org/wiki/Humidity

I think it's rather simple question.

• For a steam you may use PV= RT as equation of state.
• atmosphere is constant pressure system.
• for constant P , V is directly proportional to T. And density (d) is inversely proportional to V. This implies d is inversely proportional to T.
• So, in winter T is lesser as compared to summer hence d is greater in winter than in summer.
• Specific humidity (or moisture content) is the ratio of the mass of water vapor to the total mass of the air parcel. Since density of air is ratio of mass of vapor+air to volume of air parcel, it can be said that both specific humidity (humidity in general) and d are measures of same thing 'moisture content'. Hence increase in density can also be expressed as increase in specific humidity ( or other forms of humidity).
• At core the main cause of more visible steam in winters as compared to summer is due to less volume occupied by steam.

Hope this helps.

• How does that work? The density of water vapor (18 g/mol) is lower than the density of dry air (~28.8 g/mol). Nov 25, 2023 at 8:32
• @PM2Ring can you explain what you mean? Nov 25, 2023 at 9:36