im sure this has been asked before but i couldnt find it.
why does water condense on the outside of a cold bottle of water?
It depends on the water temperature, the air temperature, and the humidity.
As air gets hotter, it can hold more water vapor. Usually humidity is given as “relative humidity”. Relative humidity is the amount of water vapor in the air relative to the amount of water vapor the air could hold at whatever temperature the air is. When the air is 100 degrees F and the relative humidity is 50%, there’s more water in the air than there is when the air is 50 degrees F and the relative humidity is 50%.
There’s another measure of humidity, that you seldom hear on the weather forecast, but it’s more relevant to your question. It’s the “dew point”. The dew point is the temperature at which the relative humidity would be 100% for the amount of water vapor that is in the air. The more water vapor there is in the air, the higher the dew point.
The cold water cools the glass, and the glass cools the air around it. If the glass cools the air below the dew point, then there’s more water vapor in the air than the air can hold. This is possible but unstable; the water tends to condense out of the air wherever it can find a “nucleation site” to start condensing. The outside of the glass provides lots of nucleation sites and voila … water on the outside of the glass.
The cooled air is also heavier than the warmer air far from the glass, so it tends to fall and set up mild convenction currents in the air which replenish the air near the glass with more warm air, which lets the process continue. Wind and moving the glass do the same thing.
Dew on the grass in the morning is caused by the same process.