If the air is at 100% humidity and I leave out a glass half- filledwater, will the glass eventually dry out, become more full, or stay the same level, or cause it to rain?
If the glass is the same temperature as the air, it will stay the same. The air can’t hold more moisture, so it won’t evaporate. Unless the air takes on more moisture from some other source, it won’t become unable to hold its moisture and rain into the glass.
The glass is at equilibrium. Technically, molecules are evaporating off the water surface, and molecules from the air are condensing onto it, but these processes are in balance.
If the glass is cold, it will cool the surrounding air and form condensation, increasing the water in the glass and around it. If the glass is warmer than the air, it will evaporate and lose a little water before it cools to the surrounding temperature.
Assumption: you are talking about relative humidity for clarity.
100% humidity means that atmosphere is holding all the moisture it is capable of at that temperature. So long as everything is at steady state, that means no air currents, no additional energy coming into or leaving the system; sort of all the usual stuff you would expect in a classroom physics problem to make it easy.
What will happen in this instance is that some higher energy water molecules will leave the glass to the air and some lower energy water molecules will leave the air and enter the glass. The exchange rate will be balanced and the water level will not change.
Because there is no change in energy it will also not rain.
This is an interesting experiment because it demonstrates the concept of thermal mass as it is used in heating and cooling buildings.
If the glass is kept in a chamber that maintains 100% humidity at temperature T0, when the glass is first put in the chamber, the water is at T1.
If T1 is higher than T0, the water surface will be warmer than the air above it and so evaporation will occur until the water reaches equilibrium with the surrounding. If T1 is lower than T0, the air will condense into the glass until equilibrium temperature is reached.
Thermal mass is used in buildings to reduce energy cost.
Assume a large concrete mass (e.g. transfer floor in a tower). Each morning it is still cool from the overnight cooler temperatures. During the day, the occupied space is warmer than the concrete mass and so the floor will absorb heat from the occupied space and reduce the cooling load required by the HVAC system. At night, the outside cools off and the occupied space may need heat but guess what, now the heat absorbed during the day is will be absorbed back into the occupied space reducing the heating load overnight.
To get back to your glass at 100% humidity, no matter how good your chamber is, it will cycle in temperature as the heater comes on and off as it tries to adjust for outside ambient conditions. The temperature of the water in the glass will lag behind the temperature change in the air around it so evaporation and condensation will happen during the lag periods.
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If the glass is the same temperature as the air, it will stay the same. The air can’t hold more moisture, so it won’t evaporate. Unless the air takes on more moisture from some other source, it won’t become unable to hold its moisture and rain into the glass.
The glass is at equilibrium. Technically, molecules are evaporating off the water surface, and molecules from the air are condensing onto it, but these processes are in balance.
If the glass is cold, it will cool the surrounding air and form condensation, increasing the water in the glass and around it. If the glass is warmer than the air, it will evaporate and lose a little water before it cools to the surrounding temperature.
Assumption: you are talking about relative humidity for clarity.
100% humidity means that atmosphere is holding all the moisture it is capable of at that temperature. So long as everything is at steady state, that means no air currents, no additional energy coming into or leaving the system; sort of all the usual stuff you would expect in a classroom physics problem to make it easy.
What will happen in this instance is that some higher energy water molecules will leave the glass to the air and some lower energy water molecules will leave the air and enter the glass. The exchange rate will be balanced and the water level will not change.
Because there is no change in energy it will also not rain.
This is an interesting experiment because it demonstrates the concept of thermal mass as it is used in heating and cooling buildings.
If the glass is kept in a chamber that maintains 100% humidity at temperature T0, when the glass is first put in the chamber, the water is at T1.
If T1 is higher than T0, the water surface will be warmer than the air above it and so evaporation will occur until the water reaches equilibrium with the surrounding. If T1 is lower than T0, the air will condense into the glass until equilibrium temperature is reached.
Thermal mass is used in buildings to reduce energy cost.
Assume a large concrete mass (e.g. transfer floor in a tower). Each morning it is still cool from the overnight cooler temperatures. During the day, the occupied space is warmer than the concrete mass and so the floor will absorb heat from the occupied space and reduce the cooling load required by the HVAC system. At night, the outside cools off and the occupied space may need heat but guess what, now the heat absorbed during the day is will be absorbed back into the occupied space reducing the heating load overnight.
To get back to your glass at 100% humidity, no matter how good your chamber is, it will cycle in temperature as the heater comes on and off as it tries to adjust for outside ambient conditions. The temperature of the water in the glass will lag behind the temperature change in the air around it so evaporation and condensation will happen during the lag periods.