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R&D MINUTE

BETTER UNDERSTAND THE CONDENSATION PHENOMENON

 

 
As condensation is a recurrent issue for bivouac enthusiasts, Samaya offers a reading of this natural phenomenon, to better understand its formation and thus try to prevent it during mountain outings.
 
Condensation in bivouac tents is a natural phenomenon that occurs when warm, moist air comes into contact with a cold surface, changing from a gaseous to a liquid state. During a bivouac, this happens mainly when the outside temperature drops during the night, while the inside air remains warmer. This temperature difference causes water droplets to form on the cooler tent walls.
 
The triptych of absolute humidity, relative humidity and vapor pressure deficit
 
Relative humidity (RH) and absolute humidity are fundamental concepts for understanding condensation dynamics, as is vapor pressure deficit (VPD). These three concepts interact in complex ways to determine when and how condensation occurs in bivouac tents.
 
Relative humidity is a measure of the amount of water vapor present in air, expressed as a percentage of the maximum amount of water vapor that air can hold at a given temperature. The capacity of air to hold water vapour is highly dependent on temperature. Warm air can hold much more water vapor than cold air. As a result, when the temperature drops, the air can no longer hold as much water vapor, thus increasing relative humidity.
 
When relative humidity reaches 100%, the air is saturated. At this point, the dew point is reached, i.e. the temperature at which the air must be cooled for water vapor to begin condensing into liquid water. For example, if the air inside a tent cools to the dew point, condensation forms on the cooler surfaces of the tent.
 
Absolute humidity, on the other hand, is a measure of the actual amount of water vapour in the air, independent of temperature. It is generally expressed in grams of water vapour per kilogram of air. Unlike relative humidity, absolute humidity does not change directly with temperature. However, as the air warms or cools, relative humidity will change, because the capacity of the air to hold water vapor varies with temperature.

 

 
Vapor pressure deficit (VPD) is a measure of the difference between the amount of water vapor the air can hold when saturated (at 100% relative humidity) and the actual amount of water vapor present in the air. VPD is crucial to understanding condensation dynamics in tents. VPD is calculated as the difference between saturation vapor pressure (PVS) and actual vapor pressure (PVR). PVS is the pressure exerted by water vapor in saturated air at a given temperature, while PVR is the pressure exerted by water vapor actually present in the air.
 
What impact does VPD have on condensation? A high VPD means that the air is far from saturation, indicating low relative humidity. However, condensation can still occur even when relative humidity is below 100%, particularly when humid air comes into contact with a much cooler surface. In this case, the surface temperature may be low enough to reach the dew point locally, causing water vapour to condense on this cold surface.
 
The interaction of condensation-causing factors
 
In a bivouac tent, interactions between relative humidity, absolute humidity and VPD determine if and when condensation occurs.
 
At night, the outside temperature drops more rapidly than inside the tent. Indoor air, which can be charged with water vapor through respiration and perspiration, sees its relative humidity rise as the temperature drops. What's more, the walls of the tent, which are colder than the air inside, can reach or fall below the dew point. If this happens, even if the relative humidity inside the tent has not reached 100%, condensation will form on these surfaces.

 

 
Condensation reduction strategies
 
Correctly ventilating the tent can reduce relative humidity by allowing humid air to escape and drier air to enter, thus increasing the VPD. This helps prevent condensation by keeping indoor air away from saturation. The choice of bivouac location is also important: avoiding proximity to bodies of water, dense vegetation and damp ground helps reduce this phenomenon. Keeping damp equipment outside the tent and wiping the walls with a microfiber towel are also good habits to adopt. Nights with clear skies and morning dew, as well as nights spent in clouds or fog, increase ambient humidity and the likelihood of condensation.
 
Condensation in bivouac tents is unavoidable under certain conditions, but understanding its mechanisms and applying moisture management and ventilation strategies can be keys to spending a comfortable, dry night out in the wilderness.

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