Your Questions Answered "The Relative Humidity level is high so it must be condensation"
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"The Relative Humidity level is high so it must be condensation"
This is a very common misunderstanding. Whether or not relative humidity at a given level is a contributory factor to the formation of condensation ‘depends’. It is very common for people to quote high relative humidity levels when trying to explain condensation but relative humidity levels on their own are virtually meaningless.Imagine for a moment the lounge and a bedroom in your property. If the relative humidity in the lounge is 58% and the relative humidity in the bedroom is 73% which room would you think is ‘wetter’?
Most people automatically think it is the bedroom but the real answer in our example is that they are both equally ‘wet’. How is this possible? The answer is down to temperature. If the lounge air temperature is 23°C it is able to support more water vapour than the air within the bedroom which is at 18°C (See The Football Explanation above). Because of the differing temperatures both rooms will have different relative humidity levels but the amount of water vapour present in each room will be similar. For this reason it is better to work with vapour pressure as well when evaluating condensation.
To explain explain this another way. If you imagine two identical buckets each half full of water which for the purpose of this explanation represents 50% relative humidity. One of the buckets is heated causing it to expand so the water now occupys 40% of the available space. The other bucket is cooled and contracts causing the water to occupy 60% of the available space. If the cooling continues causing the bucket to shrink further it will eventually be 100% full. If it shrinks further still it will overflow which represents the formation of condensation. In both cases the volume of water has remained identical but the space available to contain it has varied because of the influence of temperature on the size of the buckets. This same principle applies to warm and cool air.
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