Entrainment is the mixing of drier, often cooler environmental air into a rising cumulus cloud. This dilutes the cloud's liquid water content and, critically, causes some cloud droplets to evaporate (since the entrained air is unsaturated), which cools the air parcel evaporatively and reduces its temperature excess (and hence buoyancy) relative to the environment.
When drier ambient air is entrained into a cumulus cloud, it mixes with the saturated cloudy air, lowering the overall relative humidity of the mixture...
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