The surface layer is not necessarily saturated when precipitation occurs. The public often thinks that the relative humidity is 100% at the surface when it is precipitating. This may not be the case and is often not the case. The length of time it takes air to become saturated when precipitation falls into a sample of "stagnant" air depends on 4 primary factors. The term "stagnant" refers to the air neither rising nor sinking nor advecting to change moisture carrying capacity. These factors include: 1) available moisture to be evaporated, 2) the pre-existing moisture content of the air, 3) temperature and 4) precipitation type. 1) AVAILABLE MOISTURE- If precipitation is light, there may not be enough moisture content to saturate the air. Light precipitation can completely evaporate into dry air as it falls to produce virga. When light precipitation reaches the surface when the lower PBL is initially dry, the RH may increase a little but often is not able to rise to 100%. There may not be enough available moisture to reach the air's holding capacity of water vapor. The DURATION of the precipitation is important. A longer duration precipitation event adds more available moisture to be evaporated into the air. Light precipitation will only slowly increase the relative humidity of a dry PBL. However, if given a sufficient time the relative humidity could rise to at or near 100%. During heavy precipitation, the lower troposphere will saturate much more quickly. 2) PRE-EXISTING MOISTURE- Air that is near saturation will be able to reach saturation upon the arrival of precipitation more quickly as compared to air that is fairly dry. Also, if the soil is wet, that moisture will help contribute to saturating the air at the surface. 3) TEMPERATURE- Temperature contributes heavily in determining the moisture carrying capacity of air. Warm dry air can evaporate much more of water vapor as compared to cold dry air. For example, air at 33 C when saturated will have 10 times the moisture content of air that is negative 3 C when saturated. When heavy precipitation falls into dry air, warm air will cool at a greater rate than cold air since more latent heat will be absorbed due to the evaporation process. Cold air can be saturated more quickly than warm air since it does not take much water vapor to saturate cold air. Limiting this is the fact that the rate of evaporation is less in cold air (cold water and ice have a slower molecular motion than warm water). 4) PRECIPITATION TYPE- The limiting factors to ice (sleet, snow) sublimating is due to the rigid structure of the ice (low energy water/ice molecules resist phasing to water vapor more than high energy water/ice molecules) and due to the ice crystals having to sublimate into cold air (low vapor pressure gradient). When a dry snow event occurs when the PBL is previously dry, you may notice the relative humidity remaining well below 100% throughout the entire precipitation event. Sleet and dry snow do not lose their mass to the air as water vapor nearly as effectively as rain. Rain falling into warm / dry air will evaporate very quickly. The warmth increases the molecular energy of the water and the warm air (being able to evaporate large amount of moisture) produces a large vapor pressure gradient between the drop and the surrounding dry air. During future precipitation events, take note of how the relative humidity changes during the event. Determine the effect that precipitation intensity, duration and type have on increasing the relative humidity of the air at the surface. |