Some particles are better than others at seeding rain, while others can be better at seeding snow or ice. There are different types of seeds it’s usually thin particles of dust or clay, but soot or black carbon from fires can also play this role. The ability of these different types of particles to form cloud droplets varies according to their size and also their exact composition, as the hygroscopic properties of these different constituents are very different. Those objects are called nuclei, or to be more exact, cloud condensation nuclei.Ĭloud condensation nuclei or CCNs (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100th the size of a cloud droplet on which water can condens. They need another substance, a ‘seed’ with a radius of at least one micrometer (one millionth of a meter) on which they can form a bond. However, water particles are simply too small to bond together for the formation of cloud droplets. In one form or another, water is always present in the atmosphere. Let’s look at this phenomenon in more detail. So in a way, raindrops form just like pearls. In consequence, we conclude that δD p is a correct candidate to examine and to reconstruct large-scale atmospheric processes from past to present time scales.At the center of every raindrop there is an impurity (dust, clay, etc) – basically all raindrops have something like that at its core, just like pearls do. We show that large-scale dynamic along air masses history is dominant (nearly 80%) to explain δD p whereas local effects are dominant to explain deuterium excess in precipitation. Lastly, we explore how local processes affect δD p. Other local processes such as rain type, condensation conditions and surface water recycling appear as better candidates to explain ΔD p_eq. The review of possible causes to explain the disequilibriums shows that below-cloud rain evaporation and diffusive exchanges are little involved. Although equilibrium state does not prevail at the individual rain event scale, a strong relationship is observed between δD p and δD p_eq over the whole period of field samplings (r 2 = 0.86, n = 70, p < 0.001). They are significantly correlated to δD p (r 2 = 0.30, n = 70, p < 0.001) suggesting that controls on δD p also impact ΔD p_eq. Disequilibriums (ΔD p_eq = δD p - δD p_eq) are mostly negative (73%), indicating that precipitation is more depleted than a condensate that would have been formed from surface water vapor, and half of them are between −10 and + 10‰. Our observations show that the observed isotopic composition of precipitation (δD p) deviates from the theoretical isotopic composition of precipitation at equilibrium with water vapor (δD p_eq). This study examines the isotopic equilibrium state from event-based precipitation and daily near-surface water vapor samples collected during the onset and the termination of the 2005–2006 wet season in the Bolivian Andes (Zongo valley, 16☀9′S, 68☀7′W). However, the paucity of field observations limits the validation of this assumption. The isotopic equilibrium state between precipitation and low-level water vapor is a common assumption in numerous paleoclimate and atmospheric studies based on water stable isotopes.
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