The modern era of precipitation enhancement began with , , and 's investigations of the effect of artificial sublimation nuclei. Glaciogenic cloud seeding relies on the scarcity of naturally occurring ice nucleating particles, and on the difference in vapor pressure over liquid water and ice at the same temperature. A few artificial nuclei introduced into a cloud can trigger ice formation, leading to a cascade of events that result in precipitation from a cloud that would not have rained or snowed in the absence of the intervention. For a more complete overview of cloud seeding, see , , or .

Though most precipitation over the continents in the mid-latitudes involves ice , rain does result from warm phase processes as well. Accelerating those processes is the basis for hygroscopic seeding, wherein relatively large condensation nuclei are introduced into a cloud to jump-start the collision-coalescence process by creating large droplets . Initial hygroscopic seeding experiments were conducted using table salt, milled to a nominal size of five micrometers in diameter, though problems with caking were reported . (Caking is the formation of clumps caused by moisture absorption, which hinders the dispersal of fine particles.) Hygroscopic flares were developed to surmount those difficulties. They were designed to produce large particles of calcium chloride, but also released large quantities of smaller potassium chloride particles as part of the combustion process . In randomized tests, both methods were shown to be successful in enhancing rainfall, as judged through radar returns. Subsequent work has established that milled salt might be more effective if the problems with dispersal (caking) can be overcome .

In a typical hygroscopic seeding operation, the material of interest is dispersed in an updraft below cloud base. One of the primary conceptual models of hygroscopic seeding, the competition effect, relies on large hydrophilic particles at cloud base to depress the peak supersaturation, resulting in fewer, larger droplets . There are experiments, however, where hygroscopic material was dispersed directly into a cloud . For example, used in-cloud seeding via hygroscopic flares in stratocumulus as a way to test hypotheses on aerosol-cloud interactions. They found an initial increase in small cloud droplets, presumably from the smaller potassium chloride particles typical of the Ice Crystal Engineering flares they used . Later interception of the flare plume within the cloud showed larger droplets had been enhanced. Though cloud-base hygroscopic seeding remains the dominant paradigm in many programs due to flight safety, logistics, and regulatory considerations, modeling efforts have indicated that in-cloud seeding may be more effective than below cloud .

Laboratory studies of glaciogenic cloud seeding have a robust history, going back to Schaefer's original work, as noted above. Subsequent studies have primarily focused on the efficacy of silver iodide as an ice nucleating particle (see and references therein). There have been far fewer laboratory studies of hygroscopic seeding. and are two notable examples. Here, we present results from laboratory studies of the effect of milled salt, an NaCl-TiO₂ core-shell material , and mineral dust on pre-existing, steady-state clouds in the Michigan Tech Pi Chamber .

Experiments were conducted in the Pi Cloud Chamber, which produces mixing clouds by a temperature gradient between two plates separated by 1 m . In these experiments, the temperature gradient was 15 °C, with the bottom panel at 27.5 °C, the top panel at 12.5 °C, and the walls at the average temperature of 20 °C. The 20 °C average temperature is warmer than typical operational hygroscopic seeding conditions (≈ 10–15 °C), and extrapolation to field conditions are interpreted accordingly. A steady state cloud, referred to from here on as the “baseline” cloud, was first established by the injection of 130 nm dry diameter NaCl particles. Baseline NaCl aerosol was generated by creating a polydisperse distribution from solution using a TSI 3076 constant output atomizer, drying in a Nafion dryer, and finally size selecting using a differential mobility analyzer (TSI 308100). The baseline aerosols were diluted to a concentration of 10⁵ cm⁻³ and injected into the chamber at a rate of 2 liters per minute (L min⁻¹). The injection aerosol concentration and flow rate together equate to an injection of approximately 1 cm⁻³ s⁻¹ when mixed into the full volume of the chamber. After the injection of baseline aerosols began, sufficient time was allowed for the chamber to reach steady-state conditions, in which cloud droplet removal by settling balances cloud droplet formation by activation .

Cloud droplet size distributions (DSDs) for the baseline cloud, as well as DSDs for the following experiments, were measured using a Welas Digital 2000 optical particle counter and a Mesa Photonics Cloud Droplet Measuring System (CDMS), version 2. Both counters derive sizing information from the scattered light of particles passing through the sensing volume. This scattering signal is inverted to a size based on an assumption of sphericity and refractive index. Because of this, there are inherent uncertainties in the size distributions immediately after dispersing dry salt and dust into the chamber. Neither material is spherical in the dry state, and of course, neither has the refractive index of water.

The sonic virtual temperature was measured using a high-speed sonic temperature sensor (Applied Technologies, Inc.). This sensor works in the same way as a sonic anemometer and derives the sonic virtual temperature from the speed of sound in air. The bulk temperature was measured using a 100 Ω ± 0.12 %, thin film platinum resistance thermometer. Both the sonic virtual temperature and bulk temperature measurements were co-located and sampled at a rate of 1 Hz.

With the baseline cloud established, a test aerosol could then be injected into the chamber. Test aerosols included a recently developed seeding powder composed of NaCl with a TiO₂ shell (core-shell nanomaterial, CSNT) and jet-milled NaCl, both provided by the United Arab Emirate’s Rain Enhancement Program. The jet milled salt was intended to match the size distribution of the CSNT powder and serve as a reference for its performance as a cloud seeding agent. Ultrafine Arizona test dust (Powder Technology Inc.) was also used as a non-hygroscopic reference.

Because the milled salt is hygroscopic and prone to caking as it absorbs trace amounts of moisture, each sample was baked in a tube furnace for a minimum of 8 h at 400 °C and periodically shaken in a test tube shaker to break up the largest clumps. Dry size distributions of the powders, dispersed into the chamber using the method described below, indicate that this process was successful in returning a portion of the NaCl to its original state. However, the distributions also show that a portion of the NaCl powder was present as large particles which quickly settled out of the chamber. No caking was observed in the CSNT powder, despite its being packed and shipped in the same manner as the milled NaCl, and it was deemed that baking would be unnecessary. Therefore, the CSNT injection cases discussed here were conducted with unbaked CSNT powder. The testing masses for milled NaCl and CSNT powder were chosen based on preliminary testing in dry conditions with the goal of injecting the same concentration of particles in each case. Due to caking, it was necessary to inject 1 g of NaCl powder to match the injected concentration from 0.2 g of CSNT. Further details of the powders' appearance and the dry size distributions are given in Appendix .

To preserve the core/shell structure of the CSNT and to minimize sintering and caking in the milled salt, the powders were kept dry up to the moment they were injected into the chamber. This was achieved by loading a measured quantity of powder into a latex balloon for each test. The latex balloons were then connected to a compressed air line and lowered into the chamber, and popped by a burst of compressed air, dispersing the powder into the chamber. Tests were conducted with empty balloons (i.e. no powder) to confirm that the entrainment of dry air from the air used to burst the balloon had minimal impact on the existing cloud. No change in the DSD or droplet concentration as measured by the Welas was observed after the bursting of empty balloons. Tests with Arizona test dust were also conducted using the burst balloon method.

The primary goal of hygroscopic cloud seeding for precipitation enhancement is to broaden the droplet size distribution, preferably in the large droplet tail, thus promoting collision-coalescence. The results above indicate that injection of a sufficient amount of hygroscopic seeding powder into a pre-existing cloud does result in creation of large droplets, but those droplets are not solely the redistribution of existing liquid water onto fewer, larger droplets. The powder stimulates an initial increase in the liquid water content by absorbing water from the vapor phase as it deliquesces and grows. The large droplets that are created fall out of the volume quickly. This leads to the following questions.

How much powder should be dispersed to produce this effect?

Our discussion here is focused toward an understanding of the processes in our experimental system, but the results should be applicable to seeding clouds in the atmosphere with a hygroscopic powder as well. We start with the relevant time scales for the problem. The first time scale to consider is the time required for mixing the dry air containing the powder with the cloudy air into which it is dispersed. As noted in Sect. , we dispersed the powder using a balloon burst, which mixed the powder and cloudy air quickly. Visually, the balloon inflated with approximately five liters of dry air before bursting. Unless the individual grains of the powder are excessively large, they will typically persist in the volume in which they are dispersed long enough to mix with the cloudy, humid air and deliquesce. The deliquescence time scale is less than a millisecond , and is not considered further here.

For seeding to be effective, the hydrated salt must be larger than the droplets in the existing cloud, which ensures that they will fall relative to the existing cloud and act as collectors in collision-coalescence. However, the salt must persist in the volume long enough to take up the available water.

Expressed in terms of time scales, this is 3τphase<τfall,powder<τfall,cloud Here, τphase is the phase relaxation time for the dispersed powder, which is a characteristic time for the vapor field to relax to a new equilibrium when the dry powder is dispersed into it. τfall,powder is the characteristic residence time of the hydrated salt in the volume, and can be expressed as τfall,powder=Hvt where H is a characteristic dimension of the volume and vt is the terminal fall speed. τfall,cloud is the characteristic residence time of the existing cloud droplets in the volume.

The characteristic residence time for the powder in the volume is given by 4τfall,powder=18ηHρlgG2Ds2 where η is the viscosity of air, ρl is the density of the liquid, and g is the acceleration due to gravity. We assume that the hydrated salt is still in the regime where the Stokes fall speed is applicable. The factor G in the denominator is the hygroscopic growth factor, which describes the increase in size upon taking up water . Ds is the dry diameter of the salt, so that GDs is the diameter of the resulting solution droplet. Note that G is a function of the saturation ratio; G for NaCl at deliquescence is 2.28 .

The residence time for the cloud droplets in the volume is given by an expression identical to Eq. () with (GDs)2 replaced by Dc2, the diameter of the cloud droplets in the volume. Imposing the requirement that the hydrated powder have a shorter residence time than the cloud leads immediately to the unsurprising conclusion that the hydrated powder should be larger than the mean size of the cloud droplets in the volume. This leads to the desired differential fall speed and a higher likelihood of collision-coalescence. If NaCl is dispersed into a cloud with a mean droplet diameter of 10 µm, the salt grains should have diameters greater than 4 µm.

The lower size limit is set by the requirement that the hydrated powder fall relative to the cloud droplets. The upper size limit can be derived by requiring that the dispersed powder persist in the dispersal volume long enough to take up the available water vapor.

Equating the time scale for the powder in the volume and the powder's phase relaxation time is 518ηHρlgG2Ds2=12πD′NsGDs where D′ is a diffusion coefficient which incorporates the effects of the transport of heat and vapor and Ns is the number concentration of the dispersed powder.

This equation can be solved for GDs. 6GDs=36πηHD′Nsρlg

However, Ns can be eliminated from Eq. () by imposing the further requirement that enough powder be dispersed to reduce the saturation ratio in the volume to the powder's deliquescence point. The mass of vapor then converted to liquid is 7mv=(1-RHdel)qs,T where RHdel is the deliquescence relative humidity and qs,T is the saturation vapor density of the cloud. The corresponding liquid water is 8mw=43πGDs23ρlNs The difference in the density of water and the saturated solution is neglected in Eq. (). We have also neglected the contribution of the mass of the dry powder, and have assumed that the saturation vapor density is much greater than the liquid water content of the cloud, which is typically true.

Combining Eqs. (), (), and () leads to an expression for the critical hydrated diameter 9GDs=216ηHD′(1-RHdel)qs,Tρl2g14 Particles larger than this value will not persist in the dispersal volume long enough to come into equilibrium with the vapor field. Using the saturation vapor density for 20 °C (the temperature of the chamber in the tests shown above), and the deliquescence of NaCl, GDs≈80 µm. For atmospheric dispersal, temperatures are more likely to be in the range of 10 to 0 °C where the critical diameters of the hydrated powder are reduced to approximately 70 and 60 µm respectively. These results show that, for example, milled NaCl with a dry diameter of 25 µm will be an effective hygroscopic seeding agent for dry powder dispersal.

Experiments in the Pi Chamber demonstrate that direct, in-situ seeding of both milled NaCl and NaCl-TiO₂ powders can effectively stimulate growth of a large droplet tail and an increase in the local liquid water content in warm clouds. The observed vapor depletion effect aligns with simulations and fog dispersal experiments by , who demonstrated that large salt solution droplets can locally modify the vapor field. Our work extends these findings by showing that dry powder dispersal produces similar effects, complementing earlier investigations by . However, care must be taken to prevent sintering of the salt grains in the bulk material, as such caking can significantly reduce effectiveness of the powder dispersal.

While these chamber experiments capture the microphysical response under controlled conditions, they do not fully represent atmospheric cloud dynamics at the spatial and temporal scales of realistic precipitation development. Future work should combine controlled laboratory studies with parcel and large-eddy simulations to map these observed microphysical responses onto realistic cloud lifecycles, quantify the required seeding rates and injection heights, and assess the domain of applicability across different cloud regimes. Because of the limitations of the optical particle counters used in this study, our reported liquid water contents are uncertain, especially in the moments directly after injection of the powders. For future investigations, adapting a hot-wire probe for use in the chamber is a promising avenue for verifying LWC without the ambiguity introduced from optical methods, since the hot-wire would not respond to a dry powder. Overall, our findings suggest that hygroscopic seeding with dry powders warrants reconsideration as an operational precipitation enhancement strategy. The demonstrated mechanism of vapor depletion through rapid deliquescence offers a pathway to accelerate droplet growth beyond classical condensational growth timescales, potentially triggering earlier onset of warm rain processes. This has implications for both operational weather modification programs and fundamental understanding of how aerosol perturbations can alter warm cloud microphysics and precipitation efficiency.