His Majesty King Bhumibol Adulyadej had initiated the Royal rainmaking operation and monitored the result of the experiments from daily report. He also studied from scientific documents, closely observed weather condition and phenomena on the atmosphere whether there was the experiment or not. Because of his devotion, he could progressively develop the process of cloud seeding. In 1973, His Majesty King Bhumibol Adulyadej delivered the principle of cloud seeding to the officers and this principle has been continuously used until 1999. There are 6 steps of warm and cold cloud seeding.
Warm Cloud Seeding
Step I: Triggering
This step begins when the sky in the target area is clear or only a few cumulus clouds have formed in the morning and the average relative humidity is at least 60 percent. Powder of sodium chloride, hereafter NaCl, is dispersed from aircraft in the upwind side of the target area at the altitude of 7,000-8,000 ft. In order to activate cloud formation, each particle of Sodium Chloride acts as cloud condensation nuclei to absorb moisture and then change the status from vapour to liquid or cloud droplets. The clouds can be formed to be bigger clouds and the cloud top can reach the altitude of 10,000 ft.
Step II: Fattening
This step is to make clouds in the previous step as well as the natural clouds grow bigger. It begins when the cloud tops of the cumulus clouds formed from Step I reach the heights of 10,000 ft. This step is done by an aircraft which disperses powder of calcium chloride, hereafter CaCl2, into clouds at the level of 8,000 ft. The heat from the chemical reaction of CaCl2 and moisture will expedite or enhance the updraft of air mass in the cloud. Therefore, the cloud growth increases continuously and the cloud top is higher than the previous step. The cloud top can reach the heights of 15,000 ft. where it is ‘warm cloud’ (the temperature in the cloud top is higher than 00c). If the updraft is enough, then the cloud top can develop until it reaches the heights of 20,000 ft. where it becomes cold cloud (the temperature in the cloud top is below 0๐c).
Step III: Attacking (Sandwich Technique)
This is the case for warm cloud seeding which begins when the cumulus clouds formed from Step II move along the wind approaching the target area and the cloud tops reach the heights of 10,000 ft. or higher. One aircraft disperses NaCl at the top or the shoulder of the clouds (an attitude of 9,000 ft.) on the upwind side. At the same time, another aircraft applies urea at the height of 1,000 ft. above the base clouds. These results in two aircrafts flying simultaneously in parallel, making an angle of 45 degrees to the horizontal line. This flying technique is called ‘sandwich’ flying. After seeding, raindrops in cloud become densely bigger and level down to cloud base. The cloud becomes mature and starts to fall as rain but not too much.
Step IV: Enhancing
This step is to maintain Step III and enhance rainfall onto the ground. After operating step III, the rain may not fall too much because the atmosphere below the cloud base is drier and the temperature is higher than that in the cloud. Thus, the raindrops that fall through the cloud base rapidly evaporate into vapor. This step is done by dispersing dry ice flakes (-78๐c) at the heights of 1,000 ft. below the cloud base to lower air mass’s temperature and increase relative humidity. This action will help reducing evaporation of raindrops. The rainfall rate increases gradually and a greater number of giant raindrops reach the ground. This causes heavy rain which provides more amount of rainfall than that of natural rain.
Cold Cloud Seeding
Step V: Attacking by cold cloud seeding or Silver Iodide (AgI) Seeding
This step is operated when the cloud top formed from Step II reach the heights of 20,000 ft. This process is done by ejecting flares of AgI into the cloud top at the heights of 21,500 ft. After operation, the amount of ice will be formed especially in the cloud top. Freezing of droplets enhances the release of latent heat, increases cloud buoyancy and updraft, and induces moister air into the cloud base. The ice will grow much better before melting to be raindrops.
Step Vl: Attacking Technique (Super Sandwich)
This step is used when the cloud top from Step II reach over the heights of 20,000 ft. and the cloud condition is the same as that in step V. This step involves warm and cold cloud attacking techniques at the same time with at least 3 aircrafts. The cold cloud seeding aircraft ejects flares of AgI into the cloud top (at the heights of 21,500 ft.) and the two warm cloud seeding aircrafts fly by using sandwich technique. The first one disperses NaCl at the mid-cloud level (about 10,000 ft.), another one disperses urea at the cloud base level. However, the warm cloud seeding aircraft may disperse dry ice in step IV in order to enhance rainfall. After seeding by the Super Sandwich Attacking Technique, the treated clouds start to rain heavily and continuously for a long time resulting in large amount.