By Skip Rigney

It rained Thursday. It may come as a surprise that dust, and perhaps even germs, played an essential role in forming the clouds from which the rain fell.
Clouds are made of water. Some clouds, such as the puffy white ones called cumulus and the low gray overcast called stratus, are composed entirely of liquid water droplets. Fog is essentially just a stratus cloud at ground level. Others clouds, such as the wispy, white streaks called cirrus, are all ice crystals. Ice crystals may form directly from water vapor or from the freezing of liquid droplets. Many clouds, especially rain clouds, are a mixture of both liquid droplets and ice crystals.
Way back in the 1800s, scientists figured out that when air, which always contains some amount of water as an invisible gas, is sufficiently cooled, that invisible vapor condenses into liquid droplets or ice. Those scientists knew that the maximum amount of invisible water vapor that air can hold depends on the temperature. The warmer the air, the more water vapor it can hold.
Consider a blob of air. (Meteorologists call them “parcels,” but that’s just a fancy word for blob.) Now cool the blob. The cooling can happen in a variety of ways. One way is to take away the source of heat, which happens every night. The blob will also cool if you decrease it’s pressure. This happens when a blob is lifted upward to higher altitudes. The pressure on the blob decreases, the blob expands, and its temperature drops.
Whatever the mechanism for cooling, as the temperature of the blob drops, the relative humidity increases. That’s because, even though the amount of water in the blob stays the same, cool air cannot hold as much invisible water vapor as warm air. When the relative humidity reaches 100 percent, the water vapor begins to turn into liquid water or ice.
At least that’s what common sense might tell you. But, there’s an additional ingredient required for vapor to condense into droplets. It took the experiments of a 19th century Scottish meteorologist named John Aitken to uncover what was going on at the microscopic level. Aitken’s colleague, physicist C.G. Knott, gave this summary of Aitken’s surprising discoveries: “1) when water vapour condenses in the atmosphere, it always does so on some solid nucleus; 2) the dust particles in the air form the nuclei on which it condenses; 3) if there was no dust in the air there would be no fogs, no clouds, no mists, and probably no rain.”
Thankfully for life on Planet Earth, there are thousands of tiny floating particles of dust, soot, and salt in a typical blob of “clean” air the size of your pinky finger. Those particles act as the “nuclei” necessary for droplets to form.
Over a century after Aitken, scientists continue to be surprised by the complexity of clouds. Researchers are now investigating the role that airborne bacteria may play in raising the freezing point of liquid cloud droplets, which in turn causes ice to form, a step that’s often needed to turn clouds into rainmakers.
Rain is so commonplace that it’s easy to take for granted what an amazing process brings us the freshwater we need to sustain our lives.