For any precipitation to fall there is a need for a cloud. Clouds are formed when the relative humidity reaches 100% and with enough hygroscopic nuclei so that condensation can take place. Cloud droplets initially grow quickly, but as they become larger the process slows down. The condensation process itself releases latent heat in the air reducing the rate of condensation and instability.
But condensation alone is not enough to produce precipitation. Cloud droplets also join together (coalescence) and water vapor will deposit on ice crystals if they are present. Which eventually can produce precipitation, but in frozen form.
We will discuss the rain or droplet formation processes needed for precipitation to fall eventually and possibly become a issue for your flight. Temperatures below zero will create a different precipitation type, commonly know as snow or hail.
But these are creating a whole new set of challenges for the crew.
Deposition and Coalescence
Deposition is the process of water vapor molecules condensing on ice crystals, ice means subzero temperatures so for that to happen we need a cold cloud. Temperatures around -15°C are most favorable for that. But for ice crystals to form we also need condensation nuclei. Clay, fine soil and volcanic dust particles seem to be the most popular types to create ice crystals.
If the air is in motion then ice crystals will grow by colliding with each other (coalescence), which helps the process so that eventually precipitation will fall if these droplets become heavy enough.
Coalescence is the fusion of cloud droplets. For this process we need large droplets, created by large condensation nuclei. Salt particles seem to be the most predominant source for this. Oceanic air is an abundant source for salt particles, and research has shown that heavy precipitation is not uncommon from clouds that have their origins over the oceans (cold fronts). Plus that evaporating water is readily available too.
Big droplets have a greater mass and will fall with a relatively higher velocity, during their descent they overtake and collide with smaller lighter droplets. This will make them grow even larger and in their wake they tend to take along other small droplets. At this point precipitation has started.
If there are not sufficient large drops or their size is not big enough and the temperature in the cloud is above zero then we can say that droplet growth is restricted and precipitation may not occur at all. Flying through such a cloud will get your windows wet though.
For drops to continue to fall they must be heavier (by gravitational force) than buoyancy and drag combined.
Precipitation can take many forms, it depends a bit on the temperature and the stability of the air. The rate of precipitation is usually indicated in light (-), moderate and heavy (+). It is determined by the amount of water falling into a rain gauge or on a flat surface during a certain amount of time. The type or character of the precipitation can be continuous or intermittent, as we know from showers.
Continuous precipitation indicates usually a thick extensive cloud layer (Nimbostratus cloud) and not likely to clear in a short period of time. A stable warm front is associated with this type of weather. Intermittent, showery precipitation usually comes with a unstable cold or occluded cold front with Cumuliform clouds and you may expect clear skies between the, sometimes heavy, precipitation.
This type is found in low level stable type stratus clouds in combination with Nimbostratus clouds. The drops are fine and close together, visibility can be reduced significantly if the drizzle becomes heavier (+).
This is the most common type of precipitation when temperatures are above zero, its a liquid water drop and they are thousands of times larger than a cloud drop (which normally stays afloat). If temperatures are subzero then snow flakes are created. The intensity may vary from light (-), moderate to heavy (+) depending on the stability and cloud type associated with the rain and the latitude as cloud water content (and temperature) is lower in the higher latitudes.
When flying in winter time this form of precipitation is a real challenge for most general aviation pilots as this may form clear ice on aircraft surfaces.
Consist of star or branch shaped ice crystals, and no two crystals are alike. This is not to be confused with hail. Snow pellets are little grains of ice with a milky white appearance and they are ball shaped (spherical). Sometimes associated with sleet (rain and snow) if temperatures at the surface are just above zero.
Usually associated with the mature and dissipating stages of Cumulonimbus clouds (Cb). Hail takes the form of irregular ball shaped pieces of layered rime or clear ice with an ice crystal core. Sizes may vary from millimeters to even several inches! The weight of a hail stone can be considerable. Damage to an aircraft done by hail can be extensive, even to the point of a total write off.