this lecture discusses precipitation formation and types of precipitation precipitation is the generic term for any form of water that comes from the sky technically those objects are called hydrometeors the likelihood of precipitation depends on several factors but the main factor is the amount of water available in the atmosphere mixing ratio quantifies the amount of water so higher mixing ratios mean there's more water available to condense other factors include the base height of the cloud because the lower the cloud is to the surface the more likely any droplet that forms can make its way through the atmosphere without evaporating the droplet must grow large enough to lose suspension from the turbulence in the atmosphere and drop under the force of gravity droplets will grow larger the longer they're held in suspension if the updrafts in the cloud are strong the droplets will grow much larger that explains why tall clouds with strong vertical updrafts can create large sizes of precipitation that happens in cumulonimbus clouds stratus clouds that are shallow with weaker updrafts create smaller sizes of precipitation based on the overall temperature profile and mixing ratio of the cloud different droplet formation processes occur we categorize clouds as either warm or cold based on their vertical temperature profile warm clouds are uniformly above freezing so they contain liquid water throughout these clouds form in lower latitudes near the equator the droplet growth process in warm clouds is called collision coalescence because cloud droplets of different sizes will collide and merge to grow the raindrops most clouds outside of the tropics are cold clouds where the temperatures will move below freezing at some point in height even summer clouds in Arizona will be cold clouds because they grow to great vertical extent and have cloud tops well below freezing collectively the growth processes in cold clouds are aggregation and accretion aggregation happens more often in moderately cold clouds that have very high mixing ratios the types of crystals that tend to form in those conditions are dendrites dendrites can hook together through aggregation to get very large sometimes 3 to 4 inches across dendrites are the most common type of snowflake because crystals grow fastest around negative 15 degrees Celsius accretion is the pasting of supercooled water onto surfaces and accretion happens at the expense of nucleation supercooled water is water that exists in the liquid phase well below freezing it's difficult for nucleation at temperatures below freezing but at negative 40 degrees Celsius nucleation is forced temperatures between 0 and negative 40 will allow supercooled water to Ace or accrete onto crystals there's a limit to droplet size as the droplet approaches four to five millimeters the droplet will be torn in two as updrafts shear it apart average sized raindrops are one to two millimeters and as raindrops approach two millimeters in diameter they start to flatten on their undersides so they look more like a jellyfish than a sphere that flattening helps facilitate the shear terminal velocity is the constant rate of speed of a free-falling object when the downward directed acceleration by gravity is balanced by the frictional drag in the atmosphere as the mass of the object increases the terminal velocity increases so the larger the raindrop the faster the downward directed velocity the largest droplets ever observed were 10 millimeters which is 1 centimeter or about 1/3 of an inch they were found in tropical warm clouds over Brazil but there's no evidence that the droplets hit the ground that size there are actual definitions and parameters for some common terms used to describe precipitation sizes and intensities intensity is just a volume per time like two inches per hour drizzle refers to smaller liquid droplets at a lower intensity and rain refers to larger droplets the intensity of rain can be described as light moderate or heavy light is a tenth of an inch an hour moderate is up to a third of an inch in an hour and heavy is anything bigger than that solid precipitation like snow is also described by intensity of light moderate and heavy flurries are intermittent and light with usually no measurable accumulation winds play a factor two squalls refer to sustained winds of 25 miles per hour for at least a minute blizzards are sustained winds of at least 35 miles per hour with considerable snow that reduces visibility to less than a quarter mile for at least an hour snow is mostly air which is why snow is a good insulator for snow caves and igloos however measuring snow is difficult because it drifts with the wind and that makes snow depth a less accurate measure a better measure of snow is snow water equivalent or Sui 10 inches of snow that melts down to an inch of water wood reports we as an inch however some snows are dry and some are more moist a dry snow might have a sway of 1/10 of an inch and a moist know could have a Swee of up to four inches one snow drops out of a cloud it can undergo other changes depending on the vertical temperature profile underneath the cloud other things to know about snow is that if it moves into a warm portion of the atmosphere it will melt into liquid precipitation if the snow melts it will never turn back into snow as it moves down to the surface however if the liquid moves into a colder portion it can refreeze into sleet or freezing rain if the cold lair is deep then the liquid can refreeze into a pellet called sleet if the liquid doesn't have time to refreeze but hits a frozen surface it accretes at the surface creating freezing rain freezing rain is much more dangerous than groffle is also called soft hail or snow pellets it's formed when supercooled water rhymes around a snow crystal much like regular hail but it's smaller around one to five millimeters and it doesn't have a crystalline structure instead when you press a piece of grobble between your fingers it crushes like a piece of cereal hail forms from accretion within the cloud stronger updrafts hold the hail embryo and suspension and move it in and out of the glaciated portion of a cloud allowing the hail stone to grow when the hail stone is large enough to lose suspension from the updrafts it falls to the ground the last time someone was killed by hail in the United States was March 28 2008 a n'ajjer was killed by grapefruit sized hail stone in Fort Worth Texas that same system spawned an ef3 tornado that killed two other people and injured nearly a hundred others most times hail events in the United States caused personal property damage hail damage easily reaches 10 billion dollars annually hail events are much deadlier in lesser developed countries the largest hailstone ever measured in the US was the Aiden diameter hailstone it happened in Vivian South Dakota on July 23rd 2010 it weighed almost two pounds that means updrafts in that storm system had to be well over a hundred and 60 miles per hour