Precipitaion Patterns
There are many types of precipitation: rain, snow, sleet, and hail, to name a few. In this lesson we will learn about the mechanisms that produce various types of precipitation. It is important to note that the presence of clouds and their associated condensation nuclei alone do not always produce precipitation. Very specific conditions must occur in order for a cloud to produce precipitation. For a detailed discussion of atmospheric moisture and cloud formation click on the links. To see Idaho's annual average precipitation map click here. To see Idaho's average temperatures for July and January, click here.

In order for cloud droplet to form a non-equilibrium condition, where condensation exceeds evaporation, must exist. The curvature of a cloud droplet affects its rate of evaporation. The more curved the droplet, the more evaporation that occurs. Smaller cloud droplets will evaporate quickly unless the air is supersaturated (the relative humidity exceeds 100%). Because of the curvature effect, air that is saturated with respect to a flat surface is unsaturated with respect to a curved cloud droplet. An ordinary cloud droplet 100 times smaller than raindrop.

Though supersaturation is required in order for cloud droplets to sustain themselves, relative humidity rarely approaches 101%, even in very wet clouds. How do cloud droplets ever grow to raindrop size? The answer lies in the Hygroscopic nature of certain condensation nuclei. Recall that condensation on hygroscopic particles will commence when the relative humidity is below 100%. This is known as the solute effect.

Consider a parcel of air unsaturated air rich with condensation nuclei. As the air cools the relative humidity increases. At some point below 100% saturation, condensation commences on the most hygroscopic of the available nuclei. These nuclei continue to grow as the air cools further and the relative humidity approaches 100%. The curvature effect becomes negligible for larger droplets but remains appreciable for smaller nuclei. The rise in relative humidity within the air mass is slowed by the fact that the larger particles begin to remove lots of water vapor from the air. Soon, the particles are removing water vapor from the air as fast as it can be replaced from external sources. At this point the relative humidity actually begins to decrease. Condensation in clouds is such an inefficient precipitation producing process that it is very unlikely to produce, by itself, precipitation in any appreciable amount. Another mechanism is clearly responsible for producing precipitation from clouds. Two additional mechanisms are responsible for producing precipitation from clouds the collision-coalescence process, and the ice-crystal process.

The collision-coalescence process occurs in warm clouds. As cloud droplets form within clouds they become electrically charged. The cloud droplets grow larger by sticking to each other in the aftermath of collisions due to electrical attraction. As time passes the droplets grow larger and larger. Updrafts help keep the droplets suspended in the cloud longer. If the cloud is thick the droplets will also stay suspended longer. Finally, the droplets will grow large enough that they can no longer remain suspended and will begin to fall. As soon as they leave the cloud base they begin to shrink due to evaporation. Raindrops that reach the ground are smaller than those leaving the base of the cloud.

The ice-crystal process occurs in colder clouds that exist mainly in the middle to high-Iatitudes. Even in these extremely cold clouds there are liquid water droplets (existing well below freezing). These are referred to as supercooled water droplets. The temperature of a cloud, in fact, must exceed -4OoC in order for it to consist entirely of ice crystals. Such clouds are referred to glaciated.

When the temperature drops low enough within a cloud, large numbers of water molecules begin to bond in a rigid form within supercooled liquid water droplets. This leads to the formation of ice embryos, i.e., small ice crystals in the center of supercooled water droplets The water molecules must have very low rms speeds in order for ice embryos to remain intact since even slight thermal motions disrupt them. Even colder temperatures enable the crystal to become a freezing nucleus. The presence of these ice embryos enhances the freezing process. The presence of ice nuclei also enhance the freezing process. Ice nuclei may be clay (kaolinite), biological material, or anything that looks like an ice crystal. Contact freezing is another important method by which ice crystals to form in a cloud, involving collisions between ice nuclei (freezing nuclei) and supercooled droplets.

As we have seen, when precipitation first begins to fall it is usually in a frozen state. Often precipitation begins in the form of either graupel or snowflakes. Snowflakes are an aggregation of ice crystals. Much precipitation falling at middle latitudes, even in mid-summer, falls as snow flakes in the beginning. Graupel is formed by collisions between supercooled cloud droplets and ice crystals.

In a precipitation theory known as the Bergeron Process all raindrops begin as ice crystals. When the ratio of ice crystals to water droplets in clouds is on the order of 1:100,000, conditions are right for precipitation to begin. When there are too few ice crystals, the existing crystals grow large and fall out of the cloud, leaving it unaffected. When there are too many crystals, a cloud of ice crystals is formed, and no precipitation occurs because the individual crystals are all too small to fall to the ground.

Cloud seeding is an important process used quite often in the winter to create precipitation. The object is to find clouds that are deficient with ice crystals and inject artificial ice nuclei to produce the ratio of 1:100,000. (Silver iodide is usually the artificial ice nuclei used because it resembles an ice crystal so well.) A cold cloud is needed for this to work effectively.

Rain is liquid drop precipitation with diameter greater than or equal to 0.5mm.

Drizzle is a liquid drop with diameter less than 0.5mm. Virga is precipitation that doesn't reach the ground. If updrafts in a cloud change to downdrafts rainfall amount may increase to a shower. If a shower is excessively heavy it is referred to as a cloudburst.

Snow consists of frozen ice crystals falling to the ground. Because snow scatters light more effectively than rain one may easily observe where snow changes to rain below a cloud (above the freezing line is darker). If, however, one looks directly up into the precipitation from below the snow appears lighter because it scatters light in all directions below the cloud. As a result the bottom of a rain cloud appears much darker than a cloud with snow in it.

Fallstreaks are a virgalike phenomenon consisting of snow rather than rain.

Flurries are brief snow showers, typically from cumuliform clouds. A snow squall is a more intense snow shower, essentially the equivalent of a cloudburst. Continuous snowfall is associated with nimbostratus and altostratus clouds. A blizzard is a snowstorm accompanied by low temperatures, strong winds, blowing and drifting snow.

Sleet is melted snow that re-freezes into a tiny ice pellet. Freezing rain occurs when raindrops fall through a freezing layer that supercools them and subsequently freeze on contact with the ground.

Freezing drizzle is freezing rain with drop diameters less than 0.5mm. Rime is an accumulation of small, supercooled cloud droplets that are milky and granular in appearance. Snow grains and snow pellets are the solid equivalent to drizzle. Snow grains have a diameter of less than lmm and stick upon hitting a surface, while snow pellets have diameters of greater than 5mm and bounce upon hitting a surface.

Hail is produced when large, frozen raindrops, graupel, etc. act as accretion nuclei. In order for a hailstone to form, the accretion nuclei must remain in a cloud a long time and thus travel a large distance within the cloud. This process is facilitated by strong updrafts of the type common within cumulonimbus clouds. Hail is most often associated with such clouds and is therefore more common during the spring and summer than in winter. Hailstreaks are long narrow bands of land struck by hail as the precipitating cloud moves along.

About 80 per cent of the precipitation that falls on Idaho each year is in the form of snow. It takes about one foot of snow to make one inch of water when it melts. Since water is Idaho's single most important resource a system has been developed to measure snow depths in the mountains of Idaho. This system almost guarantees that water will be used efficiently, and that it will be well conserved so that everyone will have enough water each year. We all rely on the water that falls on our state each year, not just the farmers who use it for irrigation. We also use water for power, to fish in, and to help wildlife survive. Idaho's industries need water to operate and you and I need it to drink, to bathe in, to do our dishes and to water our lawns. In addition to water supply, precipitation plays a significant role in shaping the landscape around us.

The state precipitation map at left underscores the greatest natural deficiency suffered by the West. The region lacks sufficient precipitation for most of the basic needs of human beings. It has been responsible for the treeless plains and, naturally, the desert. In the Snake River Valley for example there is a yearly average of only eight inches. Where the annual amount is less than fifteen inches and irrigation is not possible, dry farming and grazing are the dominant agricultural activities

The highest amount of precipitation ever recorded in Idaho was on Deadwood Summit in Valley County in the winter of 1964-65. Precipitation of 98.6 inches was recorded that year. Much of that precipitation was in the form of snow (if it takes one foot of snow to make one inch of water when it melts, imagine how much snow fell on Deadwood Summit that winter).

Just 75 miles to the east of Deadwood Summit, Challis has the lowest average yearly precipitation in Idaho, just 7.09 inches. Of the larger cities and towns in Idaho, Boise has an average precipitation of less than 12 inches and Wallace in Shoshone County has the heaviest annual precipitation of 41.64 inches. Snow depths vary widely throughout the state, ranging from skiffs in the lower dry areas to very deep in the central mountains.