Precipitation

Cloud Formation -

Clounds consist of billions of minute water drops/ice crystals that a re suspended above the earth’s surface. For clouds to form the air must reach saturation and there must be a surface on which water vapor can condense to form liquid drops.

Cloud Condensation Nuclei -

Sources of CCN include volcanic eruptionns, dust storms, pollen, fires, exhaust pipes, etc where water vapor can condese onto these particles. The most efficient CCN have an affinity for water (hygoscopic) while a lack of CCN requires higher humidity for condensation to occur.

*Growth of cloud droplets = amount of precipitation

Classificiation -

Clouds are classified on how high they form at and how they are formed (vertical, horizontal). High level clouds (cirro-) are wispy and light. 20k ft or higher and almost completly composed of ice crystals. Cirrus are feathery, cirrocumulus are clumpy and cotton ball shaped, and cirrostratus and sheet formed.

*Contrails are condensation trails that leave the back of jet engines. Hot exhaust mixes with cold, dry air which allows water vapor to condense on the exhaust particles.

Middle Clouds (alto-) occur between 6.5k and 20k ft. Altocumulus lenticularis have been mistaken for UFO’s where the are very wide and circular. Occur in the present of a mountain range, down wind.

Low level Clouds (strato) which cover a wide range of sky. Stratocumulus (clumpy), nimbostratus (pricipitation). Very little temperature variability in the environment with these clouds.

Clouds of vertical development form where there is localized convention or convergence making the atmosphere unstable, making air parcels rise vertically. The bases are low. Cumulus (popcorn) and cumulonimbus (thunderstorms).

Fog - A cloud with the base near the ground that forms when air cools to its dew point or water vapor is added to the ground. Reduced visibility. national weather service issues advisories when visibility is reduced to 0.25 miles or less.

*Radiation fog is formend by cooling air to its dew point. Common in mountain valley

*Advection fog is also formed by cooling air to its dew point on coastal areas. As warm and moist air masses blow over cool surfaces, it creates fog.

*Upslope fog is ALSO formed by cooling air to its dew point when cool and moist air goes up a mountain.

*Steam fog is when water vapor is added to the air (evaporation). When cold dry air moves across warm water.

How does precipitation form -

Diameter of a raindrop is 100 times that of a typical cloud droplet, but volume is 1 million times larger. Increasing the rate of condensation will only produce more cloud droplets, not increase their size.

Precipitation in Warm Clouds form through collision coalescence where droplets collide with each other, velocity of the fall icnreases which changes the shape of the drop, and the the big drops breaks into small drops while falling.

Precipitation in cold clouds occurs through the bergeron process. These are clouds have vertical development where there is a mixture of ice crystals, drops, and water vapor. There is a zone between 1.5mb and 2mb where water is unsaturated and ice is saturated that occurs at temps. of -10 to -20deg celcius. This allows water vapor to deposit onto ice crystals which decrease relative humidity and water drops evaporate. The size of the water drop then gets smaller and the ice crystlas get bigger through deposition.