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Meteorology Today UF Chapter 7 Precipitation

Typical size of a condensation nucleus

0.2 mu(m)

Typical size of a cloud droplet


Typical size of a raindrop

2000 mu(m)

equilibrium vapor pressure

the number of vapor molecules surrounding a droplet remains fairly constant so the droplet does not grow or shrink

curvature effect

the smaller a droplet is, the greater the curvature it has, which makes it easier for water to evaporate because of weak bonds

because smaller droplets have greater curvature, it requires ___ to maintain equilibrium.

greater vapor pressure is req'd to maintain equilibrium, and the pressure is provided by a RH > 100.1%

To keep a droplet in equilibrium...

vapor molecules must be around it to replace molecules that have evaporated

When the air is ____ with respect to flat surfaces, it is ______ with respect to curved surfaces

saturated, unsaturated

If small droplets are to grow, they require...

supersaturation of the air and condensation nuclei

Condensation Nuclei

particles around which water condenses to start the formation of cloud droplets

Hygroscopic Nuclei

water-attracting particles, like salt, that allow condensation to begin in unsaturated air

Solute Effect

Salt particles partially dissolve in water droplets, decreasing the saturation vapor pressure

Condensation can form rain drops, but _____.

It is so slow that it takes days and if they fall by chance before they get big enough, they will evaporate before reaching the ground

Two ways to grow raindrops besides condensation are____.

Collision-Coalescence process and Bergeron process

Terminal Velocity

when the gravity and the air drag on a falling object become equal and the object begins to fall at a constant rate

_____ drops fall faster than ______ drops.

larger, smaller


larger drops fall faster, overtake and absorb smaller drops

_____ drops grow faster than ______ drops.

larger, smaller

Why are the first drops of rain large and heavy?

Because the larger drops fall fastest and reach the ground first.

Why don't smaller drops make it to the ground?

Because they evaporate on the way down or are absorbed into larger drops.

4 important factors in the production of raindrops

liquid water content of clouds, cloud thickness, updrafts of cloud, electric charge of droplets and electric field

warm clouds

above freezing

Thick clouds with strong updraft will produce _______.

heavy rain, large raindrops

Thin clouds with weak updraft will produce ______.

at most a drizzle

cold clouds

below freezing

If water droplets are below 0C...

they are supercooled.

The smaller the droplet, the ____ the temperature at which it will freeze.


Below _____C almost all droplets freeze and form ice crystals.


Ice Nuclei

Same geometry as ice, small particles around which ice crystals form

Deposition nuclei

ice nuclei on which vapor directly deposits as ice without becoming liquid first

Freezing nuclei

supercooled liquid droplets freeze when the nuclei is immersed

Contact Nuclei

supercooled liquid droplets freeze when they collide with the nuclei

homogeneous freezing

freezing of pure water without an ice nucleus, using an ice embryo

ice embryo

enough water molecules join together within the droplet in a rigid pattern to form a tiny ice pattern and it acts as an ice nucleus

There are more ____ nuclei than _____ nuclei

condensation, ice

The saturation pressure above a water surface is ___ than an ice surface. Why?

greater, because water molecules evaporate more easily.

Ice-crystal - Bergeron Process

water evaporate from water droplets and migrates to ice crystals, so ice crystals grow at expense of water droplets


falling ice crystals collide with supercooled water droplets which freeze on contact and thus the ice crystal grows forming graupel

Fracturing ice crystals cause...

a greater number of ice particles to fall and grow by aggregation and accretion, like a chain reaction


a.k.a. rime or snow pellets: icy matter that forms from accretion


while falling, many ice crystals collide and stick together to make snowflakes


ice matter that forms from aggregation

Bergeron Process was proposed by ...

Wegener, Bergeron, and Findeisen

For ice crystals to grow large enough to produce precipitation, there must be a ratio of...

1 ice crystal to 100,000 - 1,000,000 water droplets

Clouds with low liquid h20 content will create ___ through ___.

snowflakes, aggregation

Clouds with high liquid h20 content will create ____ through ____.

graupel, accretion


an excessively heavy shower

Continuous rain

falls from a layered cloud that covers a large area and has smaller vertical air currents (nimbostratus)


raindrops are < 0.5 mu(m) produced by thin stratus clouds


raindrops are > 0.5 mu(m) caused by strong upward winds, or an updraft that suddenly weakens or changes direction- short and intensive precipitation

Acid rain

atmospheric pollutants dissolve into the water droplets and form an acidic solution


rain drops evaporate before they reach the ground, making white streaks below clouds (happens in dry, warm atmosphere)

Rainfall Intensity

is the amount of rain measured in inches per hour

Most precipitation starts as...

snow and melts on the way down

snow flurries

light snow showers that do not accumulate

snow squalls

intense snow showers with a short duration


low temps and strong winds blow falling snow around creating poor visibility

ground blizzard

drifting and blowing snow after snowfall has ended creates poor visibility

Snowfall Intensity

measured in visibility distance

Snow Blanket

good insulator (poor conductor) traps heat underneath and keeps things from freezing


small ice crystals sublimate before they reach the ground, causing white streaks below clouds


change from ice to vapor

Most snow occurs in ____ regions of US.

Northwest, Northeast, and West

Least snow occurs in ____ regions of US.

Southeast, Southwest


tiny ice pellets formed when ice crystals fall and melt in the warmer atmosphere below the cloud, but then refreeze in the surface layer which is subfreezing.

freezing rain

supercooled liquid droplets fall and freeze when they make contact with an object, causing a glaze of translucent ice


when small liquid cloud or fog droplets land on frozen objects and freeze on contact (milky ice)


pieces of translucent or opaque ice of spherical or irregular shape ranging from pea to golf ball sized or larger

Hail is formed when...

particles or graupel grow by accretion, and stay in the cloud for 5-10 minutes

rain gauge

any instrument that can collect and measure rainfall

standard rain gauge

funnel collector catches rain in a tube to be measured

tipping bucket rain gauge

automated gauge that collects water in 1/10 inch collectors, and each time a collector fills, it tips to empty and a remote recorder registers the 1/10 in. rain

weighing-type rain gauge

measures the weight of collected rainfall

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