MET 1010 Ch.7
Meteorology Today UF Chapter 7 Precipitation
Typical size of a condensation nucleus
Typical size of a cloud droplet
Typical size of a raindrop
equilibrium vapor pressure
the number of vapor molecules surrounding a droplet remains fairly constant so the droplet does not grow or shrink
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
If small droplets are to grow, they require...
supersaturation of the air and condensation nuclei
particles around which water condenses to start the formation of cloud droplets
water-attracting particles, like salt, that allow condensation to begin in unsaturated air
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
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 drops fall faster, overtake and absorb smaller drops
_____ drops grow faster than ______ drops.
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
Thick clouds with strong updraft will produce _______.
heavy rain, large raindrops
Thin clouds with weak updraft will produce ______.
at most a drizzle
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.
Same geometry as ice, small particles around which ice crystals form
ice nuclei on which vapor directly deposits as ice without becoming liquid first
supercooled liquid droplets freeze when the nuclei is immersed
supercooled liquid droplets freeze when they collide with the nuclei
freezing of pure water without an ice nucleus, using an 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
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 ___.
Clouds with high liquid h20 content will create ____ through ____.
an excessively heavy shower
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
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)
is the amount of rain measured in inches per hour
Most precipitation starts as...
snow and melts on the way down
light snow showers that do not accumulate
intense snow showers with a short duration
low temps and strong winds blow falling snow around creating poor visibility
drifting and blowing snow after snowfall has ended creates poor visibility
measured in visibility distance
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.
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.
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
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