Chapter 7
Clouds, Precipitation, and Weather Radar
Clouds
• A cloud is the visible product of condensation or deposition of water vapor in the atmosphere
• Need more than just saturation to form clouds
• When the relative humidity of the air is >100% we say the air is supersaturated:
and a cloud forms; vapor in excess of 100%
Nuclei
• Recall that that the atmosphere is composed of gases and aerosols
• Nuclei – naturally occurring particles that promote condensation or deposition in the atmosphere
• Nuclei have a radius greater than 1.0 μm - droplets grow at RH near 101%, which does occur in the atmosphere
• Sources: volcanoes, forest fires, pollution, soil erosion, and sea spray
Nuclei
• Cloud Condensation Nuclei (CCN)– Condensation of water vapor at temperatures
above and below the freezing point of water
• Ice Forming Nuclei (IN)– Formation of ice crystals at temperatures well
below freezing• Freezing nuclei – water vapor condenses and freezes
• Deposition nuclei – water vapor deposits directly as ice
Supercooled Water
• Water that cools below freezing, but does not freeze (as cold as –38.2oF)
• Homogeneous Nucleation– Supercooled water drops collect on a tiny ice crystal
spontaneously at a temperature less than –38.2oF
• Heterogeneous Nucleation– Supercooled water drops collect on a foreign particle at a
temperature less than freezing, but warmer than –38.2oF
Classification of Clouds
• General Appearance• Altitude of Cloud Base
– Stratiform: high, middle, or low level– Cumuliform: clouds with vertical development
• Temperature– Warm cloud > 0oC– Cold cloud at or below 0oC
• Composition– Ice crystals, supercooled droplets, or water droplets
International Visual Cloud Classification-• Cumulus (literally, heap or pile)• Stratus (literally, flattened out or covered with a layer)• Cirrus (literally, a lock of hair or a tuft of horsehair)• Nimbus (precipitating cloud)• Altum (height)
High Clouds
• Altitude: above 5000m - stratiform
• Temperature: -25oC
• Composition: almost entirely ice crystals
• Appearance: – Thin and wispy– Transparent to allow sunlight through– Rarely cover the entire sky– No precipitation
Cirrus (Ci)
Cirrostratus (Cs)
Cirrocumulus (Cc)
Middle Clouds
• Altitude: 2000-7000m• Temperature: Between 0oC and -25oC• Composition: ice crystals, water droplets, or a
combination of both• Appearance:
– Thicker and larger than cirrus clouds– Sun is dimly visible– Completely or partially cover the sky– Rarely produce precipitation that reaches the ground
Altocumulus (Ac)
Altostratus (As)
Low Clouds
• Altitude: ground to 2000m
• Temperature: temperatures above -5oC
• Composition: mostly water droplets
• Appearance:– Low lying thick gray clouds– Sun is obscured– Completely cover the sky– Generally light, but steady precipitation
Stratocumulus (Sc)
Nimbostratus (Ns)
Stratus (St)
Clouds With Vertical Development• Altitude: height of Convective Condensation Level
(CCL)– generally about 1000-2000m– Clouds tops can be as high as 20,000m (stratosphere)
• Composition: water drops, supercooled water drops, and ice crystals
• Appearance:– White puffy clouds– Cotton, Cauliflower– No precipitation with “fair weather” cumulus– Significant storms with cumulonimbus
Cumulus (Cu)
Cumulus Congestus (CuCon)
Cumulonimbus (Cb)
Mountain Wave Cloud
Cap Cloud
Lee-Wave Clouds
Lenticular
Mammatus
Nacreous Clouds
Noctilucent Cloud
Fog
• Fog– a cloud (stratus) in contact with the ground – Restricts visibility to 1000m or less– If this visibility restriction is not met, then it is
called mist
• The air needs to be saturated for fog to develop
Radiation Fog
• Air becomes saturated due to radiational cooling• Conditions for development:
– Clear night sky
– Light winds (calm winds would favor dew)
– Humid air at the ground with dry air aloft
• Generally occurs over land where rain or snowmelt has occurred the day before
• Is often burned off by the sun a few hours after sunrise
Advection Fog
• Air becomes saturated due to advective cooling
• Conditions for development:– Warm humid air advecting over a cold surface– The cold surface chills the air to its saturation
point at the lowest layers
• Warm air flowing over snow covered ground or a cold water surface (Great Lakes)
Steam Fog
• Air becomes saturated due to the addition of water vapor
• Conditions for development:– In the winter when cold dry air flows over an unfrozen
lake
– The lower layer warms and becomes more humid due to evaporation this mixes with the cold dry air aloft to form fog
• Fog resembles smoke coming out of a smokestack
Upslope Fog
• Air becomes saturated due to expansional cooling
• Conditions for development– As humid air ascends up a mountain it expands
and cools, thus reaching saturation
• Sometimes the fog reaches the top of the hill and spreads as a stratus cloud over a valley – this is called high fog
Precipitation Processes
• Most clouds do not bring any rain or snow
• For clouds to precipitate the cloud particles must be large enough for their terminal velocity to be greater than the updraft in the cloud
• For this to happen drops need to be about 2mm in diameter, but cloud drops are only 10-20μm in diameter – so how does it rain (or snow)?
Warm Air Clouds
• Collision-Coalescence Process– Droplets that grow by colliding and then
coalescing (merging) with one another
• Droplets with larger diameters have a larger terminal velocity, so as they move through the cloud, they “pick up” smaller droplets
• As droplets become large enough they fall out of the cloud as precipitation
Cold Air Clouds
• Bergeron-Findeisn Process– The growth of ice crystals in a cloud at the
expense of supercooled water droplets
• Same idea as warm air clouds as the frozen particles grow they overtake more droplets and fall out of the cloud
Virga
• Once a large droplet leaves the base of the cloud there is no guarantee that it will reach the surface
• Often the drop will evaporate
• Virga– Water or ice particles that vaporize before they
reach the earth’s surface
Types of Precipitation
• Rain– Diameters between 0.5 and 6mm
• Drops break apart if diameter gets too large
• Drizzle– Diameters between 0.2 and 0.5mm– Generally occurs in stratus clouds– Occur with fog and contribute to low visibility
Types of Precipitation• Snow
– An agglomeration of ice crystals in the form of flakes– Crystals come in 4 types
• Needles• Dendrites• Plates• Columns
– Snow flakes vary in size, but can be as big as 5-10 centimeters in diameter
– Snow Pellets: supercooled droplets that collide and freeze on an ice crystal
– Snow Grains: like drizzle, except they freeze before reaching the ground
Types of Precipitation• Ice Pellets
– Also called sleet– Snowflakes that partially or completely melt and then refreeze
before hitting the ground
• Freezing Rain (or drizzle)– Liquid drops that supercool and partially freeze on contact on
cold surfaces at the ground– This forms a coat of ice on road, trees, and stuff
• Hail– Chunks of ice– Forms in thunderstorms with strong updrafts that cause ice
rock to grow– Mostly melt before hitting the surface
Weather Radar
• A remote sensing tool for determining the location, movement, and intensity of areas of precipitation
• National Weather Service uses a WSR-88D – WSR – weather surveillance radar
• Reflectivity Mode– Location, movement, and intensity of areas of precipitation– Maximum range of 285 miles
• Velocity (Doppler) Mode– Air motions directly toward or away from the radar associated
with the circulation of the weather system– Maximum range of 143 miles
Doppler Effect
• A shift in the frequency of sound waves emanating from a moving source– How the sound of a train or ambulance changes as it
moves towards and then away from you
• Doppler radar monitors the motion of precipitation toward or away from the radar– Meteorologists can detect circulations and rotations
(tornados) and thus give advanced warnings
Reflectivity Mode
Doppler Effect
Radar Stuff
• Clear Air Mode– Very sensitive radar setting– Radar can detect dust particles or swarms of
bugs that collect along boundaries of air masses– These boundaries are potential sites for
thunderstorms development
• Ground Clutter– Nearby objects (buildings, trees) that reflect
back to the radar