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