Outline Further Reading: Chapter 06 of the text book - what is a cloud? - cloud classifications -...
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Transcript of Outline Further Reading: Chapter 06 of the text book - what is a cloud? - cloud classifications -...
Outline
Further Reading: Chapter 06 of the text book
- what is a cloud?
- cloud classifications
- clouds and precipitation
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(1 of 11)
Introduction
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(2 of 11)
• Previously,
– We learned that when air is displaced vertically, it cools due to adiabatic processes
– Discussed causes of vertical motion
• Convection
• Orographic lifting
– We also discussed what happens once the temperature cools to the dew point temperature, namely, we expect clouds to form as water vapor changes to liquid
• Today,
– We want to discuss what forms these clouds take
• Classify cloud formations
• Discuss characteristics that allow us to identify different types of clouds
Cloud Type by Form
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(3 of 11)
• Clouds:
– Optically thick mass of suspended water drops or ice crystals• Clouds can be classified by some simple, but subjective, criteria that also provides
information on the atmospheric conditions
• One form of classification is based on appearance or form
Cirrus: Thin wispy clouds Stratus: Layered clouds with fairly continuous coverage
Cumulus: Individual clouds characterized by heaped, puffy appearance
Have vertical extent, from 1-3km up to tropopause
Cloud Type by Altitude-01
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(4 of 11)
• Can also classify them based on their altitude
• “Cirro”
• “Alto”
• “Strato”
• “Fog” CirroHigh clouds (7-18) km
Cold (<-25 C) with exclusively ice crystals
Cirrocumulus: high, puffy cloudsCirrus: high, wispy clouds
Cloud Type by Altitude-02
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(5 of 11)
Altostratus: thin, layered cloudsAltocumulus: individual or “rolls” of clouds
AltoMiddle level clouds (2-7 km)
0-25 C composed of both water and ice crystals
Cloud Type by Altitude-03
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(6 of 11)
Stratus: Dense, uniform gray layers Stratocumulus: groups of dense, puffy clouds
StratoLow level clouds (0 - 4 km)
> 5 C composed of water
Cloud Type by Altitude-04
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(7 of 11)
FogClouds at ground level
Radiation fog: forms at night when cold ground cools the air above it (in valleys)Advection fog: forms when warm, moist air moves over colder surface and cools
(in coastal areas)
Cloud Type by Rain
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(8 of 11)
• Finally, we can classify them based on the presence of rain
• Nimbus: any cloud that rains
Cumulonimbus: vertical clouds that produce rain Nimbostratus: low, flat clouds that drizzle
Thunderstorms
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(9 of 11)
– Thunderstorms form when we have an unstable, moist atmosphere resulting in strong vertical motions
– Actually composed of many individual circulation “cells”
• Updraft region is where the upward motion is intense and where the rain typically falls
• Downdraft region is associated with downward motion which can also be intense -> leads to downbursts
• Cells are typically 10-20km across and circulation within them occurs over 20 minutes or so, i.e. new cells can form in 20 minutes
– Can produce hail: this is when an ice particle is continuously cycled through the convection cell before becoming heavy enough to fall out
– Can also produce lightening: as water is moved within the cell, it develops a fictional charge; the discharge occurs through a spark, i.e. lightening
Clouds and Precipitation-01
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(10 of 11)
• Rain (or any precipitation)– Requires
• Vertical motions -> cooling• Presence of condensation nuclei -> something for the rain drops to form on
– Condensation does not necessarily mean rainfall; could just form clouds
– In order to form precipitation, the drops must be large enough to fall against the upward motion of the air
• This requires that the aggregation (or coalescence) of lots of water molecules into drops• Cloud droplets are typically 50-100 micro-meters in diameter• In contrast, rain drops are typically 500 micro-meters before they begin to fall (drizzle)• At 1000-2000 micro-meters, they are considered rain drops
Clouds and Precipitation-02
Natural Environments: The AtmosphereGG 101 – Spring 2005
Boston University
MyneniLecture 15: Clouds
Feb-28-05(11 of 11)
• There are also other forms of precipitation
• Snow - crystallized water formed by the aggregation of frozen water
• Sleet - rain falling through a colder lower layer and freezing
• Hail - the aggregation of liquid water onto frozen water and subsequent freezing
• Note that precipitation does not necessarily fall straight to the ground - it might start to fall then get caught in an updraft and cycle through; during this process more aggregation occurs and we get very large rain or hail falling out (i.e. ‘golf-ball size hail’)