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” Cirro

High clouds (7-18) kmCold (<-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’)