Outline

1. Myneni Lecture 15: Clouds Feb-23-07 (1 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Further Reading: Chapter 06 of the text book Outline - what is a cloud? - cloud classifications - clouds and precipitation

2. Myneni Lecture 15: Clouds Feb-23-07 (2 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University 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 Introduction

3. Myneni Lecture 15: Clouds Feb-23-07 (3 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cloud Type by Form • 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 • Clouds: • Optically thick mass of suspended water drops or ice crystals 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

4. Myneni Lecture 15: Clouds Feb-23-07 (4 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Can also classify them based on their altitude “Cirro” “Alto” “Strato” “Fog” Cloud Type by Altitude-01 Cirro High clouds (7-18) km Cold (<-25 C) with exclusively ice crystals Cirrus: high, wispy clouds Cirrocumulus: high, puffy clouds

5. Myneni Lecture 15: Clouds Feb-23-07 (5 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cloud Type by Altitude-02 Alto Middle level clouds (2-7 km) -25 to 0 C composed of both water and ice crystals Altocumulus: individual or “rolls” of clouds Altostratus: thin, layered clouds

6. Myneni Lecture 15: Clouds Feb-23-07 (6 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cloud Type by Altitude-03 Strato Low level clouds (0 - 4 km) > 5 C composed of water Stratocumulus: groups of dense, puffy clouds • Stratus: Dense, uniform gray layers

7. Myneni Lecture 15: Clouds Feb-23-07 (7 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cloud Type by Altitude-04 Fog “Clouds 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)

8. Myneni Lecture 15: Clouds Feb-23-07 (8 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cloud Type by Rain • 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

9. Myneni Lecture 15: Clouds Feb-23-07 (9 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Thunderstorms • 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 frictional charge; the discharge occurs through a spark, i.e. lightening

10. Myneni Lecture 15: Clouds Feb-23-07 (10 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Rain (or any precipitation) Requires Vertical motions -> cooling Presence of condensation nuclei -> something for the rain drops to form on (for example a small dust particle) 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-01

11. Myneni Lecture 15: Clouds Feb-23-07 (11 of 11) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Clouds and Precipitation-02 • 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’)