Part 2. Water in the Atmosphere

Chapter 6

Cloud Development and Forms

IntroductionClouds form as parcels of air lift and cool

Clouds are instrumental to the Earth’s energy and moisture balances

Mechanisms that Lift AirOrographic lifting

Frontal lifting

Convergence

Localized lifting

Orographic uplift (right) and orographically induced clouds(below)

Windward side is rainy Leeward side is dry(rainshadow)

Frontal lifting• A front is a boundary between unlike air

masses • Warm/moist air rises to form clouds• Cold (a) and warm (b) fronts occur

Convergence• Air converging into low pressure regions

Localized convection• Free • Forced (mechanical)

Static Stability and the Environmental Lapse Rate (ELR)

Static stability = air’s susceptibility to uplift• The environmental lapse rate compared to

the dry and moist adiabatic lapse rates determines the static stability of air

Types of static stability• Absolutely unstable air• Absolutely stable air• Conditionally stable air

Absolutely unstable air = dry adiabatic lapse rate is less than environmental lapse rate (positive bouyancy)

(Box 6.1.1, ELR 1)

Absolutely unstable, unsaturated air

Absolutely unstable, saturated air

Rising air parcel stays warmer than outside environmental air

(See Box 6.1.1 environmental lapse rate 1)

Absolutely stable air = moist adiabatic lapse rate is greater than the environmental lapse rate (negative bouyancy)

(Box 6.1.1, ELR 3)

Absolutely stable,unsaturated air

Absolutely stable, saturated air

Rising air parcel stays colder than outside environmental air

(See Box 6.1.1 environmental lapse rate 3)

Conditionally stable air = dry adiabatic lapse rate is greater than the environmental lapse rate, but the moist adiabatic lapse rate is less than the environmental lapse rate

(Box 6.1.1, ELR 2)

Conditionally unstable situation,unsaturated air

Conditionally unstable situation,saturated air

Rising moist air parcel stays warmer than outside environmental air (unstable)

(See Box 6.1.1 environmental lapse rate 2)

Rising dry air parcel stays cooler than outside environmental air (stable)

Heating/cooling the lower atmosphere changes the ELR

A diurnal profile of the ELR

Advection of Cold/Warm Air at Different Levels can change the ELR

Advection of an Air Mass with a Different ELR

Limitations on the Lifting of Unstable AirStable layers aloft will limit how high air can be lifted

This stable layer is also called an inversion layer

Profile of a frontal inversion

Profile of a subsidence inversion

Cloud types are based on appearance and/or height

Cirrus with fall streaks

Cirrus clouds are composed entirely of ice crystals

Altocumulus

Stratus

“Alto” level clouds are composed of both water droplets and ice crystals

Low clouds are composed entirely of water droplets

Stratocumulus

Clouds with vertical development• Cumulus

– Cumulus humilis, cumulus congestus, cumulonimbus

Cumulus humilis

Cumulus congestus

Formation of fair weather cumulus

Cumulonimbus

Unusual clouds• Lenticular • Banner clouds • Mammatus• Nacreous clouds (mother of pearl) • Noctilucent clouds

Lenticular

Banner cloud

Nacreous

Noctilucent

(a) = Visible image(b) = Infrared image(c) = Color-enhanced infrared