Part 2. Water in the Atmosphere Chapter 6 Cloud Development and Forms

Introduction Clouds form as parcels of air lift and cool Clouds are instrumental to the Earth’s energy and moisture balances Mechanisms that Lift Air Orographic 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 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 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 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 Air Stable 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