Part 2. Water in the Atmosphere Chapter 6 Cloud Development and Forms
Dec 16, 2015
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)
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
Altocumulus
Stratus
“Alto” level clouds are composed of both water droplets and ice crystals
Low clouds are composed entirely of water droplets
Clouds with vertical development• Cumulus
– Cumulus humilis, cumulus congestus, cumulonimbus
Cumulus humilis
Unusual clouds• Lenticular • Banner clouds • Mammatus• Nacreous clouds (mother of pearl) • Noctilucent clouds