Download (See Box 6.1.1 environmental lapse rate 3) Conditionally stable air

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
Windward side is rainy
Orographic uplift (right)
and orographically
induced clouds
(below)
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
Rising air parcel stays
warmer than outside
environmental air
Absolutely unstable,
saturated 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
Rising air parcel stays
colder than outside
environmental air
Absolutely stable,
saturated 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
Rising dry air parcel stays
cooler than outside
environmental air (stable)
Rising moist air parcel stays
warmer than outside
environmental air (unstable)
Conditionally unstable situation,
saturated air
(See Box 6.1.1
environmental lapse rate 2)
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
“Alto” level clouds are
composed of both water
droplets and ice crystals
Stratus
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