Part 2. Water in the Atmosphere

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Part 2. Water in the Atmosphere

• Chapter 6
• Cloud Development and Forms

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Introduction

• Clouds form as parcels of air lift and cool
• Clouds are instrumental to the Earths energy and
  moisture balances
• Mechanisms that Lift Air
• Orographic lifting
• Frontal lifting
• Convergence
• Localized lifting

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Leeward side is dry (rainshadow)
Windward side is rainy
Orographic uplift (right) and orographically
induced clouds (below)
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• 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

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• Convergence
• Air converging into low pressure regions
• Localized convection
• Free
• Forced (mechanical)

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• Static Stability and the Environmental Lapse Rate
  (ELR)
• Static stability airs 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

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• Absolutely unstable air dry adiabatic lapse
  rate is less than environmental lapse rate
  (positive bouyancy)
• (Box 6.1.1, ELR 1)

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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)
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• Absolutely stable air moist adiabatic lapse
  rate is greater than the environmental lapse rate
  (negative bouyancy)
• (Box 6.1.1, ELR 3)

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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)
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• 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)

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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)
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• Heating/cooling the lower atmosphere changes the
  ELR

A diurnal profile of the ELR
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• Advection of Cold/Warm Air at Different Levels
  can change the ELR

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• Advection of an Air Mass with a Different ELR

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• 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
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Profile of a frontal inversion
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Profile of a subsidence inversion
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Cloud types are based on appearance and/or height
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Cirrus with fall streaks
Cirrus clouds are composed entirely of ice
crystals
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Altocumulus
Alto level clouds are composed of both water
droplets and ice crystals
Stratus
Low clouds are composed entirely of water droplets
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Stratocumulus
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• Clouds with vertical development
• Cumulus
• Cumulus humilis, cumulus congestus, cumulonimbus

Cumulus humilis
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Cumulus congestus
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Formation of fair weather cumulus
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Cumulonimbus
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• Unusual clouds
• Lenticular
• Banner clouds
• Mammatus
• Nacreous clouds (mother of pearl)
• Noctilucent clouds

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Lenticular
Banner cloud
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Nacreous
Noctilucent
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• Visible image
• Infrared image
• Color-enhanced infrared