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Clouds
Water is strange stuff!
• Gas - water vapour (invisible)
• Liquid - water droplets (visible)
• Solid - ice crystals, hail, snow
Water can occur in 3 states:
What are clouds?
• Clouds are formed of tiny droplets of water or ice.
• Clouds form when water vapour cools and condenses.
• The temperature at which condensation occurs is called dew point.
• Condensation also requires nucleii (small particles) such as dust.
What happens when air is cooled?
• Relative humidity increases.• Dew point is reached (100% relative
humidity).• Water vapour (invisible) condenses to
form water droplets (visible).• Sometimes the water vapor may
change straight into solid state: called Deposition.
• Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy.
◆Localized Convective Lifting
◆ Convergence• Convergence is when air flows together and rises.
Processes that lift air
Cloud Formation
Orographic Lifting
• Orographic lifting occurs when mountains act as barriers to the flow of air, forcing the air to ascend. The air cools adiabatically; clouds and precipitation may result.
Cloud Formation
• When air is allowed to expand, it cools, and when it is compressed, it warms.
◆ Adiabatic Temperature Changes
• Dry adiabatic rate is the rate of cooling or heating that applies only to unsaturated air.
◆ Expansion and Cooling
• Wet adiabatic rate is the rate of adiabatic temperature change in saturated air.
Air Compression and Expansion
Cloud Formation by Adiabatic Cooling
Cloud Formation
• A front is the boundary between two adjoining air masses having contrasting characteristics.
◆ Frontal Wedging
Processes That Lift Air
Cloud Types and Precipitation
◆ Clouds are classified on the basis of their form and height.• Cirrus (cirrus = curl of hair) are clouds that are
high, white, and thin.• Cumulus (cumulus = a pile) are clouds that
consist of rounded individual cloud masses.• Stratus (stratus = a layer) are clouds best
described as sheets or layers that cover much or all of the sky.
Types of Clouds
Clouds can be classified by their shape and height
Flat clouds eg Stratus (St)
Fluffy cloudseg Cumulus (Cu)
Wispy cloudseg Cirrus (Ci)
Cloud shapes
Clouds can also be classified by height of cloudbase
Low level (below 2000m) eg stratus
Middle level - alto (2,000 to 6, 000m) eg alto-cumulus
High level - cirro (above 6,000m) eg cirrus
Cloud heights
Cloud Classification
Spot the cloud!
Low and flat …..
Stratus
Spot the cloud!Middle level, fluffy …..
Alto -Cumulus
Spot the cloud!
High level, wispy …..
Cirrus
Spot the cloud!Rain bearing, storm clouds …..
Cumulonimbus
World cloud patterns
Can you explain the major world cloud belts?