Clouds and Rain Formation

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Rain is a very important climate element which affects plant growth and development especially in areas that rely on rain for crop production. This is highly varied with time and location.

For example, it may rain in a particular area, while it may not rain in an area nearby. If a particular area has a certain amount of rainfall, the other area may have more rainfall.

This is understandable because the rain is formed from water evaporation which condenses and becomes a cloud that always moves, depends on where the wind blows (TOT CFS' Hand note).

Basic Question

Why does the air rise on some occasions and not on others?
Why does the size and shape of clouds vary so much when the air rises?
(Ahrens, "Meteorology") alert-info

The goals of this topic are  that we should be able to learn and understand :

• Clouds formation process and role of condensation nuclei
• Evaporation, condensation, precipitation, water cycle

Cloud Formation and Dissipation Processes

You know, we can find water in the atmosphere in 3 phases, solid, liquid and gas. Note, only water ! The basic processes how the water changing its form,  evaporation and condensation.

Evaporation is the changing of water from liquid to gas. By this process liquid water on earth entering the atmosphere as water vapour. Evaporation is caused by 2 factors, heat and wind force.

While condensation is the opposite of the evaporation process, condensation is the changing of water from gas to liquid. Water vapour in the atmosphere condensing into liquid water droplets in a cloud (when energy is released = cooled).

When the water reaches the dew point, or saturation point, it will begin to condense into water droplets that form a cloud.

Dew points represent the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur.

The dew point is a good indicator of the air’s actual water vapour content. High dew point indicates high water vapour content; low dew points, low water vapor content. Addition of water vapour to the air increases the dew point; removing water vapour lowers it.

How can the water reach its dew point ? There are for ways:

• Contacting a colder surface: 
• similar with cold water in a glass.
• the warmer air in the outside of the glass condenses in outside of the wall of the glass.

• When air releases its heat, it starts to cool until its dew point. The air starts to condense.

• Mixing with colder air
• when the cold air flows and passes the warm air. then they mix together. the warm air will be cool. If it reaches its dew point, it will condense.

• Expanding when it rises
• when the air is warmer than its surroundings, it will expand.
• its density will decrease and the air starts to rise.
• when air rises, its temperature decreases.

Before a true cloud can form without the condensation nuclei, the water vapour is difficult to condense. it is still in a super cool liquid water form.

Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals.

The changing of water vapour into water droplets is helped by tiny particles floating around in the air - such as salt and dust - these are called aerosols.

We know that the air expands when it rises. So how does the air rise? There are four processes cause the air to rise, they are:

How the air rises

Convection

• Convection is associated with surface heating of the air at the ground surface. 
• If enough heating occurs, the mass of air becomes warmer and less dense (lighter) than the surrounding environment air, and just like a hot air balloon, it begins to rise, expand, and cool. 
• When sufficient cooling has taken place, the saturation occurs forming clouds.
• This process is active in the interior of continents and near the equator forming cumulus and or cumulonimbus (thunderstorms) clouds. 
• The rain that is associated with the development of thunderstorm clouds is delivered in large amounts over short periods of time in extremely localized areas.

Orographic Lift

• when moist air is forced to rise over a mountain area.
• As the air parcel rises it cools and condenses to form water droplets, raindrops, and eventually rainfall.

Convergence

• Streams of air flow from different directions are forced to rise where they meet together or converge. 
• When air is forced to converge or meet each other (such as at the center of a low-pressure system), it can only go upward (can’t go downward). An example would be the air masses flow to the center of the low-pressure area is forced to rise. This can cause cumulus cloud and showery conditions. 

Front

• Clouds are formed when a mass of warm air rises up over a mass of cold, dense air over large areas along fronts.
• A 'front' is the boundary between warm, moist air and cooler, drier air. 
• In most cases, the two air masses have different temperature and moisture characteristics. 
• One of the air masses is usually warm and moist, while the other is cold and dry. 
• The leading edge of the cold and dry air mass acts as an inclined wall or front causing the moist warm air to be lifted. 
• The lifting then causes the warm moist air mass to cool due to expansion resulting in saturation.

Atmospheric stability

Atmospheric stability is a measure of the atmosphere’s susceptibility to vertical motion. In a stable atmosphere, a lifted parcel of air will be colder (heavier) than its surrounding air. Because of this fact, the lifted parcel will tend to sink back to its original position.

In an unstable atmosphere, a lifted parcel of air will be warmer (lighter) than the air surrounding it, and thus will continue to rise upward, away from its original position. Cloud are more likely to form in unstable condition.

Cloud Type

Cloud type (Source : Pearson Education, Inc)

Cloud types, Based on their height are:

• High clouds – above 6,000 meters: cirrus, cirrostratus, Cirrocumulus

• Middle clouds – 2,000 to 6,000 meters: altostratus and altocumulus

• Low clouds – below 2,000 meters: stratus, stratocumulus, and nimbostratus (nimbus means “rainy”)

• Clouds of vertical development: From low to high altitudes
• Called cumulonimbus
• Often produce rain showers andthunderstorms

Formation of water vapours to be water droplets (rain)

How do droplets grow and become raindrops ? It all begins with clouds.

Clouds are composed of tiny water droplets from condensation on Cloud Condensation Nuclei. Initial growth is triggered by condensation, but this is limited by diffusion.

They never get a chance to grow into raindrops by condensation alone – this process would take D A Y S.

There are 2 main processes the droplets grow and become raindrops:

• In  ‘warm’ clouds
• Collision
• When cloud droplets collide with each other
• Falling droplet bumped tiny droplet and form larger droplet
• It takes about 106 small cloud droplets (10 mm) to form one large raindrop (1000 mm)

• Coalescence 
• droplet with the same size merge and stick together 
• Coalescence efficiency is assumed to be near 100% (all drops stick together if they collide)

• In ‘cold’ clouds
• Bergeron process occurs in Cold Cloud also known as the cold rain or ice crystal process
• Temperature in the cloud is below freezing
• Ice crystals collect water vapor
• Large snowflakes form and fall to the ground or melt during descent and fall as rain

Precipitation

The falling of any form of water from the air to Earth’s surface called as Precipitation. When will water leave a cloud and fall to the ground as precipitation?

- Basically, when gravity tells it to. 

- When cloud droplets grow into drops heavy enough to fall to Earth, precipitation occurs.

Four major types of precipitation, are:

• Rain
• Large droplets (up to 0.25 cm in diameter) that are far apart and fall rapidly

• Snow
• Clumps of 6-sided crystals that grow by collision
• The most common type of solid ppt

• Sleet
• Ice pellets that form when rain falls through a layer of freezing air
• What about “black ice” or “glare ice”? Sometimes in an ice storm, supercooled rain drops will freeze instantly as they come in contact with a cold surface such as roads, roofs, and power lines

• Hail
• Solid ppt in the form of lumps of ice
• Begins as raindrops falling from a cumulonimbus cloud
• Convection currents toss the droplet high up into the cloud where it freezes. It then falls to a lower level and water condenses on it as a liquid. The tossing and freezing process repeats itself and the hailstone keeps adding layers and growing larger.

References about Clouds and Rain Formation:

• Ahrens, Essential of Meteorology
• www.atmo.ttu.edu/bancell/lecnotes/Chapter6.ppt
• www.atmo.ttu.edu/bancell/lecnotes/Chapter7.ppt

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