pdfclouds fog precip

7/25/2019 PdfClouds Fog Precip

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Clouds, Fog, & Precipitation

Clouds and fog are physically the samejust location is different

Fog is considered a stratus cloud at or near the surface

What does one see when looking at clouds or fog?

Liquid droplets or ice crystals! Water vapor is an invisible gas.

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3 ingredients to make clouds or fog:

1. Water vapor - evaporated from earth's surface into air.2. A cooling mechanismto condense the water vapor into liquid or ice. Rising airis the

cooling mechanism to create clouds. The rising air cools due to the adiabatic process.

Fog is formed byfive differentcooling mechanisms. The name of the fog relates to that

specific cooling mechanism.

3. Condensation nucleisuch as smoke, pollen, dust, pollution, etc. that provides a surface

for vapor to condense onto.

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As water vapor is cooled it does not have enough energy to remain in the gas phase

When the air temperature (T) equals the dew point temperature (TD), water vapor

condenses into liquid droplets

When T = TD aloft clouds form. When T = TD at the surface fog forms.

physicalgeography.net
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Collision-Coalescence occurs in clouds when larger liquid cloud

droplets fall through the cloud and collide with smaller

droplets that are either suspended in the cloud or falling more

slowly than the larger droplets

Once the collision occurs, the large and small droplets coalesce(join) to create a new larger droplet.

This larger droplet then repeats the process as it falls even

faster through the cloud.

The end result is an extremely large droplet called a raindropleaving the cloud at the bottom.

vsc.edu


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Bergeron Processoccurs when a cloud is supercooled.

A supercooled cloud is one that contains both liquid droplets and also ice crystals.

The cloud temperature must be at or below freezing in order for this process to

occur.

Lutgens & Tarbuck
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Bergeron process occurs because water vapor molecules "prefer" to deposit onto anice crystal (deposition) rather than to condense into a liquid droplet.

Therefore, when water leaves a liquid droplet in the form of vapor, that vapor will

deposit itself onto a nearby ice crystal.

Once water vapor begins to deposit onto the ice crystal there are two results: the ice

crystals growand cloud droplets evaporate (thus getting smaller). When the ice

crystal leaves the cloud it will be large enough to be called a snowflake.

Lutgens & Tarbuck


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It is easier for water vapor to deposit onto an ice crystal (deposition) than it is for

water to leave an ice crystal (sublimation).

Therefore, the ice crystal will accept water vapor molecules at a faster rate than it

will give them awaycrystal grows over time.

Lutgens & Tarbuck


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Rain- must be liquid before hitting the surface and must not freeze onto surface. Therefore, the

surface temperatures must be above freezing and the air above the ground must be above

freezing.

Snow- must begin as a snowflake (Bergeron) and remain a snowflake until reaching the ground. Ifflake melts into a drop it can never return to flake form again. Therefore, the cloud and all of the air

beneath the cloud must be below freezing.

Freezing Rain- rain that hits a below-freezing surface to form a glaze. The surface must be at or

below freezing while the air above the surface must be warmer than freezing.

Sleet- rain that freezes in the air and hits the ground as a small pellet. Typically, there is a warm

layer of air aloft that keeps precipitation as rain but the layer of air above the surface is below

freezing. When rain falls into that below-freezing layer of air, it turns to an ice pellet.
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Clouds are classified by altitude & appearance.

There are four main groups of clouds: high, middle, low,and vertically-developed.


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Clouds can easily be identified by knowing these five cloud name fragments:

cirr- high, ice, all white, very thinalto- middle, ice and/or liquid, white and grey, thin

cumu- puffy appearance, all vertically-developed clouds begin with "cumu"

nimb- cloud must be precipitating

strat- cloud is spread out, most likely covers the sky

Important:The terms above are NOT actual cloud types. One must combine these

fragments to form an actual cloud type. For example, cirrostratuscombines cirr- and

strat-. The means a cirrostratus cloud is high, all white, made of ice, very thin, and is

spread across the sky.

Often the best way to identify a cloud is to use process of elimination. Start with

color (is it all white [high], white and gray [middle], or gray [low]?) Then decide if it is

vertically developed or not. Then consider if it has any "puffiness" (cumu) to the

shape or if it is more flat and spread across the sky (strat).


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Fog is denoted by the cooling mechanism involved

There are five main types of fog: Radiation Fog

Advection Fog

Steam Fog

Upslope Fog

Frontal Fog

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Advection Fog- occurs when warm, humid air is cooled to its dew point by coming

into contact with a cooler surface below. Snow, ice, and cold water are common

surfaces that cause advection fog.

williamsclass.com


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Steam Fog- occurs when cooler air rests above warmer water and vapor that

evaporates into the air cools to its dew point. Sometimes calledArctic sea smoke.

lsc.edu


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Upslope Fog-occurs when air is forced to rise up a large slope and cools

(adiabatically) to its dewpoint. Common in mountainous states.

williamsclass.com


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Frontal Fog- also knows asprecipitation fogand occurs when rain drops fall into

unsaturated, cooler air below. As the drops evaporate, water vapor is introduced

into the cooler air. Very quickly, the vapor condenses into a small fog droplet.

charliesweatherforecasts.blogspot.com

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