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Handout
Meteorologie

Kantonschule Baden (Schweiz)

5 (deutsch: 2) , W.Zierhofer, 2018

Olga B. ©
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Loubna D. Tanisha H. B. // 06.01.18

Geography

Clouds

Clouds bring precipitation and are with that important for our climate. Moreover, statements about the state of the atmosphere and about the weather development can be made thanks to their appearance.

  • But although condensation creates clouds, clouds don’t necessarily produce precipitation.

Where insolation meets water, the water heats up and water evaporates constantly. That water vapour ascends into the sky, where it cools to dew point and condenses to water droplets and/or ice crystals. These form clouds as we know them.

Types of clouds

There are many different kinds of clouds. But generally we distinguish them by shape and height.

There are various shapes of clouds. They can be streaky, or puffy like cotton balls, or they can look one big blanket in the sky.

The four main groups (by height) are:

  • low clouds (strato-): (0 - 2 km)

  • middle clouds (alto-): (2 - 7km)

  • high clouds (cirro-): (7 - 13km)

  • clouds with vertical growth: (0 - 13km)

Low clouds include: stratus, (nimbostratus), stratocumulus

Middle clouds include: altocumulus, altostratus, (cumulus)

High clouds include: cirrus, cirrocumulus, cirrostratus, stratocumulus

Clouds with vertical growth include: cumulus, cumulonimbus, nimbostratus


We can also interpret clouds in terms of weather (especially precipitation) prediction.

  • Cirrus no precipitation

  • Cirrocumulus no precipitation

  • Cirrostratus no precipitation

  • Stratocumulus possible drizzle & snow

  • Altocumulus no precipitation

  • Altostratus possible drizzle & snow

  • Cumulus no precipitation

  • Stratus possible drizzle & snow

  • Nimbostratus rain, snow or graupel

  • Cumulonimbus storms / rain-, hail-, graupel- or snow showers

Precipitation

Precipitation is basically just water falling to the ground in various forms especially as rain and snow.

The tiny water droplets that form clouds are being held up in the air by rising air currents. To fall to the ground as precipitation they have to become sufficiently bigger to overcome the uplifting mechanism.

  • The ice crystal mechanism / the Bergeron – Findeisen mechanism:

The temperature of the air is between -5°C and -25°C supercooled water droplets and ice crystals exist at the same time. The water droplets evaporate and then condense again onto snowflakes, which grow in size, either by further condensation or by fusion with other snowflakes.

They also increase in number due to splinters of ice breaking off. If the air temperature rises above freezing point as the snow falls, they melt into raindrops.


  • The collision and coalescence process:

Warm” clouds (i.e. those containing no ice crystals) contain numerous water droplets of different sizes. The water droplets are swept up at different velocities and while doing so they collide with each other or coalesce (fuse) and grow in size.

Different types of precipitation

  • Snow

  • Sleet: (mixture of snow and rain // Schneeregen)

  • Hail: (clumps of ice // Hagel)

  • Graupel: (smaller, softer version of hail // Graupel)

  • Rain:

    • Convergent & cyclonic (frontal) rainfall

Two air streams meet and the warmer, less dense air is forced to move upward over the colder, denser air stream. The uplifted, warm air cools the water, condenses and rain falls. (Often in equatorial places)


Near-saturated, warm maritime air is forced to rise when confronted by a coastal mountain barrier. Mountains reduce the water-holding capacity of the rising air by forcing it to cool down.


    • Convectional rainfall

Look under Thunderstorms

Thunderstorms

Def: a storm with thunder and lightning and typically also heavy rain or hail.

Convectionalrainfall results from the rapid upward movement of warm, moist air due to overheating of the ground. During the upward movement the air remains warmer than the surrounding air by releasing latent heat by condensing, enabling it to rise further as the air cools, and forms a cumulonimbus cloud.

The falling droplets create a downdraught, pulling cold air with it. This cold air spreads out at the earth’s surface, occasionally causing strong winds.

It’s the exact condition for thunderstorms the only thing missing is the lighting and the thunder.

The life of a thunderstorm is divided into three stages:

  1. The cumulus stage

The cumulonimbus cloud forms

  • The cumulonimbus cloud grows vertically like a heap and has a characteristic anvil-shaped, flat top


  1. The mature stage

The downdraft creates a gust front which forces more air into the updraft


  1. The dissipating stage

Rain starts falling into the updraft and that cuts off the rising air (the cycle is interrupted)

The storm eventually dies


Lighting: it is the visible discharge of electricity between several clouds or between cloud and ground

Thunder: it is the sound of the pressure wave created by the heating of the air by a flash




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