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Page 1: 1. Effect of precipitation on visibility and cloud base

1. Effect of precipitation on visibility and cloud base

Page 2: 1. Effect of precipitation on visibility and cloud base

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Depth of cloud required for precipitation

UKMO Technique

Convective and non-convective clouds

Cloud top temperature not considered

Page 3: 1. Effect of precipitation on visibility and cloud base

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Effect of Precipitation on Visibility

LiquidVisibility is inversely proportional to:

− Amount of water− Number of drops

Lowest visibility in drizzle & heavy rain

SolidRapid fall as intensity increasesModerate snow, visibility usually < 1500mHeavy snow, visibility < 500m

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Effect of Precipitation on cloud base - RAIN

Patchy stratus forms during prolonged moderate rain

Rarely 8 oktas

Rough guide− 2hrs continuous rain - base 800 feet− 3hrs continuous rain - base 400 feet

No allowance for other processes

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Effect of Precipitation on cloud base - SNOW

LOW LEVEL AIR TEMPERATURE BELOW 0°C

8 oktas stratus commonly forms during prolonged moderate snow

Rough guide− 2hrs continuous snow - base 400 feet− 3hrs continuous snow - base 200 feet

Heavy snow – Sky obscured.

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Effect of precipitation on cloud base - SNOW

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Melting starts as 0°C isotherm is reached

Snow falls into the lower levels of this atmosphere

Snow falling into a layer with an above freezing temperature melts and may evaporate if layer is unsaturated

Large amounts of latent heat required

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LOW LEVEL AIR TEMPERATURE ABOVE 0°C

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Effect of precipitation on cloud base - SNOW

Cooling occurs as snow melts just below this level

Temperature profile changes

Profile starts to follow the 0°C isotherm down toward the surface

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Effect of precipitation on cloud base - SNOW

If this process continues the air can become saturated and cloud can form beneath the layer cloud

Lowest levels become unstable and some thin cumulus cloud can also form. This is known as Fractocumulus

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Effect of precipitation on cloud base - SNOW

Rough guide− 1hr continuous snow -

600 feet of isothermal− 4hrs continuous snow

- 1200 feet of isothermal

So rain can turn to snow

Haze

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