1. Effect of precipitation on visibility and cloud base

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<ul><li><p>1. Effect of precipitation on visibility and cloud base</p></li><li><p> Crown copyright 2004 Page 2</p><p>Depth of cloud required for precipitation</p><p>UKMO Technique</p><p>Convective and non-convective clouds</p><p>Cloud top temperature not considered</p></li><li><p> Crown copyright 2004 Page 3</p><p>Effect of Precipitation on Visibility</p><p>LiquidVisibility is inversely proportional to:</p><p> Amount of water Number of drops</p><p>Lowest visibility in drizzle &amp; heavy rain</p><p>SolidRapid fall as intensity increasesModerate snow, visibility usually &lt; 1500mHeavy snow, visibility &lt; 500m</p></li><li><p> Crown copyright 2004 Page 4</p><p>Effect of Precipitation on cloud base - RAIN</p><p>Patchy stratus forms during prolonged moderate rain</p><p>Rarely 8 oktas</p><p>Rough guide 2hrs continuous rain - base 800 feet 3hrs continuous rain - base 400 feet</p><p>No allowance for other processes</p></li><li><p> Crown copyright 2004 Page 5</p><p>Effect of Precipitation on cloud base - SNOW</p><p>LOW LEVEL AIR TEMPERATURE BELOW 0C</p><p>8 oktas stratus commonly forms during prolonged moderate snow</p><p>Rough guide 2hrs continuous snow - base 400 feet 3hrs continuous snow - base 200 feet</p><p>Heavy snow Sky obscured.</p></li><li><p> Crown copyright 2004 Page 6</p><p>Effect of precipitation on cloud base - SNOW</p><p>0 10</p><p>850</p><p>1000</p><p>2 3 5</p><p>79</p><p>Melting starts as 0C isotherm is reached</p><p>Snow falls into the lower levels of this atmosphere</p><p>Snow falling into a layer with an above freezing temperature melts and may evaporate if layer is unsaturated</p><p>Large amounts of latent heat required</p><p>0 C</p><p>LOW LEVEL AIR TEMPERATURE ABOVE 0C</p></li><li><p> Crown copyright 2004 Page 7</p><p>Effect of precipitation on cloud base - SNOW</p><p>Cooling occurs as snow melts just below this level</p><p>Temperature profile changes</p><p>Profile starts to follow the 0C isotherm down toward the surface</p><p>Dew point increases slightly0 10</p><p>850</p><p>1000</p><p>2 3 5</p><p>79</p><p>0 C</p></li><li><p> Crown copyright 2004 Page 8</p><p>Effect of precipitation on cloud base - SNOW</p><p>If this process continues the air can become saturated and cloud can form beneath the layer cloud</p><p>Lowest levels become unstable and some thin cumulus cloud can also form. This is known as Fractocumulus</p><p>0 10</p><p>850</p><p>1000</p><p>2 3 5</p><p>79</p><p>0 C</p></li><li><p> Crown copyright 2004 Page 9</p><p>Effect of precipitation on cloud base - SNOW</p><p>Rough guide 1hr continuous snow -</p><p>600 feet of isothermal 4hrs continuous snow </p><p>- 1200 feet of isothermal</p><p>So rain can turn to snow</p><p>Haze</p><p>0 10</p><p>850</p><p>1000</p><p>2 3 5</p><p>79</p><p>0 C</p><p>1. Effect of precipitation on visibility and cloud baseEffect of Precipitation on VisibilityEffect of Precipitation on cloud base - RAINEffect of Precipitation on cloud base - SNOWEffect of precipitation on cloud base - SNOWEffect of precipitation on cloud base - SNOWEffect of precipitation on cloud base - SNOWEffect of precipitation on cloud base - SNOW</p></li></ul>

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