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
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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
0 C
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
Dew point increases slightly0 10
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0 C
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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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0 C
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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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0 C