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The scale of dBZ values can be seen along the bottom of the image. Decibel relative to Z, or dBZ, is a logarithmic dimensionless technical unit used in radar. It is mostly used in weather radar, to compare the equivalent reflectivity factor (Z) of a remote object (in mm 6 per m 3) to the return of a droplet of rain with a diameter of 1 mm (1 mm 6 per m 3). [1]
When classified according to the rate of precipitation, rain can be divided into categories. Light rain describes rainfall which falls at a rate of between a trace and 2.5 millimetres (0.098 in) per hour. Moderate rain describes rainfall with a precipitation rate of between 2.6 millimetres (0.10 in) and 7.6 millimetres (0.30 in) per hour.
Drizzle is a light precipitation which consists of liquid water drops that are smaller than those of rain – generally smaller than 0.5 mm (0.02 in) in diameter. [1] Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from drizzle are on the order of a millimetre (0.04 in) per day or less at the ...
Find the depth of rainfall from a storm of duration 6 hours and return period 10 years on a catchment of 5 km 2 in Sheffield. From the FSR maps, the M5-60 minutes rainfall is 20.5mm, and "r" = 0.4. Divide 20.5mm by 0.4 to get 51.3mm, which is the M5-2 days rainfall depth. Factor Z1 = 0.64, so multiply 51.3mm by 0.64 to get 32.8mm.
= 4.1 R-0.21 mm −1 (equivalent to 41 R-0.21 cm −1 in the reference [4]), R being the rainrate in stratiform precipitation in millimeters per hour; D = raindrop diameter in mm; The units of N 0 are sometimes simplified to cm −4 but this removes the information that this value is calculated per cubic meter of air.
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They use three levels of warning. The amber warning indicates that a rainfall intensity of 30 millimetres (1.2 in) per hour is expected. The red warning indicates rainfall amounts of 50 millimetres (2.0 in) per hour are anticipated. The black warning indicates that rainfall rates of 70 millimetres (2.8 in) are possible. [19]