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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.
= 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.
Environment Canada reports a chance of precipitation (COP) that is defined as "The chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any random point of the forecast region during the forecast period." [7] The values are rounded to 10% increments, but are never rounded to 50%. [8]
An intensity-duration-frequency curve (IDF curve) is a mathematical function that relates the intensity of an event (e.g. rainfall) with its duration and frequency of occurrence. [1] Frequency is the inverse of the probability of occurrence. These curves are commonly used in hydrology for flood forecasting and civil engineering for urban ...
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]
The theoretical return period between occurrences is the inverse of the average frequency of occurrence. For example, a 10-year flood has a 1/10 = 0.1 or 10% chance of being exceeded in any one year and a 50-year flood has a 0.02 or 2% chance of being exceeded in any one year.
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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]