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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]
The FSR includes values for two key variables mapped across the UK: the M5-60 minutes rainfall, and the ratio "r". M5-60 minutes rainfall is the expected depth of rainfall in millimetres (mm) from a storm lasting 60 minutes (1 hour) with a return period of 5 years (hence M5).
= 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.
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]
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]
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The measurement day begins with the pan filled to exactly two inches (5 cm) from the pan top. At the end of 24 hours, the amount of water to refill the pan to exactly two inches from its top is measured. If precipitation occurs in the 24-hour period, it is taken into account in calculating the evaporation.