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The probability of precipitation can also be expressed using descriptive terms instead of numerical values. For instance, the NWS might describe a precipitation forecast with terms such as "slight chance" meaning 20% certainty and "scattered" meaning 30–50% areal coverage. [10] The precise meaning of these terms varies. [11]
In areas where imperial units are used (primarily the United States), liquid precipitation (rain and drizzle) is measured in intervals of 0.01 inches (0.25 mm), while snow, ice pellets, and most other precipitation types are measured in intervals of 0.1 inches (2.5 mm). [1]
The scale of dBZ values can be seen along the bottom of the image. 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]
Where a and b are related to the type of precipitation (rain, snow, convective (like in thunderstorms) or stratiform (like from nimbostratus clouds) which have different , K, N 0 and . The best known of this relation is the Marshall-Palmer Z-R relationship which gives a = 200 and b = 1.6. [ 6 ]
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]
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. Heavy rain describes rainfall with a precipitation rate above 7.6 millimetres (0.30 in) per hour, and violent rain has a rate more than 50 millimetres (2.0 in) per hour. [11]
Time of concentration is a concept used in hydrology to measure the response of a watershed to a rain event. It is defined as the time needed for water to flow from the most remote point in a watershed to the watershed outlet. [1] It is a function of the topography, geology, and land use within the watershed.
Rainfall in these regions averages between 300 and 600 millimeters (11.8 and 23.6 in) per year, with lower amounts across Baja California Norte. Average rainfall totals are between 600 and 1,000 millimeters (23.6 and 39.4 in) in most of the major populated areas of the southern altiplano, including Mexico City and Guadalajara.