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About This Map
Precipitation is the total liquid content of rain, snow, sleet (ice pellets), and freezing rain which falls out of the atmosphere in a given time period. Precipitation rate is the rate at which precipitation is falling at a given moment. On our weather maps, precipitation is given in millimeters or inches, and precipitation rate is given in millimeters per hour or inches per hour. Mean sea level pressure is the atmospheric pressure adjusted for altitude at a given moment. When weather reports mention surface pressure or barometric pressure, they are usually referring to mean sea level pressure. Mean sea level pressure anomaly is the departure of the mean sea level pressure from average. On our weather maps, mean sea level pressure is given in millibars or inches of mercury. Thickness is the difference between the geopotential heights at two levels of the atmosphere at a given moment. The black line on each map denotes roughly where the freezing line is for that section of the atmosphere. Thickness anomaly is the departure of the thickness from average. On our weather maps, thickness and thickness anomaly are given in decameters.