Because of the low temperatures in the Arctic, especially during winter, the amount of moisture in the atmosphere is quite limited. Near the surface, mean specific humidity (the mass of water vapor per unit mass of air, including the water vapor) for winter as averaged for the region north of 70° N is only about 1 g kg-1 compared to about 3-4 g kg-1 in summer. The total water column vapor (or precipitable water, the equivalent liquid depth of water vapor in the atmospheric column) between the surface and 300 hPa, averaged for 70-90° N, ranges from about 2.5 mm in January to 14 mm in July (Serreze et al., 1995a). This compares to a global mean annual average of about 25 mm. About 80% of the total water vapor in all months is concentrated between the surface and 700 hPa and 95% of the total is below 500 hPa. During the summer half-year, the spatial pattern of precipitable water is essentially zonal, whereas in the winter half-year there is greater asymmetry. Amounts are least over the Canadian sector and largest over the Norwegian Sea as a result of patterns of evaporation, the planetary wave structure and associated vapor transports by synoptic-scale eddies. On average, the strongest moisture transports into the Arctic tend to be found near the prime meridian in association with the North Atlantic cyclone track (Chapter 6). As shown by Gyakum (2000), moisture transports associated with significant precipitation events over the Arctic drainage basins are often organized into well defined "tropospheric rivers" (Zhu and Newell, 1998).
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