North American Constant Pressure Charts

North American Constant Pressure Charts is a product that is available from the National Climatic Data Center (NCDC). The geographic coverage is the Northern Hemisphere and North America. These analyzed constant pressure charts are prepared by the National Weather Service, National Centers for Environmental Prediction as part of their routine analyses and forecasting procedures. Data are constant pressure charts as follows: 1. January 1, 1946 through December 31, 1949 at 04 and 16 GMT for pressure levels 1000, 850, 700, 500, 300, and 200 millibars. 2. January 1, 1950 through May 31, 1957 at 03 and 15 GMT for pressure levels 1000, 850, 700, 500, 300, 200, and 150 millibars. 3. June 1, 1957 through December 31, 1966 for 00 and 12 GMT for 850, 700, 500, 300, 200, and 150 millibars. 4. January 1, 1967 through the present for 00 and 12 GMT for 850, 700, 500, 300, and 200 millibars, and 12 GMT only for 100, 50, 30, and 10 millibars. 4 and 1 millibars are sometimes included for 12 GMT. Analyzed heights (decameters) of each pressure level for each chart are drawn as solid lines at 60-meter intervals. Isotachs at 20-knot intervals are drawn as dashed lines. High and low pressure centers, troughs, ridges, and sometimes jet streams are depicted. Station observational data with wind-barbs are also plotted.