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By Terry Nathans he weather and climate of the trans-Mississippi west was virtually unknown at the begin- Tning of the nineteenth century. This changed dramatically shortly after the Louisiana P- chase was signed in 1803, which set the stage for acquiring the first systematic weather measurements of the trans-Mississippi west. The framework for obtaining these measurements was outlined in the now famous June 20, 1803 letter from President Thomas Jefferson to his protégé and personal secretary, Captain Meriwether Lewis. In that letter, Jefferson instructed Lewis to plan and carry out an overland expedition to the Pacific Ocean for the purposes of commerce, and to observe and record a broad range of natural history subjects, including the …climate, as characterised by the thermometer, by the proportion of rainy, cloudy & clear days, by lightning, hail, snow, ice, by the access & recess of frost, by the winds prevailing at different s- sons, the dates at which particular plants put forth or lose their flower, or leaf… (Jackson 1978, p. 63). Jefferson’s instructions to Lewis, which were part of his decades-long ambition of laun- ing an expedition to explore the interior of North America, were made at the threshold of what Fleming (1990) has called the “expanding horizons” in meteorology. During this period, more reliable meteorological instruments began to emerge allowing for a more comprehensive and systematic acquisition of weather data.