Total number of maximum daily high temperatures recorded in the United States between 1 January 2000 and 30 September 2009, according to NOAA's National Climatic Data Center (NCDC). Only about half as many record lows were recorded in the same period-142,420, to be exact. That 2-to-1 ratio reflects overall warming in the United States compared to the previous five decades, according to a study led by NCAR's Gerald Meehl that was published in Geophysical Research Letters on 1 December. With colleagues from Climate Central, The Weather Channel, and NCDC, Meehl found that the expected drop in record highs and lows over time was far steeper for lows than for highs. As noted in an NCAR news release, simulations with the Community Climate System Model hint that the 2-to-1 ratio this past decade could soar to as high as 50-to-1 by century's end.

Below: This graphic shows the ratio of record daily highs to record daily lows observed at about 1,800 weather stations in the 48 contiguous United States from January 1950 through September 2009. Each bar shows the proportion of record highs (red) to record lows (blue) for each decade. The 1960s and 1970s saw slightly more record daily lows than highs, but in the last 30 years record highs have increasingly predominated, with the ratio now about two-to-one for the 48 states as a whole. (Graphic by Mike Shibao.)

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