When it was warm for a region, temperatures tended to be record or near-record warm. has not been consistent, with locations that were experiencing warm or cool conditions changing nearly every month. Throughout much of 2017, the general weather pattern across the contiguous U.S. Let’s first look at the regional and seasonal differences in temperature and precipitation across the country in 2017. Animation by NOAA, based on originals by Jake Crouch, NCEI. It's more common for a very warm year to result from more persistent warmth-and eventually dryness-over a consistent area. Notice how the very warm areas contributing to this year's near-record warmth shift around from place to place. Monthly average temperatures (left) and percent of average precipitation (right) compared to the twentieth-century average for January-July 2017. A couple years of note are highlighted, showing how the relationship between these two variables in the past have generally followed the rule of warm-and-dry and cool-and-wet, with 2017 clearly an outlier. Plotting the data for all 123 January-July periods on record, the relationship between the national temperature rank and the percent area of U.S. Among the 10 warmest January-July periods, only 2017 had less than 10 percent of the nation experiencing very dry conditions. While the national precipitation rank isn’t as clearly correlated with the seven-month temperature, the percent area of dryness is. The table shows the ten warmest January-July periods for the Lower 48, the precipitation rank for that seven-month period, and the July percent area of the nation moderately to extremely dry. Connection between national temperature ranks & the extent of "very dry" areas The percent area statistic is based on the Palmer Drought Index, which takes into account accumulated dryness over the past several months and is a good gauge of longer-term dry conditions. Using this percent area statistics helps to normalize the variability between regions. Using the percent area statistic in this comparison instead of actual rainfall shortfalls helps to take into account each location’s climate.įor example, if the Midwest usually receives a lot of precipitation in the summer and a particular year is wetter than average for the region, it will have a different impact on national-scale statistics compared to the desert areas of the West having a relatively wet summer compared to its own history. experiencing moderate to extremely dry conditions to the nationally-averaged temperature and temperature rank. One way to look at the impacts of dryness on national temperature outcomes is to compare the percent area of the contiguous U.S. Testing the rule: examining warm and dry years on national scale During the warm season, the sun’s high angle in the sky brings more direct sunlight, and precipitation and cloud cover-or their absence-have larger impacts on temperatures. The relationship between precipitation and temperature holds true for most times of the year, but the biggest influence tends to be during the warmer parts of the year. Also, below-average precipitation usually means less cloud cover, which allows more of the sun’s energy to make it to the ground. The additional warmth will also speed up evaporation, causing the ground to dry out even more. That will cause temperatures to be warmer compared to when the ground is wet. When there is below-average precipitation, and the ground is dry, more of the sun’s energy warms the ground and the air. Also, above-average precipitation usually means more clouds, which prevents the sun’s energy from even reaching the ground, and that also keeps temperatures cooler. When the sun’s energy hits wet ground, a portion of it evaporates the water instead of warming the ground and the air, keeping temperatures lower. When there is above-average precipitation, the ground holds more water. To understand why warm years tend to be dry, we have to examine how the sun’s energy interacts with water that is in or on the ground. This Beyond the Data post will examine why 2017 has been so warm and wet while in the past most of the warm years have tended to be dry for large parts of the country.
It is typically the dry years that are unusually warm, while in the past, wet years were generally cooler. This warm-wet pattern is somewhat contrary to the general rule of thumb about how temperature and precipitation go together in the contiguous United States. Temperature and precipitation ranks for 2017 to date Year-to-date (January-July 2017) temperature and precipitation ranks for the contiguous United States.
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