After averaging over a particular month or season of interest, the coldest 10 percent of years are considered “unusually cold” and the warmest 10 percent are “unusually hot.” For example, if last year’s summer highs were the 10 th warmest on record for a particular location with more than 100 years of data, that year’s summer highs would be considered unusually warm. At each station, the recorded highs and lows are compared with the full set of historical records. These graphs are based on daily maximum temperatures, which usually occur during the day, and daily minimum temperatures, which usually occur at night. This method ensures that the results are not biased toward regions that happen to have many stations close together.įigures 1 and 2 show trends in the percentage of the country’s area experiencing unusually hot temperatures in the summer and unusually cold temperatures in the winter. National patterns can be determined by dividing the country into a grid and examining the data for one station in each cell of the grid. The data come from thousands of weather stations across the United States. Changes in record high and low temperatures (Figure 5).The change in the number of days with unusually hot and cold temperatures at individual weather stations (Figures 3 and 4).Unusually hot summer temperatures and cold winter temperatures nationwide (Figures 1 and 2).This indicator examines trends in unusual temperatures from several perspectives: 3 This change will provide less opportunity to cool off and recover from daytime heat. In most locations, scientists expect daily minimum temperatures-which typically occur at night-to become warmer at a faster rate than daily maximum temperatures. Conversely, cold spells are expected to decrease. 2Higher heat index values (which combine temperature and humidity to describe perceived temperature) are expected to increase discomfort and aggravate health issues. As the Earth’s climate warms overall, however, heat waves are expected to become more frequent and more intense. Record-setting daily temperatures, heat waves, and cold spells are a natural part of day-to-day variation in weather. Extreme heat can lead to power outages as heavy demands for air conditioning strain the power grid, while extremely cold weather increases the need for heating fuel. In addition, prolonged exposure to excessive heat and cold can damage crops and injure or kill livestock. 1 People can also die from exposure to extreme cold (hypothermia) (see the Cold-Related Deaths indicator). Heat waves can lead to illness and death, particularly among older adults, the very young, and other vulnerable populations (see the Heat-Related Deaths and Heat-Related Illnesses indicators). Unusually hot or cold temperatures can result in prolonged extreme weather events like summer heat waves (see the Heat Waves indicator) or winter cold spells. The decade from 2000 to 2009 had twice as many record highs as record lows. Since the 1970s, however, record-setting daily high temperatures have become more common than record lows across the United States (see Figure 5). If the climate were completely stable, one might expect to see highs and lows each accounting for about 50 percent of the records set.The number of unusually cold days has generally decreased throughout the country, particularly in the western United States (see Figure 4). Based on this way of looking at hot days, unusually high temperatures have increased in the western United States and in several areas along the Gulf and Atlantic coasts but decreased in much of the middle of the country (see Figure 3). The two maps show where changes in the number of days with unusually hot (above the 95 th percentile) and cold (below the 5 th percentile) days have occurred since 1948.Since the 1980s, though, unusually cold winter temperatures have become less common-particularly very cold nights (lows). The 20 th century had many winters with widespread patterns of unusually low temperatures, including a particularly large spike in the late 1970s (see Figure 2).This trend indicates less “cooling off” at night. The occurrence of unusually hot summer nights (lows) has increased at an even faster rate. Nationwide, unusually hot summer days (highs) have become more common over the last few decades (see Figure 1).
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