3 . Take a period of limited rainfall. Add heat. And you have what scientists call a ‘hot drought’ — dry conditions made more intense by the evaporative power of hotter temperatures.

A new study, published in the journal Science Advances, Wednesday, finds that hot droughts have become more common and severe across the western U.S. as a result of human-caused climate change. “The frequency of compound warm and dry summers particularly in the last 20 years is unprecedented,” said Karen King, lead author of the study and an assistant professor at the University of Tennessee in Knoxville.

For much of the last 20 years, western North America has been under the control of a huge drought that has strained crop producers, city planners and water managers. Scientists believe it to be the driest period in the region in at least 1,200 years. They reached that determination, in part, by studying the rings of trees collected from thousands of sites across the Western U.S..

Cross-sections or cores of trees, both living and dead, can offer scientists windows into climate conditions of the past. Dark scars can show where fires have burned. Pale rings can indicate insect outbreaks. “Narrow rings mean less water,” said King, a dendrochronologist, who specialized in tree ring dating. “Fatter rings, more water.” Scientists have looked at tree ring widths to understand how much water was in the soil at a given time. King and fellow researchers did something different. They wanted to investigate the density (密度) of individual rings to get a picture of historical temperatures. In hotter years, trees build thick walls to protect their water.

By combining that temperature data with another tree-ring-sourced dataset looking at soil moisture (湿度), the researchers showed that today’s hotter temperatures have made the current western huge drought different from its past ones.

It also suggests that future droughts will be worsened by higher temperatures, particularly in the Great Plains, home to one of the world’s largest aquifers, and the Colorado River Basin, the source of water for some 40 million people.

10 . How Weather Affects Our Mood

The weather supplies many metaphors (隐喻) for our changeable minds. Moods can brighten and darken, futures can be under a cloud, and relationships can be stormy.    1    , but does it also influence our moods?

Of the many aspects of weather, sunshine is the most closely tied to mood. Although the link is weaker than many people imagine, sunlight has repeatedly been found to increase positive moods and reduce tiredness.    2    . Happy people tend to be more favorable toward one another, and people are more helpful when the sun is shining. One study even found that diners in Minnesota, US, tipped more generously on sunny days.

    3    . A study showed that university admissions officers stressed the academic qualifications of applicants more on cloudy days, and their non-academic qualities more on sunny ones.

    4    . The more it departs (偏离) from an ideal of around 20C, the more discomfort we feel. One study found that rates of helping others decreased as temperatures dropped below or rose above this value.

Indeed, the effects of weather on mood depend on our behavior and on how we think.     5    . A study by US psychology researcher Matthew Keller and his colleagues showed that beneficial effects of warm and sunny conditions on mood were only seen in people who had spent more than 30 minutes outdoors that day. Good weather even had negative effects on mood for people confined (限制) indoors, who perhaps looked enviously (嫉妒地) outside at the solar fun they were missing.