This post presents a one-minute video. (There is also a slowed-down 20-minute version of the same thing at the end of this post.)
The video shows a map of the United States, divided into states. Those states are divided into sections. The colors of those sections change as the video proceeds. The colors indicate whether those areas are getting wetter or dryer.
Feel free to pause the video at any point. Notice key periods, including:
- The Dust Bowl drought years of the 1930s, repeated to some extent in the 1950s
- Extreme conditions during major El Niño and La Niña years
- The relatively long-term tendency toward dryness
- The much longer-term fluctuations
- Worries about the future
The data presented in those colors come from the National Climatic Data Center of the U.S. National Oceanic and Atmospheric Administration (NOAA). NOAA offers explanations of several commonly used drought severity indicators. Among these are the Palmer drought indexes.
The data in this video draw on the Palmer Hydrological Drought Index (PHDI). NOAA explains that a hydrological index addresses wetness or dryness on or in the ground, while a meteorological index covers wetness or dryness in the air and sky. For example, water levels in a nearby lake would be a hydrological concern, while amounts of rainfall would be a meteorological concern.
I chose a hydrological index because amounts of water in the ground are slower to change. There can be torrential rainfalls one month and no rain the next, so it can be difficult to see long-term trends in meteorological data. But in NOAA’s words, “hydrological impacts of drought (e.g., reservoir levels, groundwater levels, etc.) . . . take longer to develop and longer to recover from”; hence, the PHDI “responds more slowly to changing conditions.” (Note: the PHDI has limitations.)
I was interested in a less responsive index because I wanted to see longer-term trends. Those who play with the webpage that produced the accompanying video will see that other Palmer indices can yield information overload: conditions within an area can rapidly switch from being too wet to too dry, and vice versa. In the video, by contrast, you can often see it coming. Sections change color in a more sequential fashion, rarely jumping from wet to dry within the space of a single month.
I prepared this video because I was curious about the extreme droughts that some parts of the U.S. have been experiencing. I hoped that the 100-year video would contain obvious tendencies — toward drought overall, perhaps, or toward extreme events. The more I learned in the process, the more I came to appreciate how complex the climate is. The final problem mainly emphasized things I already knew, notably the extent of drought in recent years.
The following version of the video presents the same data in much slower fashion, over a period of 20 minutes:
You may also be interested in a video about the water level drop at Lake Meredith.