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Calculating A Drought: A Look At Formulas

UPDATED August 17, 2010

By WeatherBug Meteorologist, Andrew Rosenthal

Dry weather and drought can be a significant risk to communities, putting a strain on farming and water supplies, as well as putting people in danger of forest fires. There is a huge difference between a "dry week" and a drought. Over the years, drought formulas have been developed to help determine the severity of a drought.

Several major drought formulas were developed in the 1960s by U.S. Weather Bureau meteorologist Wayne Palmer. He was aware of the connection between dry weather and its effect on farming, and wanted to create some formulas to determine just how dry it was. These formulas are still in use, and thanks to his work, bear Palmer`s name.

The official index used to determine drought conditions and to issue forecasts is the Palmer Drought Severity Index, or PDSI. As drought tends to occur over long periods of time, the PDSI focuses on long-term dryness. A comparison between rainfall over the last 12 months and normal rainfall rates is used to calculate the PDSI. This gives meteorologists a chance to see where the long-term drought conditions are located, rather than those locations that just had a dry week or two.

The PDSI is based on a "standardized" index, meaning that it determines how the rainfall amounts compare to normal rainfall. It is updated once a week, with every location in the United States given a value ranging from -5 for much drier than normal conditions to +5 for very wet conditions. The midpoint, zero, is used for locations with normal rainfall conditions.

How are these values used to create official products? PDSI values are charted similar to a temperature map. Locations with values less than zero are considered to be under drought conditions. Ranges in values are then given names depending on how low they are, such as "Extreme Drought" (-4 to -5), "Severe Drought" (-3 to -4), and so on. This is the basis for the Drought Report that WeatherBug produces every week.

If you`re interested in how dry it`s been recently, you probably would want to look at the Palmer Crop Moisture Index (CMI). Also created by Wayne Palmer, the CMI can show meteorologists which areas have seen plenty of rain and which have not in recent weeks. The CMI is also updated once a week, farmers can use the index to determine whether their crops have received enough rain that week, or whether the crops need to be irrigated. Similar to the PDSI, the CMI is standardized and ranges from -5 to +5.

Another index used to measure the dryness of the ground is the Keetch-Byrum Drought Index (KBDI). The KBDI was created in the 1960s by John Keetch and George Byrum, two scientists working with forest fire control. The KBDI differs significantly from the PDSI and CMI. It instead focuses on how likely the ground is to burn in a forest fire.

The KBDI is updated every day, starting with the previous day`s index. It is updated using values based on the current day`s high temperature and rainfall. The scale starts with a value of zero when the ground is completely soaked or after the spring snowmelt. The KBDI has no top value, although values above 600 are considered to be very dry, increasing the risk of wildfires greatly. Advantages of the KBDI are that a daily record can easily be kept, and that the value of the KBDI is the amount of rain, in hundredths of an inch, needed to completely soak the ground. In other words, if the KBDI is 100, it would take 1.00 inches to soak the ground.

Other drought indexes also focus on the likelihood of fires. The most prominently used is the Fosberg Index. Fosberg determines the likelihood of a fire to spread. Components of index are humidity near the ground and wind speed. Think of Fosberg as asking if the weather will act like a hair dryer that day.

It ranges from 0, or very wet with calm winds, to 100, very dry with strong winds. Similar to this is the Haines Index. Haines determines the likelihood of a large wildfire to form, using atmospheric conditions well off the ground. Unlike the Fosberg index, the Haines scale runs from 2, very low risk, to 6, very high risk.

There are many ways to determine drought severity, depending on whether the need is for fire risk or crop and hydrological needs.

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