The major parameters in this file are sequential "Time Biased Corrected" state climatic division monthly Average Temperatures (Deg.F. to 10ths), Precipitation (Inches to 100ths), and Palmer Drought Indices (PDSI, PHDI, PMDI, and ZNDX). Period of record is 1895 through latest month available, updated monthly.
Monthly averages within a climatic division have been calculated by giving equal weight to stations reporting both temperature and precipitation within a division. In the U.S., observers at cooperative stations often take one observation per day, and the ending time of the climatological day at any station can vary from station-to-station as well as year-to-year. Differences of the 24-hour period over which each observer reports his or her maximum and minimum temperature as well as the average temperature [(max + min)/2] affects the calculated monthly mean temperature. Karl, et al. (1986), describe the biases that this introduces. These potential biases were rectified by adjusting for these varying observation times. The model described by Karl, et al. (1986), was used to adjust the climate division averages such that all stations end their climatological day at midnight; i.e., climatological day coincides with calendar day. The time of observation was determined at each station within a climate division during January of the years 1931, 1941, 1951, 1965, 1975, and 1984 for the states of California, Colorado, Illinois, Indiana, New York, North Carolina, and Washington. The fraction of observers recording at various hours of the day was calculated and interpolated for intervening years (extrapolated for subsequent years). For these seven states, the ending time of observation was grouped into three categories: AM, PM, and MD. The AM category included observers who ended their climatological day between 3 AM and 11AM; the PM category between noon and 9 PM; and the MD category between 10 PM and 2 AM; all local standard time. The fraction of observers in these categories was calculated, and it was assumed the 7 AM observation time best represented the AM category; the 5 PM observation time, the PM category; and midnight for the MD category. The reason for the simplification was to test if a faster method, requiring significantly less bookkeeping and keypunching, could not provide nearly as good results as calculating the fraction of observers at each of the 24 hours of the day.
The time of observation bias model was run by using the latitude and longitude of each of the centroids of the climate divisions. The output from the model was the time of observation bias, with respect to a midnight-to-midnight climatological day, for each of the possible ending hours of the climatological day. Each climate division's monthly average was then adjusted by weighting the bias at any given hour by the fraction of stations within the climate division observing at that hour, and subtracting the result for the reported monthly mean temperature.
Differences of the biases were small (< 0.3 Deg. F.) for those calculated by categorizing the ending time of observation into three categories compared to those obtained from calculating the fraction of stations with observation times at each of the 24 hours of the day. This is attributed to the preponderance of AM observation times falling between 6 AM and 9 AM, and PM observation times falling between 4 PM and 7 PM. As a result, by assuming 7 AM observation for all AM stations and 5 PM for all PM stations, a good estimate of the median bias is obtained for all AM or PM observations. Furthermore, nearly all the MD stations observed at midnight.
It should also be noted that the borders of the climate divisions in 1951 were not consistent with those defined in 1965. Due to the substantial additional effort it would have required locating each station within three or four climate divisions, as defined today, the change in the statewide percentage of AM, PM, and MD observation times was applied in equal proportions to all climate divisions prior to and including 1951.
Based on small differences between the two methods of estimating the time of observation bias, the simpler categorical procedure was used for all climate divisions. This should effectively eliminate most of the biases (over 2 Deg.F) in some climate divisions that have become part of the divisional averages. These biases affect both trends and actual estimates of divisional averages. Reference Karl, et al. (1986): "A model to estimate the time of observation bias associated with monthly mean maximum, minimum, and mean temperatures for the Unites States" (Thomas R. Karl, Claude N. Williams, Jr., and Pamela J. Young, National Climatic Data Center, and Wayne M. Wendland, Illinois State Water Survey, Journal of Climate and Applied Meteorology, January 1986, American Meteorological Society, Boston, MA). The time of observation bias model which can be used for the maximum, minimum, and mean monthly temperature at any location in the contiguous U.S. [program NCC*F11SRC (OBTBIAS Code) and NCC*F11REL (OBTBIAS)], is available from the NCDC.
Historical drought data have been added to this file for the period 1895 to present. The file is updated monthly. All drought data are calibrated using the period 1895-1990 (cf. Karl, 1986; Journal of Climate and Applied Meteorology, Vol. 25, No. 1, January 1986). Drought data include:
1. Palmer Drought Severity Index (PDSI)
This is the monthly value (index) that is generated indicating the severity
of a wet or dry spell. This index is based on the principles of a balance
between moisture supply and demand. Man-made changes were not considered in
this calculation. The index generally ranges from -6 to +6, with negative
values denoting dry spells and positive values indicating wet spells. There
are a few values in the magnitude of +7 or -7. PDSI values 0 to -.5 =
normal; -0.5 to -1.0 = incipient drought; -1.0 to -2.0 = mild drought; -2.0
to -3.0 = moderate drought; -3.0 to -4.0 = severe drought; and greater than -
4.0 = extreme drought. Similar adjectives are attached to positive values of
wet spells. This is a meteorological drought index used to assess the
severity of dry or wet spells of weather.
2. Palmer Hydrological Drought Index (PHDI)
This is the monthly value (index) generated monthly that indicates the
severity of a wet or dry spell. This index is based on the principles of a
balance between moisture supply and demand. Man-made changes such as
increased irrigation, new reservoirs, and added industrial water use were not
included in the computation of this index. The index generally ranges from -
6 to +6, with negative values denoting dry spells, and positive values
indicating wet spells. There are a few values in the magnitude of +7 or -7.
PHDI values 0 to -0.5 = normal; -0.5 to -1.0 = incipient drought; -1.0 to -
2.0 = mild drought; -2.0 to -3.0 = moderate drought; -3.0 to -4.0 = severe
drought; and greater than -4.0 = extreme drought. Similar adjectives are
attached to positive values of wet spells. This is a hydrological drought
index used to assess long-term moisture supply.
3. Palmer "Z" Index (ZNDX)
This is the generated monthly Z values, and they can be expressed as the
"Moisture Anomaly Index." Each monthly Z value is a measure of the departure
from normal of the moisture climate for that month. This index can respond
to a month of above-normal precipitation, even during periods of drought.
Table 1 contains expected values of the Z index and other drought parameters.
See Historical Climatology Series 3-6 through 3-9 for a detailed description
of the drought indices.
4. Modified Palmer Drought Severity Index (PMDI)
This is a modification of the Palmer Drought Severity Index. The
modification was made by the National Weather Service Climate Analysis Center
for operational meteorological purposes. The Palmer drought program
calculates three intermediate parallel index values each month. Only one
value is selected as the PDSI drought index for the month. This selection is
made internally by the program on the basis of probabilities. If the
probability that a drought is over is 100%, then one index is used. If the
probability that a wet spell is over is 100%, then another index is used. If
the probability is between 0% and 100%, the third index is assigned to the
PDSI. The modification (PMDI) incorporates a weighted average of the wet and
dry index terms, using the probability as the weighting factor. (Thomas R.
Heddinghause and Paul Sabol, 1991; "A Review of the Palmer Drought Severity
Index and Where Do We Go From Here?," Proceedings of the Seventh Conference
on Applied Climatology, pp. 242-246, American Meteorological Society, Boston,
MA). The PMDI and PDSI will have the same value during an established
drought or wet spell (i.e., when the probability is 100%), but they will have
different values during transition periods.
Classes for Wet and Dry Periods
| Approximate Cumulative Frequency (%) |
Range PHDI |
Category | Range Z |
|---|---|---|---|
| >96 | > 4.00 | Extreme wetness | >3.50 |
| 90-95 | 3.00, 3.99 | Severe wetness | 2.50, 3.49 |
| 73-89 | 1.50, 2.99 | Mild to moderate wetness | 1.00, 2.49 |
| 28-72 | -1.49, 1.49 | Near normal | -1.24, 0.99 |
| 11-27 | -1.50, -2.99 | Mild to moderate drought | -1.25, -1.99 |
| 5-10 | -3.00, -3.99 | Severe drought | -2.00, -2.74 |
| < 4 | < -4.00 | Extreme drought | < -2.75 |
For a complete description of the data see ftp://ftp.ncdc.noaa.gov/pub/data/cirs/drought.README