Studies have shown that meteorological drought is never the result of a single cause.  It is the result of many factors accumulating over a period of time.

Global Weather Patterns

A great deal of research has been conducted in recent years on the role of interacting systems, or teleconnections, in explaining
regional and even global patterns of climatic variability. These patterns tend to recur periodically with enough frequency and with
similar characteristics over a sufficient length of time that they offer opportunities to improve our ability for long-range climate
prediction, particularly in the tropics. One such teleconnection is the El Niño/Southern Oscillation (ENSO).

High Pressure

The immediate cause of drought is the predominant sinking motion of air (subsidence) that results in compressional warming or
high pressure, which inhibits cloud formation and results in lower relative humidity and less precipitation. Regions under the
influence of semipermanent high pressure during all or a major portion of the year are usually deserts, such as the Sahara and
Kalahari deserts of Africa and the Gobi Desert of Asia. Most climatic regions experience varying degrees of dominance by high
pressure, often depending on the season. Prolonged droughts occur when large-scale anomalies in atmospheric circulation
patterns persist for months or seasons (or longer). The extreme drought that affected the United States and Canada during 1988
resulted from the persistence of a large-scale atmospheric circulation anomaly.

Too Many Variables

Scientists don't know how to predict drought a month or more in advance for most locations. Predicting drought depends on the
ability to forecast two fundamental meteorological surface parameters, precipitation and temperature. From the historical record
we know that climate is inherently variable. We also know that anomalies of precipitation and temperature may last from several
months to several decades. How long they last depends on air-sea interactions, soil moisture and land surface processes,
topography, internal dynamics, and the accumulated influence of dynamically unstable synoptic weather systems at the global
scale.

The potential for improved drought predictions in the near future differs by region, season, and climatic regime.

The Tropical Outlook

In the tropics, for example, meteorologists have made significant advances in understanding the climate system. Specifically, it is
now known that a major portion of the atmospheric variability that occurs on time scales of months to several years is
associated with variations in tropical sea surface temperatures. The recently completed Tropical Ocean Global Atmosphere
(TOGA) project has produced results that suggest that it may now be possible to predict certain climatic conditions associated
with ENSO events more than a year in advance. For those regions whose climate is greatly influenced by ENSO events, TOGA
project results may help produce more reliable meteorological forecasts that can reduce risks in those economic sectors most
sensitive to climate variability and, particularly, extreme events such as drought.

The Temperate Zone Outlook

In the extratropical regions, current long-range forecasts are of very limited reliability. The ability that does exist is primarily the
result of empirical and statistical relationships. In the tropics, empirical relationships have been demonstrated to exist between
precipitation and ENSO events, but few such relationships have been confirmed above 30 degrees north latitude.
Meteorologists do not believe that reliable forecasts are attainable for all regions a season or more in advance.