What is Drought?
In Drought and Its Causes and Effects, Tannehill (1947) wrote: "We
have no good definition of drought. We may say truthfully that
we scarcely know a drought when we see one. We welcome the first
clear day after a rainy spell. Rainless days continue for a time
and we are pleased to have a long spell of such fine weather.
It keeps on and we are a little worried. A few days more and
we are really in trouble. The first rainless day in a spell of
fine weather contributes as much to the drought as the last,
but no one knows how serious it will be until the last dry day
is gone and the rains have come again... we are not sure about
it until the crops have withered and died."
The
difficulty of recognizing the onset or end of a drought is compounded
by the lack of any clear definition of drought. Drought can be defined by rainfall
amounts, vegetation conditions, agricultural productivity, soil
moisture, levels in reservoirs and stream flow, or economic impacts.
In the most basic terms, a drought is simply a significant deficit
in moisture availability due to lower than normal rainfall. However
even this simple definition is complicated when attempts are
made to compare droughts in different regions. For example, a
drought in New Jersey would make for wet conditions in the deserts
of Arizona! 
Drought, as measured by scientists, is defined by evaluating precipitation, temperature, and soil moisture data, for the present and past months. A number of different indices of drought have been developed to quantify drought, each with its own strengths and weaknesses. Two of the most commonly used are the Palmer Drought Severity Index (PDSI) and the Standard Precipitation Index (SPI). Drought conditions are monitored constantly using these and other indices to provide current information on drought-impacted regions. For more complete information about drought definitions, indices, and current drought conditions, see the Web pages of the National Drought Mitigation Center or the links found at the NOAA Drought Information Center for the U.S., and Drought Watch on the Prairies for Canada.
Because of the elusive nature of drought, we do not think of droughts in the same way as other weather-related catastrophes, such as floods, tornadoes, and hurricanes. However, although droughts may be less spectacular, they are often more costly than other types of natural disasters, and no region in North America is immune to periodic droughts.
On to... Why are we concerned?