At present, the limitations of the indices used to measure dry periods mean agencies only become aware of a drought when they are in the midst of it, said Mannava Sivakumar, director of WMO's Agricultural Meteorology Division.
Droughts have become more intense and frequent in recent years because of climate change, especially in Africa, but can now be monitored by the Standardized Precipitation Index (SPI), which uses mean rainfall over a long-term period of at least 30 years as a variable to develop an early warning scale.
"At the moment most countries use indices which are based on the percentage of normal rainfall days over period of time," said Sivakumar. A drought begins when the SPI is continuously negative for a period of two to three weeks, and ends when the numbers turn positive.
An effective early warning system for drought has been hard to put together because they are slow-onset disasters, but the SPI is also able to predict the intensity of the drought - the higher the negative number the more severe the drought.
The index has been around for some time, but last week meteorological experts from across the globe decided that all national meteorological and hydrological services should use it to characterize meteorological droughts.
The decision is expected to become effective in another six months, after it has been endorsed by the WMO board and congress.
Experts have been trying to decide on standardized indices to measure three types of droughts - meteorological, agricultural and hydrological - to help aid agencies respond to these disasters effectively.
The problem is that at least 20 kinds of drought indices - numerical scales based on data such as rainfall and temperature - are being used to measure the three types of droughts, which makes it difficult to compare the intensity and type of drought affecting various countries.
A meteorological drought measured by the SPI monitors the deficiency of precipitation; agricultural and hydrological indices take into account the interaction between a meteorological drought and human beings, such as the amount of water in the soil for crops, and access to water supplies.
By the end of 2010, WMO hopes to have identified standardized indices to monitor agricultural and hydrological droughts. Droughts were first recorded 3,000 years ago, but Sivakumar noted that it has taken a long time to develop an early warning system, and the need for it was urgent.
The pastoral nomads in Kenya used to record a drought once in every 10 years; now they experience one in every two or three years, he said. Kenya is experiencing one of its worst droughts, with more than four million people in need of food assistance.