Thailand's agricultural industry has been hard hit by the drought

He is expanding a cloud-seeding technique he patented in 2002, which involves aircraft releasing a chemical into clouds to induce rainfall.

He suggested the idea as most of Thailand's 76 provinces face their worst drought for several years.

An additional 17 aircraft have been drafted in for the scheme.

It has also been expanded from 10 to 22 bases across the country.

"The king has set up a centre for rain-making at Hua Hin [his seaside residence] and will personally command the centre to help alleviate drought," Deputy Agriculture Minister Newin Chidchob told the AFP news agency.

Disaster zones

Cloud-seeding involves firing particles, usually silver iodide, into clouds to encourage water vapour to gather round them and eventually fall as rain.

CLOUD SEEDING 1. Aircraft or artillery spray chemicals (often silver iodide or dry ice) into clouds to encourage tiny vapour droplets to coalesce

2. Droplets of supercooled water (liquid below freezing) coalesce into snow and melt as they fall

3. Heat released as the droplets freeze boosts updrafts, which pull more moist air into the cloud

King Bhumibol Adulyadej's technique involves using two aircraft to seed warm and cold clouds at different altitudes.

This technique is said to be particularly successful because it can more precisely target areas where the rain is to fall.

The monarch is said to have suggested his rain-making technique after expressing his concern about the drought during a meeting with Prime Minister Thaksin Shinawatra.

The drought has affected more than 8.3 million people, and 10 areas in the northern province of Chiang Mai have been declared disaster zones.

It also hit Thailand's fourth quarter economic results by significantly affecting output in the agricultural sector.