Park rangers, soldiers, police and volunteers are searching for a tourist missing in Phu Soi Dao National Park in Uttaradit since last week.
They faced heavy rains on Monday to continue to look for Thanwarich Mateema in the park after he was reported missing on Thursday.
So far, the searchers have found only a backpack and shoes on a steep trail, apparently abandoned by the solo tourist.
About 50 searchers had joined the operation and the Public Relations Department on Monday doubled the number to 100.
Mr Thanwarich, 27, from Maha Sarakham, visited the park on Thursday. He registered while entering the park but did not show up at the office at the end of the mountainous trail on the same day.
His parents and other family members travelled from the northeastern province to the park and prayed for the team to find him.
