Coming from Singapore, we practically experience hot and humid weather. Anything below 25°c is already cooling for us. Like sleep weather, where everyone just wanna stop what they're doing and just take a nap. Lol!
As far as my travels went, South East Asia do have pretty cold countries. Some cold enough to even get snow! Night times are mostly cool; and mountain peaks are obviously super cold. So I don't even bother going there. What I'm looking at are cold low lying places in South East Asia.
Chiang Mai/Chiang Rai, Thailand.
Once you land on its airport, a whiff of cold air just sweeps you off the ground. Prepare to withstand around 20°c-23°c daytime. Kinda like aircon weather already.
Everywhere in Laos.
Laos is surprisingly a super cold country. Probably due to its vast amounts of untouched mountaineous regions and forests. At night, it can go as low as 14°c. We went to Luang Prabang, Vang Vieng and Vientiane. Everywhere we went, its like aircon weather. Some places even worst.
Northern parts, Mui Ne, Vietnam
When we were in Vietnam in 2013, the weather is super cooling. There was even snow in some parts of northern Vietnam. Even in Mui ne, it was quite challenging to get from one place to another without shivering.
Sagada, Philippines
Temperatures can drop till 10°c.
Just be prepared when going to these places. Bring extra layers of clothings!