Doi Inthanon is the highest mountain in Thailand.
its elevation is 2,565 m
1 km of height = -5 or -6 degrees
then in the mountain's peak -12.5 or -15
You said that in the winter in Thailand temperature is 15 degrees
so in the peak of Doi Inthanon the temperature is 0 - 2.5 degrees
but for snow temperature should be less than 0