Dew Point
No maths just a simple sum.
Use the Dry Adiabatic Lapse Rate (DALR) which is 3 degree C per 1,000ft.
Thus; Surface Temp. 15 degrees C./Dewpoint 9 degrees C. therefore the dew point will be 2000ft., plus allow say 200ft - 300ft, not an exact science, for upward momentum. Answer will be in the region of 2,200-300ft cloudbase.