fixed temperature discontinuity at height 254 (y == 110)

at 254 (y=110) the formula results in zero instead of about 18

Tested with Excel (and also in SSP):
  H	  y	        OLD		NEW
252	108	17,11880854		17,20440258
253	109	17,35096795		17,52447763
254	110	0		17,84750268
255	111	18,19166667		18,00975
256	112	18,26331658		18,172