On the basis of KeepItSimpleStupid, have you considered using conditional formatting?
If you are keeping track of temperatures on an hourly basis per day, make cells A2, A3, A4……….A366(7) your dates and the hours columns are B1, C1, D1…………Z1. In cell AB1 place the figure 0.5.
Select the cells in the range B2:Z368 and format for Number with 1 decimal place. Whilst this range is selected, go to Format, Conditional Formatting, Formula Is =IF(B2-INT(B2)=$AB$1,TRUE,0) You can then select whichever format suits your needs. As you input your temperatures this formatting should apply to all temperatures ending in .5 .
Trusting that this is of some assistance, regards firstname.lastname@example.org