Using Excel Averageif with the HOUR function

I have 2 columns of data. 1 Column time (HH:MM:SS)(C3:C103) and the 2nd meter readings (D3:D103). For each hour there are 5 or 6 meter readings. I would like to average each hours readings to condense the data down to one averaged meter reading per hour. I tried =Averageif(C3:C103,Hour=10,D3:D103) but clearly have something wrong.