FOR EACH CASE, THE VARIABLES ARE RECORDED
so, for every 30 mins, humidity and temperature were recorded. therefore, time is case whereas humidity and temperature are variables. another way to look at this is .... for ech case (i.e. time) variables for two constants are recorded. which means that humidity and temperature were surely to be recorded for each case. the value or data it took were varied.
please let me know if it clears it up and let me know if there are any other doubts