Standard deviation of hourly temperatures of 2 days
You are doing fine, other than a few simple nitpicks.

You are not using read-nth-line (which is a crazy thing to do anyway), please drop it.

You should fix your indentation, it would make your code much easier to read.

You probably want to divide by N-1, not N, in standard-dev and you probably want to simplify the code there by dividing by N-1 instead of multiplying by the reciprocal; also expt is relatively expensive, so, if you were not i/o bound anyway, you might want to replace it with multiplication.

Function split-in-half should probably return multiple values instead of a list.

Global variable l1 should be local to process-file.

Function parse-string-to-float should probably be called parse-string-to-floats (it read the whole list).