Find the largest decrease in a sequence of numbers
This can be solved in a single pass, e.g., O(n).
For each temperature, subtract it from the highest temperature seen so far to get the drop.  Keep track of the largest drop.  Also update the highest temperature.
def biggest_drop(temperatures):
    highest = temperatures[0]
    largest_drop = 0

    for temperature in temperatures:
        drop = highest - temperature
        largest_drop = max(largest_drop, drop)

        highest = max(temperature, highest)

    return largest_drop