Why is earth's surface warmer in the northern hemisphere when it is summer there?

Are you asking why the summers in the northern hemisphere are warmer than the summers in the southern hemisphere? In that case the answer is that the northern hemisphere has more land than the southern hemisphere, and land heats up faster than water. So the summers get warmer relative to the southern hemisphere. For the same reason, southern hemisphere winter temperatures are generally warmer on average than northern hemisphere winter temperatures.