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

Earth is warmer in northern hemisphere because of a couple of factors, such as the the tilt of the earth, and where it is at in comparison to the sun since Earth doesn't orbit a perfect circle.

The earth's tilt gives us season, and when the northern hemisphere is tilted toward the sun, it makes it summer there, and the placement of earth is close to the sun.

Now, the closer you go to the equator, the hotter you get, because the sun us always hitting the equator.

By definition, summer is the portion of the year in which the hemisphere is tilted toward the sun, so that sunlight strikes the surface more directly. When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and vice versa