Answer:
The Sun's rays strike the Northern Hemisphere at a straighter angle.
Step-by-step explanation:
Earth's axis is tilted, which means that while orbiting, during half of the year it exposes one of the hemispheres more to the Sun, while the other is less exposed, and in the other half of the year the roles are reversed. When the Northern Hemisphere is tilted toward the Sun it experiences summer. This is due to the straighter angle of the Sun's rays and the longer duration of the days.
Having higher exposure to the Sun, and its rays coming at a straighter angle, complemented with longer days, contributes to higher temperatures in the Northern Hemisphere. This doesn't only happen to extremes in the tropical areas of the Northern Hemisphere, but in the temperate zones as well. Since some of the temperate zones are bordering the tropical ones, they have a bigger influence from them during summer, while the ones that are deeper into the continents also manage to build up a lot of heat, resulting in temperatures that can go as high as 40 or even 45 C degrees.