Why is it hot in summer and cold in winter?

Because the earth's axis is tilted.

It is all about the tilt of the Earth's axis. Many people believe that the temperature changes because the Earth is farthest from the sun in July and is closest to the sun in January.....!!!

During the summer, the sun's rays hit the Earth at a steep angle. The light does not spread out as much, thus increasing the amount of energy hitting any given spot. Also, the long daylight hours allow the Earth plenty of time to reach warm temperatures.....!!!

During the winter, the sun's rays hit the Earth at a shallow angle. These rays are more spread out, which minimizes the amount of energy that hits any given spot. Also, the long nights and short days prevent the Earth from warming up. Thus, we have winter.....!!!!

Fig01. Earth Position In different months.

Our planet Earth reaches perihelion – The word “perihelion” is from the Greek words peri meaning near, and helios meaning sun, its closest point to the sun for the year – on January 4, 2014.

Earth is closest to the sun every year in early January, when it’s winter for the Northern Hemisphere. We’re farthest away from the sun in early July, during our Northern Hemisphere summer. So you can see that Earth’s distance from the sun isn’t what causes the seasons. On Earth, it’s mostly the tilt of our world’s axis that creates winter and summer. In winter, your part of Earth is tilted away from the sun. In summer, your part of Earth is tilted toward the sun. The day of maximum tilt toward or away from the sun is the December or June solstice.

  Fig02. Tilt of Earth in Summer and winter.