In Earth's troposphere, the lowest level of the atmosphere, temperature decreases with increasing height, but in the stratosphere, the next layer above the troposphere, temperature increases with increasing height. This happens because the stratosphere

The stratosphere contains ozone which traps solar radiation

Explanation:

Increase in temperature with increase in altitudes in the stratosphere is caused by the presence of ozone which traps solar radiation thus causing temperature to increase in the stratosphere with increase in altitude.