DEBOLINA ROY
Sebaceous glands become more activated by the increased temperature of the skin surface which will promote the pathway for the production of lipids. So as it's been getting hotter, this biological process is causing you to have greasy skin.
When there is high humidity, sweat can make it hard to evaporate properly. The sweat and the natural oil from your skin (sebum) creates a too much wetness and sticky feeling. This is one of main reasons to have oily skin in summer.
Sunlight causes infrared rays to cause deep warming of the body. The heat can reach the dermis, which sends a signal to your oil glands to go into overdrive. This is a defence mechanism for the skin. However, it leads to a more oily and shiny complexion.
During summer, dust and pollution will easily adhere to the sweat on your skin. In addition, the excess oil from your skin mixes with the dust and pollution causing them to congest the pores.
The heat will dry out your skin's top layer of skin. When your moisture layer is compromised, the brain sends a signal to your oil glands to produce more oil to protect the skin from dryness produce an oily surface in hot weather.