There are still a lot of unknowns when it comes to COVID-19 so we do not know exactly how coronavirus will react to a change in weather. That said, we do know that the spread of most other viruses reduces when the weather warms up in spring.
Most viruses are transmitted through droplets of water/saliva. We believe coronavirus is transmitted this way as well. In winter, the air is dry and the lack of humidity and moisture allows those droplets to carry much further than they do in the spring and summer. This means that people spread the virus over much further distances. Cold weather also means that people gather in close indoor spaces where mechanical heating systems dry the air even further.
We hope that warmer weather in spring and summer will make the virus less contagious, but given that this is a new disease, we will have to wait and see.