In all of our DG sets ( capacity 6 MW Wartsila make) we are facing problem of high bearing temperature during winter season and during night time only. In day time the temperatures are normal in the range of 90 Deg C but in night time the temperatures increases to more than 100 deg C.
The exhaust gas temperatures also increases by 15- 20 deg C.
Rest all operating conditions are same. No change.
What can be a problem

Can you correlate the bearing temp increase with dewpoint temperature? i.e. increasing dewpoint temp means increasing bearing temps

Don't use relative humidity.

As the dewpoint rises, the ability to transfer heat to surroundings is impaired.

What about oil viscosity,or cooling thermostat