This is caused by the infrared (IR) illumination of your outdoor camera, which is what gives it night vision.
Rain and snow will reflect the IR light, causing a very obvious movement in the view of your camera.
Many insects are attracted to IR light as much as to visible light and this makes in front of your camera a great place for spiders to build their webs.
As any snow, rain, insects or spiders will now be in front of the floodlight, rather than directly in front of your camera, this will greatly reduce your chances of receiving false alerts at night.
The other method, which works well to reduce alerts particularly from rain, is to place cameras under an eave, awning or some other cover. This means that the rain is further away from the camera lens, which does reduce the number of alerts.
Note: don't forget to select the 'facing outside' function in 'fine tune smart detection', it is designed to handle outdoor monitoring.