TotaLite is measuring reflections from the infrared ring of LED lights surrounding the image sensor. The LED's flash light the same moment the image sensor is capturing images.
During rain, the measurements become less reliable due to distorted reflections.
With fog, dew and frost the signal strength is reduced which can make targets, "disappear" from the field of view, they re-appear after the weather factors clear out.
Dirt and dust will reduce the signal strength and will require for the targets to be cleaned.
With direct sunlight exposure, noise is introduced in the measurement which reduces the signal strength and makes the targets disappear, when the sun moves away the measurement resumes.
Humidity doesn't have a direct effect as long as it does not exceed 95% Relative Humidity.
Measurements are possible during night-time as the dark has no effect on TotaLite Sensor functionality.