# An Overview of Autonomous Vehicles Sensors and Their Vulnerability to Weather Conditions
## 3 Weather Effects
### Fog  
Fog is a visible aerosol consisting of small water droplets suspended in the air or near Earth's surface [39]. It is considered as a low-lying cloud. This weather phenomenon typically begins around midnight and mostly dissipates after sunrise once atmospheric temperatures raises and relative humidity decreases.  
Dust or air pollution must be present in the air for fog to occur. Water vapor condenses around these small solid particles, generating droplets ranging in size from 1 to 20 microns [40]. This being the case, LiDAR systems, for which its operating wavelengths are less than fog particles, will be subject to Mie scattering. In addition to the adverse effects from scattering, water absorption has a huge impact on the NIR spectral band. Water's contribution to the extinction coefficient for the 905 nm and the 1550 nm are 0.075 cm\({}^{-1}\) and 10.8 cm\({}^{-1}\), respectively. According to [PERSON] et al. and [PERSON] et al., under normal conditions, the overall extinction coefficient between these two frequencies  
Figure 11: Rain RCS (**top**) and received power (**bottom**) for narrow beam (**left**) and wide beam (**right**) for different rain rates.  
are double one to another juxtaposed to the two magnitudes of difference observed from fog's presence.  
Ultrasonic sensors will not be directly affected by scattering compared to most of other sensors. However, being a system working with sound waves, air constitution and temperature would influence its performance. Nonetheless, given the fact that ultrasonic sensors are used for applications that required very short-range detection, the effect of precipitation is very minimal.  
The range degradation curves presented by [PERSON] et al. in Figure 12 show the severity of the signal attenuation seen in different ratings of fog. What should be 0.5 km of visibility has diminished to about 0.20 km and 0.12 km for the 905 nm and 1550 nm wavelengths, although they have the same maximum range performance under normal conditions. Moderate continental fog and heavy maritime fog can attenuate NIR signals up to 130 dB/km and 480 dB/km, respectively [36]. Fog would exhibit slight Rayleigh scattering on millimeter waves due to the extreme magnitude difference of particle to wave size. Fog might indirectly affect RADAR should temperature requirements be met by condensing on the radome or target in question and, thus, emulating what was explained in the above section. The camera, on the other hand, is heavily affected by Mie scattering since its operating wavelengths (400-750 nm) are much smaller than fog particle size. Reference [37] examines a camera's ability within a fog chamber of 2 micron and 6 micron size fog particles. It was noted that visibility was reduced to 30 m compared to its ideal 60 m. The presence of air-light should also be taken into consideration since fog facilitates it. Air-light can be defined as the scattering of light from the interference of particles. With air-light interference, objects within the immediate vicinity of the light source are impossible to perceive. As the proximity increases, the perception of other objects is left to how the air-light interferes with the camera. Object distinction becomes increasingly difficult within fog and becomes more reliant on the reflectivity of the objects in question. The higher the reflectivity of an object, the darker it would appear in fog [37]. Ultrasonic sensors, relying on air constitution, would also be affected by fog. Depending on the type of fog, the air's water density can range from 0.01 to 0.2 g/cm\({}^{3}\). This interaction will be discussed further in the humidity section.